[clang-tools-extra] 637da9d - Revert "[clang] template / auto deduction deduces common sugar"

Alexander Kornienko via cfe-commits cfe-commits at lists.llvm.org
Tue Sep 13 03:18:25 PDT 2022


Author: Alexander Kornienko
Date: 2022-09-13T12:18:07+02:00
New Revision: 637da9de4c6619c0e179c2c2f0dbfebd08ac2a0f

URL: https://github.com/llvm/llvm-project/commit/637da9de4c6619c0e179c2c2f0dbfebd08ac2a0f
DIFF: https://github.com/llvm/llvm-project/commit/637da9de4c6619c0e179c2c2f0dbfebd08ac2a0f.diff

LOG: Revert "[clang] template / auto deduction deduces common sugar"

This reverts commit d200db38637884fd0b421802c6094b2a03ceb29e, which causes a
clang crash. See https://reviews.llvm.org/D111283#3785755

Test case for convenience:
```
template <typename T>
using P = int T::*;

template <typename T, typename... A>
void j(P<T>, T, A...);

template <typename T>
void j(P<T>, T);

struct S {
  int b;
};
void g(P<S> k, S s) { j(k, s); }
```

Added: 
    

Modified: 
    clang-tools-extra/clangd/unittests/ASTTests.cpp
    clang-tools-extra/clangd/unittests/HoverTests.cpp
    clang/include/clang/AST/ASTContext.h
    clang/include/clang/AST/Type.h
    clang/include/clang/Sema/Sema.h
    clang/lib/AST/ASTContext.cpp
    clang/lib/Sema/SemaDecl.cpp
    clang/lib/Sema/SemaDeclCXX.cpp
    clang/lib/Sema/SemaExprCXX.cpp
    clang/lib/Sema/SemaOverload.cpp
    clang/lib/Sema/SemaStmt.cpp
    clang/lib/Sema/SemaTemplate.cpp
    clang/lib/Sema/SemaTemplateDeduction.cpp
    clang/test/SemaCXX/cxx0x-initializer-stdinitializerlist.cpp
    clang/test/SemaCXX/deduced-return-void.cpp
    clang/test/SemaCXX/sugared-auto.cpp
    clang/test/SemaTemplate/deduction.cpp

Removed: 
    


################################################################################
diff  --git a/clang-tools-extra/clangd/unittests/ASTTests.cpp b/clang-tools-extra/clangd/unittests/ASTTests.cpp
index 8f67136585308..4bb3e025b87a5 100644
--- a/clang-tools-extra/clangd/unittests/ASTTests.cpp
+++ b/clang-tools-extra/clangd/unittests/ASTTests.cpp
@@ -84,7 +84,7 @@ TEST(GetDeducedType, KwAutoKwDecltypeExpansion) {
 
               ^auto i = {1,2};
           )cpp",
-          "std::initializer_list<int>",
+          "class std::initializer_list<int>",
       },
       {
           R"cpp( // auto in function return type with trailing return type

diff  --git a/clang-tools-extra/clangd/unittests/HoverTests.cpp b/clang-tools-extra/clangd/unittests/HoverTests.cpp
index a09e19ce9ff7f..d4000f31a5f7e 100644
--- a/clang-tools-extra/clangd/unittests/HoverTests.cpp
+++ b/clang-tools-extra/clangd/unittests/HoverTests.cpp
@@ -1940,7 +1940,7 @@ TEST(Hover, All) {
           [](HoverInfo &HI) {
             HI.Name = "auto";
             HI.Kind = index::SymbolKind::TypeAlias;
-            HI.Definition = "std::initializer_list<int>";
+            HI.Definition = "class std::initializer_list<int>";
           }},
       {
           R"cpp(// User defined conversion to auto

diff  --git a/clang/include/clang/AST/ASTContext.h b/clang/include/clang/AST/ASTContext.h
index 8ef7a32e6f09e..eb52bf5c736a0 100644
--- a/clang/include/clang/AST/ASTContext.h
+++ b/clang/include/clang/AST/ASTContext.h
@@ -2807,23 +2807,6 @@ class ASTContext : public RefCountedBase<ASTContext> {
     return AddrSpaceMapMangling || isTargetAddressSpace(AS);
   }
 
-  // Merges two exception specifications, such that the resulting
-  // exception spec is the union of both. For example, if either
-  // of them can throw something, the result can throw it as well.
-  FunctionProtoType::ExceptionSpecInfo
-  mergeExceptionSpecs(FunctionProtoType::ExceptionSpecInfo ESI1,
-                      FunctionProtoType::ExceptionSpecInfo ESI2,
-                      SmallVectorImpl<QualType> &ExceptionTypeStorage,
-                      bool AcceptDependent);
-
-  // For two "same" types, return a type which has
-  // the common sugar between them. If Unqualified is true,
-  // both types need only be the same unqualified type.
-  // The result will drop the qualifiers which do not occur
-  // in both types.
-  QualType getCommonSugaredType(QualType X, QualType Y,
-                                bool Unqualified = false);
-
 private:
   // Helper for integer ordering
   unsigned getIntegerRank(const Type *T) const;

diff  --git a/clang/include/clang/AST/Type.h b/clang/include/clang/AST/Type.h
index 5897453280729..88e2fb338a328 100644
--- a/clang/include/clang/AST/Type.h
+++ b/clang/include/clang/AST/Type.h
@@ -4320,9 +4320,10 @@ class FunctionProtoType final
   }
 
   using param_type_iterator = const QualType *;
+  using param_type_range = llvm::iterator_range<param_type_iterator>;
 
-  ArrayRef<QualType> param_types() const {
-    return llvm::makeArrayRef(param_type_begin(), param_type_end());
+  param_type_range param_types() const {
+    return param_type_range(param_type_begin(), param_type_end());
   }
 
   param_type_iterator param_type_begin() const {

diff  --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
index 43476f63fb96e..459c1109b852e 100644
--- a/clang/include/clang/Sema/Sema.h
+++ b/clang/include/clang/Sema/Sema.h
@@ -8771,9 +8771,7 @@ class Sema final {
     /// Deduction failed; that's all we know.
     TDK_MiscellaneousDeductionFailure,
     /// CUDA Target attributes do not match.
-    TDK_CUDATargetMismatch,
-    /// Some error which was already diagnosed.
-    TDK_AlreadyDiagnosed
+    TDK_CUDATargetMismatch
   };
 
   TemplateDeductionResult
@@ -8864,11 +8862,21 @@ class Sema final {
   TypeSourceInfo *ReplaceAutoTypeSourceInfo(TypeSourceInfo *TypeWithAuto,
                                             QualType Replacement);
 
-  TemplateDeductionResult DeduceAutoType(TypeLoc AutoTypeLoc, Expr *Initializer,
-                                         QualType &Result,
-                                         sema::TemplateDeductionInfo &Info,
-                                         bool DependentDeduction = false,
-                                         bool IgnoreConstraints = false);
+  /// Result type of DeduceAutoType.
+  enum DeduceAutoResult {
+    DAR_Succeeded,
+    DAR_Failed,
+    DAR_FailedAlreadyDiagnosed
+  };
+
+  DeduceAutoResult
+  DeduceAutoType(TypeSourceInfo *AutoType, Expr *&Initializer, QualType &Result,
+                 Optional<unsigned> DependentDeductionDepth = None,
+                 bool IgnoreConstraints = false);
+  DeduceAutoResult
+  DeduceAutoType(TypeLoc AutoTypeLoc, Expr *&Initializer, QualType &Result,
+                 Optional<unsigned> DependentDeductionDepth = None,
+                 bool IgnoreConstraints = false);
   void DiagnoseAutoDeductionFailure(VarDecl *VDecl, Expr *Init);
   bool DeduceReturnType(FunctionDecl *FD, SourceLocation Loc,
                         bool Diagnose = true);
@@ -8890,8 +8898,8 @@ class Sema final {
   TypeLoc getReturnTypeLoc(FunctionDecl *FD) const;
 
   bool DeduceFunctionTypeFromReturnExpr(FunctionDecl *FD,
-                                        SourceLocation ReturnLoc, Expr *RetExpr,
-                                        const AutoType *AT);
+                                        SourceLocation ReturnLoc,
+                                        Expr *&RetExpr, const AutoType *AT);
 
   FunctionTemplateDecl *getMoreSpecializedTemplate(
       FunctionTemplateDecl *FT1, FunctionTemplateDecl *FT2, SourceLocation Loc,

diff  --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
index d7c626adee19b..28f4e19893326 100644
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -3207,9 +3207,9 @@ bool ASTContext::hasSameFunctionTypeIgnoringExceptionSpec(QualType T,
 QualType ASTContext::getFunctionTypeWithoutPtrSizes(QualType T) {
   if (const auto *Proto = T->getAs<FunctionProtoType>()) {
     QualType RetTy = removePtrSizeAddrSpace(Proto->getReturnType());
-    SmallVector<QualType, 16> Args(Proto->param_types().size());
+    SmallVector<QualType, 16> Args(Proto->param_types());
     for (unsigned i = 0, n = Args.size(); i != n; ++i)
-      Args[i] = removePtrSizeAddrSpace(Proto->param_types()[i]);
+      Args[i] = removePtrSizeAddrSpace(Args[i]);
     return getFunctionType(RetTy, Args, Proto->getExtProtoInfo());
   }
 
@@ -12137,557 +12137,6 @@ unsigned ASTContext::getTargetAddressSpace(LangAS AS) const {
     return (*AddrSpaceMap)[(unsigned)AS];
 }
 
-// The getCommon* helpers return, for given 'same' X and Y entities given as
-// inputs, another entity which is also the 'same' as the inputs, but which
-// is closer to the canonical form of the inputs, each according to a given
-// criteria.
-// The getCommon*Checked variants are 'null inputs not-allowed' equivalents of
-// the regular ones.
-
-static Decl *getCommonDecl(Decl *X, Decl *Y) {
-  if (X == Y)
-    return X;
-  assert(declaresSameEntity(X, Y));
-  for (const Decl *DX : X->redecls()) {
-    // If we reach Y before reaching the first decl, that means X is older.
-    if (DX == Y)
-      return X;
-    // If we reach the first decl, then Y is older.
-    if (DX->isFirstDecl())
-      return Y;
-  }
-  llvm_unreachable("Corrupt redecls chain");
-}
-
-template <class T,
-          std::enable_if_t<std::is_base_of<Decl, T>::value, bool> = true>
-T *getCommonDecl(T *X, T *Y) {
-  return cast_or_null<T>(
-      getCommonDecl(const_cast<Decl *>(cast_or_null<Decl>(X)),
-                    const_cast<Decl *>(cast_or_null<Decl>(Y))));
-}
-
-template <class T,
-          std::enable_if_t<std::is_base_of<Decl, T>::value, bool> = true>
-T *getCommonDeclChecked(T *X, T *Y) {
-  return cast<T>(getCommonDecl(const_cast<Decl *>(cast<Decl>(X)),
-                               const_cast<Decl *>(cast<Decl>(Y))));
-}
-
-static TemplateName getCommonTemplateName(ASTContext &Ctx, TemplateName X,
-                                          TemplateName Y) {
-  if (X.getAsVoidPointer() == Y.getAsVoidPointer())
-    return X;
-  // FIXME: There are cases here where we could find a common template name
-  //        with more sugar. For example one could be a SubstTemplateTemplate*
-  //        replacing the other.
-  TemplateName CX = Ctx.getCanonicalTemplateName(X);
-  assert(CX.getAsVoidPointer() ==
-         Ctx.getCanonicalTemplateName(Y).getAsVoidPointer());
-  return CX;
-}
-
-static auto getCommonTypes(ASTContext &Ctx, ArrayRef<QualType> Xs,
-                           ArrayRef<QualType> Ys, bool Unqualified = false) {
-  assert(Xs.size() == Ys.size());
-  SmallVector<QualType, 8> Rs(Xs.size());
-  for (size_t I = 0; I < Rs.size(); ++I)
-    Rs[I] = Ctx.getCommonSugaredType(Xs[I], Ys[I], Unqualified);
-  return Rs;
-}
-
-template <class T>
-static SourceLocation getCommonAttrLoc(const T *X, const T *Y) {
-  return X->getAttributeLoc() == Y->getAttributeLoc() ? X->getAttributeLoc()
-                                                      : SourceLocation();
-}
-
-static TemplateArgument getCommonTemplateArgument(ASTContext &Ctx,
-                                                  const TemplateArgument &X,
-                                                  const TemplateArgument &Y) {
-  assert(X.getKind() == Y.getKind());
-  switch (X.getKind()) {
-  case TemplateArgument::ArgKind::Type:
-    return TemplateArgument(
-        Ctx.getCommonSugaredType(X.getAsType(), Y.getAsType()));
-  case TemplateArgument::ArgKind::NullPtr:
-    return TemplateArgument(
-        Ctx.getCommonSugaredType(X.getNullPtrType(), Y.getNullPtrType()),
-        /*Unqualified=*/true);
-  default:
-    // FIXME: Handle the other argument kinds.
-    return X;
-  }
-}
-
-static auto getCommonTemplateArguments(ASTContext &Ctx,
-                                       ArrayRef<TemplateArgument> X,
-                                       ArrayRef<TemplateArgument> Y) {
-  SmallVector<TemplateArgument, 8> R(X.size());
-  for (size_t I = 0; I < R.size(); ++I)
-    R[I] = getCommonTemplateArgument(Ctx, X[I], Y[I]);
-  return R;
-}
-
-template <class T>
-static ElaboratedTypeKeyword getCommonTypeKeyword(const T *X, const T *Y) {
-  return X->getKeyword() == Y->getKeyword() ? X->getKeyword()
-                                            : ElaboratedTypeKeyword::ETK_None;
-}
-
-template <class T>
-static NestedNameSpecifier *getCommonNNS(ASTContext &Ctx, const T *X,
-                                         const T *Y) {
-  // FIXME: Try to keep the common NNS sugar.
-  return X->getQualifier() == Y->getQualifier()
-             ? X->getQualifier()
-             : Ctx.getCanonicalNestedNameSpecifier(X->getQualifier());
-}
-
-template <class T>
-static QualType getCommonElementType(ASTContext &Ctx, const T *X, const T *Y) {
-  return Ctx.getCommonSugaredType(X->getElementType(), Y->getElementType());
-}
-
-template <class T>
-static QualType getCommonPointeeType(ASTContext &Ctx, const T *X, const T *Y) {
-  return Ctx.getCommonSugaredType(X->getPointeeType(), Y->getPointeeType());
-}
-
-template <class T> static auto *getCommonSizeExpr(T *X, T *Y) {
-  assert(X->getSizeExpr() == Y->getSizeExpr());
-  return X->getSizeExpr();
-}
-
-static auto getCommonSizeModifier(const ArrayType *X, const ArrayType *Y) {
-  assert(X->getSizeModifier() == Y->getSizeModifier());
-  return X->getSizeModifier();
-}
-
-static auto getCommonIndexTypeCVRQualifiers(const ArrayType *X,
-                                            const ArrayType *Y) {
-  assert(X->getIndexTypeCVRQualifiers() == Y->getIndexTypeCVRQualifiers());
-  return X->getIndexTypeCVRQualifiers();
-}
-
-// Merges two type lists such that the resulting vector will contain
-// each type (in a canonical sense) only once, in the order they appear
-// from X to Y. If they occur in both X and Y, the result will contain
-// the common sugared type between them.
-static void mergeTypeLists(ASTContext &Ctx, SmallVectorImpl<QualType> &Out,
-                           ArrayRef<QualType> X, ArrayRef<QualType> Y) {
-  llvm::DenseMap<QualType, unsigned> Found;
-  for (auto Ts : {X, Y}) {
-    for (QualType T : Ts) {
-      auto Res = Found.try_emplace(Ctx.getCanonicalType(T), Out.size());
-      if (!Res.second) {
-        QualType &U = Out[Res.first->second];
-        U = Ctx.getCommonSugaredType(U, T);
-      } else {
-        Out.emplace_back(T);
-      }
-    }
-  }
-}
-
-FunctionProtoType::ExceptionSpecInfo
-ASTContext::mergeExceptionSpecs(FunctionProtoType::ExceptionSpecInfo ESI1,
-                                FunctionProtoType::ExceptionSpecInfo ESI2,
-                                SmallVectorImpl<QualType> &ExceptionTypeStorage,
-                                bool AcceptDependent) {
-  ExceptionSpecificationType EST1 = ESI1.Type, EST2 = ESI2.Type;
-
-  // If either of them can throw anything, that is the result.
-  for (auto I : {EST_None, EST_MSAny, EST_NoexceptFalse}) {
-    if (EST1 == I)
-      return ESI1;
-    if (EST2 == I)
-      return ESI2;
-  }
-
-  // If either of them is non-throwing, the result is the other.
-  for (auto I :
-       {EST_NoThrow, EST_DynamicNone, EST_BasicNoexcept, EST_NoexceptTrue}) {
-    if (EST1 == I)
-      return ESI2;
-    if (EST2 == I)
-      return ESI1;
-  }
-
-  // If we're left with value-dependent computed noexcept expressions, we're
-  // stuck. Before C++17, we can just drop the exception specification entirely,
-  // since it's not actually part of the canonical type. And this should never
-  // happen in C++17, because it would mean we were computing the composite
-  // pointer type of dependent types, which should never happen.
-  if (EST1 == EST_DependentNoexcept || EST2 == EST_DependentNoexcept) {
-    assert(AcceptDependent &&
-           "computing composite pointer type of dependent types");
-    return FunctionProtoType::ExceptionSpecInfo();
-  }
-
-  // Switch over the possibilities so that people adding new values know to
-  // update this function.
-  switch (EST1) {
-  case EST_None:
-  case EST_DynamicNone:
-  case EST_MSAny:
-  case EST_BasicNoexcept:
-  case EST_DependentNoexcept:
-  case EST_NoexceptFalse:
-  case EST_NoexceptTrue:
-  case EST_NoThrow:
-    llvm_unreachable("These ESTs should be handled above");
-
-  case EST_Dynamic: {
-    // This is the fun case: both exception specifications are dynamic. Form
-    // the union of the two lists.
-    assert(EST2 == EST_Dynamic && "other cases should already be handled");
-    mergeTypeLists(*this, ExceptionTypeStorage, ESI1.Exceptions,
-                   ESI2.Exceptions);
-    FunctionProtoType::ExceptionSpecInfo Result(EST_Dynamic);
-    Result.Exceptions = ExceptionTypeStorage;
-    return Result;
-  }
-
-  case EST_Unevaluated:
-  case EST_Uninstantiated:
-  case EST_Unparsed:
-    llvm_unreachable("shouldn't see unresolved exception specifications here");
-  }
-
-  llvm_unreachable("invalid ExceptionSpecificationType");
-}
-
-static QualType getCommonType(ASTContext &Ctx, const Type *X, const Type *Y) {
-  Type::TypeClass TC = X->getTypeClass();
-  assert(TC == Y->getTypeClass());
-  switch (TC) {
-#define UNEXPECTED_TYPE(Class, Kind)                                           \
-  case Type::Class:                                                            \
-    llvm_unreachable("Unexpected " Kind ": " #Class);
-
-#define NON_CANONICAL_TYPE(Class, Base) UNEXPECTED_TYPE(Class, "non-canonical")
-#define TYPE(Class, Base)
-#include "clang/AST/TypeNodes.inc"
-
-#define SUGAR_FREE_TYPE(Class) UNEXPECTED_TYPE(Class, "sugar-free")
-    SUGAR_FREE_TYPE(Builtin)
-    SUGAR_FREE_TYPE(Decltype)
-    SUGAR_FREE_TYPE(DeducedTemplateSpecialization)
-    SUGAR_FREE_TYPE(DependentBitInt)
-    SUGAR_FREE_TYPE(Enum)
-    SUGAR_FREE_TYPE(BitInt)
-    SUGAR_FREE_TYPE(ObjCInterface)
-    SUGAR_FREE_TYPE(Record)
-    SUGAR_FREE_TYPE(SubstTemplateTypeParmPack)
-    SUGAR_FREE_TYPE(TemplateTypeParm)
-    SUGAR_FREE_TYPE(UnresolvedUsing)
-#undef SUGAR_FREE_TYPE
-#define NON_UNIQUE_TYPE(Class) UNEXPECTED_TYPE(Class, "non-unique")
-    NON_UNIQUE_TYPE(TypeOfExpr)
-    NON_UNIQUE_TYPE(VariableArray)
-#undef NON_UNIQUE_TYPE
-
-    UNEXPECTED_TYPE(TypeOf, "sugar")
-
-#undef UNEXPECTED_TYPE
-
-  case Type::Auto: {
-    const auto *AX = cast<AutoType>(X), *AY = cast<AutoType>(Y);
-    assert(AX->getDeducedType().isNull());
-    assert(AY->getDeducedType().isNull());
-    assert(AX->getKeyword() == AY->getKeyword());
-    assert(AX->isInstantiationDependentType() ==
-           AY->isInstantiationDependentType());
-    auto As = getCommonTemplateArguments(Ctx, AX->getTypeConstraintArguments(),
-                                         AY->getTypeConstraintArguments());
-    return Ctx.getAutoType(QualType(), AX->getKeyword(),
-                           AX->isInstantiationDependentType(),
-                           AX->containsUnexpandedParameterPack(),
-                           getCommonDecl(AX->getTypeConstraintConcept(),
-                                         AY->getTypeConstraintConcept()),
-                           As);
-  }
-  case Type::IncompleteArray: {
-    const auto *AX = cast<IncompleteArrayType>(X),
-               *AY = cast<IncompleteArrayType>(Y);
-    return Ctx.getIncompleteArrayType(getCommonElementType(Ctx, AX, AY),
-                                      getCommonSizeModifier(AX, AY),
-                                      getCommonIndexTypeCVRQualifiers(AX, AY));
-  }
-  case Type::DependentSizedArray: {
-    const auto *AX = cast<DependentSizedArrayType>(X),
-               *AY = cast<DependentSizedArrayType>(Y);
-    return Ctx.getDependentSizedArrayType(
-        getCommonElementType(Ctx, AX, AY), getCommonSizeExpr(AX, AY),
-        getCommonSizeModifier(AX, AY), getCommonIndexTypeCVRQualifiers(AX, AY),
-        AX->getBracketsRange() == AY->getBracketsRange()
-            ? AX->getBracketsRange()
-            : SourceRange());
-  }
-  case Type::ConstantArray: {
-    const auto *AX = cast<ConstantArrayType>(X),
-               *AY = cast<ConstantArrayType>(Y);
-    assert(AX->getSize() == AY->getSize());
-    return Ctx.getConstantArrayType(getCommonElementType(Ctx, AX, AY),
-                                    AX->getSize(), getCommonSizeExpr(AX, AY),
-                                    getCommonSizeModifier(AX, AY),
-                                    getCommonIndexTypeCVRQualifiers(AX, AY));
-  }
-  case Type::Atomic: {
-    const auto *AX = cast<AtomicType>(X), *AY = cast<AtomicType>(Y);
-    return Ctx.getAtomicType(
-        Ctx.getCommonSugaredType(AX->getValueType(), AY->getValueType()));
-  }
-  case Type::Complex: {
-    const auto *CX = cast<ComplexType>(X), *CY = cast<ComplexType>(Y);
-    return Ctx.getComplexType(getCommonElementType(Ctx, CX, CY));
-  }
-  case Type::Pointer: {
-    const auto *PX = cast<PointerType>(X), *PY = cast<PointerType>(Y);
-    return Ctx.getPointerType(getCommonPointeeType(Ctx, PX, PY));
-  }
-  case Type::BlockPointer: {
-    const auto *PX = cast<BlockPointerType>(X), *PY = cast<BlockPointerType>(Y);
-    return Ctx.getBlockPointerType(getCommonPointeeType(Ctx, PX, PY));
-  }
-  case Type::ObjCObjectPointer: {
-    const auto *PX = cast<ObjCObjectPointerType>(X),
-               *PY = cast<ObjCObjectPointerType>(Y);
-    return Ctx.getObjCObjectPointerType(getCommonPointeeType(Ctx, PX, PY));
-  }
-  case Type::MemberPointer: {
-    const auto *PX = cast<MemberPointerType>(X),
-               *PY = cast<MemberPointerType>(Y);
-    return Ctx.getMemberPointerType(
-        getCommonPointeeType(Ctx, PX, PY),
-        Ctx.getCommonSugaredType(QualType(PX->getClass(), 0),
-                                 QualType(PY->getClass(), 0))
-            .getTypePtr());
-  }
-  case Type::LValueReference: {
-    const auto *PX = cast<LValueReferenceType>(X),
-               *PY = cast<LValueReferenceType>(Y);
-    return Ctx.getLValueReferenceType(getCommonPointeeType(Ctx, PX, PY),
-                                      PX->isSpelledAsLValue() ||
-                                          PY->isSpelledAsLValue());
-  }
-  case Type::RValueReference: {
-    const auto *PX = cast<RValueReferenceType>(X),
-               *PY = cast<RValueReferenceType>(Y);
-    return Ctx.getRValueReferenceType(getCommonPointeeType(Ctx, PX, PY));
-  }
-  case Type::DependentAddressSpace: {
-    const auto *PX = cast<DependentAddressSpaceType>(X),
-               *PY = cast<DependentAddressSpaceType>(Y);
-    return Ctx.getDependentAddressSpaceType(getCommonPointeeType(Ctx, PX, PY),
-                                            PX->getAddrSpaceExpr(),
-                                            getCommonAttrLoc(PX, PY));
-  }
-  case Type::FunctionNoProto: {
-    const auto *FX = cast<FunctionNoProtoType>(X),
-               *FY = cast<FunctionNoProtoType>(Y);
-    assert(FX->getExtInfo() == FY->getExtInfo());
-    return Ctx.getFunctionNoProtoType(
-        Ctx.getCommonSugaredType(FX->getReturnType(), FY->getReturnType()),
-        FX->getExtInfo());
-  }
-  case Type::FunctionProto: {
-    const auto *FX = cast<FunctionProtoType>(X),
-               *FY = cast<FunctionProtoType>(Y);
-    FunctionProtoType::ExtProtoInfo EPIX = FX->getExtProtoInfo(),
-                                    EPIY = FY->getExtProtoInfo();
-    assert(EPIX.ExtInfo == EPIY.ExtInfo);
-    assert(EPIX.ExtParameterInfos == EPIY.ExtParameterInfos);
-    assert(EPIX.RefQualifier == EPIY.RefQualifier);
-    assert(EPIX.TypeQuals == EPIY.TypeQuals);
-    assert(EPIX.Variadic == EPIY.Variadic);
-
-    // FIXME: Can we handle an empty EllipsisLoc?
-    //        Use emtpy EllipsisLoc if X and Y 
diff er.
-
-    EPIX.HasTrailingReturn = EPIX.HasTrailingReturn && EPIY.HasTrailingReturn;
-
-    QualType R =
-        Ctx.getCommonSugaredType(FX->getReturnType(), FY->getReturnType());
-    auto P = getCommonTypes(Ctx, FX->param_types(), FY->param_types(),
-                            /*Unqualified=*/true);
-
-    SmallVector<QualType, 8> Exceptions;
-    EPIX.ExceptionSpec = Ctx.mergeExceptionSpecs(
-        EPIX.ExceptionSpec, EPIY.ExceptionSpec, Exceptions, true);
-    return Ctx.getFunctionType(R, P, EPIX);
-  }
-  case Type::ObjCObject: {
-    const auto *OX = cast<ObjCObjectType>(X), *OY = cast<ObjCObjectType>(Y);
-    assert(llvm::equal(OX->getProtocols(), OY->getProtocols()));
-    auto TAs = getCommonTypes(Ctx, OX->getTypeArgsAsWritten(),
-                              OY->getTypeArgsAsWritten());
-    return Ctx.getObjCObjectType(
-        Ctx.getCommonSugaredType(OX->getBaseType(), OY->getBaseType()), TAs,
-        OX->getProtocols(),
-        OX->isKindOfTypeAsWritten() && OY->isKindOfTypeAsWritten());
-  }
-  case Type::ConstantMatrix: {
-    const auto *MX = cast<ConstantMatrixType>(X),
-               *MY = cast<ConstantMatrixType>(Y);
-    assert(MX->getNumRows() == MY->getNumRows());
-    assert(MX->getNumColumns() == MY->getNumColumns());
-    return Ctx.getConstantMatrixType(getCommonElementType(Ctx, MX, MY),
-                                     MX->getNumRows(), MX->getNumColumns());
-  }
-  case Type::DependentSizedMatrix: {
-    const auto *MX = cast<DependentSizedMatrixType>(X),
-               *MY = cast<DependentSizedMatrixType>(Y);
-    assert(MX->getRowExpr() == MY->getRowExpr());
-    assert(MX->getColumnExpr() == MY->getColumnExpr());
-    return Ctx.getDependentSizedMatrixType(
-        getCommonElementType(Ctx, MX, MY), MX->getRowExpr(),
-        MX->getColumnExpr(), getCommonAttrLoc(MX, MY));
-  }
-  case Type::Vector: {
-    const auto *VX = cast<VectorType>(X), *VY = cast<VectorType>(Y);
-    assert(VX->getNumElements() == VY->getNumElements());
-    assert(VX->getVectorKind() == VY->getVectorKind());
-    return Ctx.getVectorType(getCommonElementType(Ctx, VX, VY),
-                             VX->getNumElements(), VX->getVectorKind());
-  }
-  case Type::ExtVector: {
-    const auto *VX = cast<ExtVectorType>(X), *VY = cast<ExtVectorType>(Y);
-    assert(VX->getNumElements() == VY->getNumElements());
-    return Ctx.getExtVectorType(getCommonElementType(Ctx, VX, VY),
-                                VX->getNumElements());
-  }
-  case Type::DependentSizedExtVector: {
-    const auto *VX = cast<DependentSizedExtVectorType>(X),
-               *VY = cast<DependentSizedExtVectorType>(Y);
-    return Ctx.getDependentSizedExtVectorType(getCommonElementType(Ctx, VX, VY),
-                                              getCommonSizeExpr(VX, VY),
-                                              getCommonAttrLoc(VX, VY));
-  }
-  case Type::DependentVector: {
-    const auto *VX = cast<DependentVectorType>(X),
-               *VY = cast<DependentVectorType>(Y);
-    assert(VX->getVectorKind() == VY->getVectorKind());
-    return Ctx.getDependentVectorType(
-        getCommonElementType(Ctx, VX, VY), getCommonSizeExpr(VX, VY),
-        getCommonAttrLoc(VX, VY), VX->getVectorKind());
-  }
-  case Type::InjectedClassName: {
-    const auto *IX = cast<InjectedClassNameType>(X),
-               *IY = cast<InjectedClassNameType>(Y);
-    return Ctx.getInjectedClassNameType(
-        getCommonDeclChecked(IX->getDecl(), IY->getDecl()),
-        Ctx.getCommonSugaredType(IX->getInjectedSpecializationType(),
-                                 IY->getInjectedSpecializationType()));
-  }
-  case Type::TemplateSpecialization: {
-    const auto *TX = cast<TemplateSpecializationType>(X),
-               *TY = cast<TemplateSpecializationType>(Y);
-    auto As = getCommonTemplateArguments(Ctx, TX->template_arguments(),
-                                         TY->template_arguments());
-    return Ctx.getTemplateSpecializationType(
-        ::getCommonTemplateName(Ctx, TX->getTemplateName(),
-                                TY->getTemplateName()),
-        As, TX->getCanonicalTypeInternal());
-  }
-  case Type::DependentName: {
-    const auto *NX = cast<DependentNameType>(X),
-               *NY = cast<DependentNameType>(Y);
-    assert(NX->getIdentifier() == NY->getIdentifier());
-    return Ctx.getDependentNameType(
-        getCommonTypeKeyword(NX, NY), getCommonNNS(Ctx, NX, NY),
-        NX->getIdentifier(), NX->getCanonicalTypeInternal());
-  }
-  case Type::DependentTemplateSpecialization: {
-    const auto *TX = cast<DependentTemplateSpecializationType>(X),
-               *TY = cast<DependentTemplateSpecializationType>(Y);
-    assert(TX->getIdentifier() == TY->getIdentifier());
-    auto As = getCommonTemplateArguments(Ctx, TX->template_arguments(),
-                                         TY->template_arguments());
-    return Ctx.getDependentTemplateSpecializationType(
-        getCommonTypeKeyword(TX, TY), getCommonNNS(Ctx, TX, TY),
-        TX->getIdentifier(), As);
-  }
-  case Type::UnaryTransform: {
-    const auto *TX = cast<UnaryTransformType>(X),
-               *TY = cast<UnaryTransformType>(Y);
-    assert(TX->getUTTKind() == TY->getUTTKind());
-    return Ctx.getUnaryTransformType(
-        Ctx.getCommonSugaredType(TX->getBaseType(), TY->getBaseType()),
-        Ctx.getCommonSugaredType(TX->getUnderlyingType(),
-                                 TY->getUnderlyingType()),
-        TX->getUTTKind());
-  }
-  case Type::PackExpansion: {
-    const auto *PX = cast<PackExpansionType>(X),
-               *PY = cast<PackExpansionType>(Y);
-    return Ctx.getPackExpansionType(
-        Ctx.getCommonSugaredType(PX->getPattern(), PY->getPattern()),
-        PX->getNumExpansions(), false);
-  }
-  case Type::Pipe: {
-    const auto *PX = cast<PipeType>(X), *PY = cast<PipeType>(Y);
-    assert(PX->isReadOnly() == PY->isReadOnly());
-    auto MP = PX->isReadOnly() ? &ASTContext::getReadPipeType
-                               : &ASTContext::getWritePipeType;
-    return (Ctx.*MP)(getCommonElementType(Ctx, PX, PY));
-  }
-  }
-  llvm_unreachable("Unknown Type Class");
-}
-
-static auto unwrapSugar(SplitQualType &T) {
-  SmallVector<SplitQualType, 8> R;
-  while (true) {
-    QualType NT = T.Ty->getLocallyUnqualifiedSingleStepDesugaredType();
-    if (NT == QualType(T.Ty, 0))
-      break;
-    SplitQualType SplitNT = NT.split();
-    SplitNT.Quals += T.Quals;
-    R.push_back(T);
-    T = SplitNT;
-  }
-  return R;
-}
-
-static bool removeDifferentTopLevelSugar(SplitQualType &SX, SplitQualType &SY) {
-  auto Xs = ::unwrapSugar(SX), Ys = ::unwrapSugar(SY);
-  if (SX.Ty != SY.Ty)
-    return true;
-  while (!Xs.empty() && !Ys.empty() && Xs.back().Ty == Ys.back().Ty) {
-    SX = Xs.pop_back_val();
-    SY = Ys.pop_back_val();
-  }
-  return false;
-}
-
-QualType ASTContext::getCommonSugaredType(QualType X, QualType Y,
-                                          bool Unqualified) {
-  assert(Unqualified ? hasSameUnqualifiedType(X, Y) : hasSameType(X, Y));
-  if (X == Y)
-    return X;
-  if (!Unqualified) {
-    if (X.isCanonical())
-      return X;
-    if (Y.isCanonical())
-      return Y;
-  }
-
-  SplitQualType SX = X.split(), SY = Y.split();
-  if (::removeDifferentTopLevelSugar(SX, SY))
-    SX.Ty = ::getCommonType(*this, SX.Ty, SY.Ty).getTypePtr();
-
-  if (Unqualified)
-    SX.Quals = Qualifiers::removeCommonQualifiers(SX.Quals, SY.Quals);
-  else
-    assert(SX.Quals == SY.Quals);
-
-  QualType R = getQualifiedType(SX);
-  assert(Unqualified ? hasSameUnqualifiedType(R, X) : hasSameType(R, X));
-  return R;
-}
-
 QualType ASTContext::getCorrespondingSaturatedType(QualType Ty) const {
   assert(Ty->isFixedPointType());
 

diff  --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index b1c453951f064..58774c0fa44a3 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -4109,9 +4109,10 @@ bool Sema::MergeFunctionDecl(FunctionDecl *New, NamedDecl *&OldD, Scope *S,
         // The old declaration provided a function prototype, but the
         // new declaration does not. Merge in the prototype.
         assert(!OldProto->hasExceptionSpec() && "Exception spec in C");
-        NewQType = Context.getFunctionType(NewFuncType->getReturnType(),
-                                           OldProto->getParamTypes(),
-                                           OldProto->getExtProtoInfo());
+        SmallVector<QualType, 16> ParamTypes(OldProto->param_types());
+        NewQType =
+            Context.getFunctionType(NewFuncType->getReturnType(), ParamTypes,
+                                    OldProto->getExtProtoInfo());
         New->setType(NewQType);
         New->setHasInheritedPrototype();
 
@@ -12373,10 +12374,7 @@ QualType Sema::deduceVarTypeFromInitializer(VarDecl *VDecl,
                                     Type.getQualifiers());
 
   QualType DeducedType;
-  TemplateDeductionInfo Info(DeduceInit->getExprLoc());
-  TemplateDeductionResult Result =
-      DeduceAutoType(TSI->getTypeLoc(), DeduceInit, DeducedType, Info);
-  if (Result != TDK_Success && Result != TDK_AlreadyDiagnosed) {
+  if (DeduceAutoType(TSI, DeduceInit, DeducedType) == DAR_Failed) {
     if (!IsInitCapture)
       DiagnoseAutoDeductionFailure(VDecl, DeduceInit);
     else if (isa<InitListExpr>(Init))

diff  --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index 6463d0678007d..ac34e42f92773 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -11582,9 +11582,7 @@ QualType Sema::BuildStdInitializerList(QualType Element, SourceLocation Loc) {
   Args.addArgument(TemplateArgumentLoc(TemplateArgument(Element),
                                        Context.getTrivialTypeSourceInfo(Element,
                                                                         Loc)));
-  return Context.getElaboratedType(
-      ElaboratedTypeKeyword::ETK_None,
-      NestedNameSpecifier::Create(Context, nullptr, getStdNamespace()),
+  return Context.getCanonicalType(
       CheckTemplateIdType(TemplateName(StdInitializerList), Loc, Args));
 }
 

diff  --git a/clang/lib/Sema/SemaExprCXX.cpp b/clang/lib/Sema/SemaExprCXX.cpp
index 375e1285f8af8..bc950ab7c4a7a 100644
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/clang/lib/Sema/SemaExprCXX.cpp
@@ -1506,17 +1506,12 @@ Sema::BuildCXXTypeConstructExpr(TypeSourceInfo *TInfo,
           Diag(Deduce->getBeginLoc(), diag::err_auto_expr_init_paren_braces)
           << ListInitialization << Ty << FullRange);
     QualType DeducedType;
-    TemplateDeductionInfo Info(Deduce->getExprLoc());
-    TemplateDeductionResult Result =
-        DeduceAutoType(TInfo->getTypeLoc(), Deduce, DeducedType, Info);
-    if (Result != TDK_Success && Result != TDK_AlreadyDiagnosed)
+    if (DeduceAutoType(TInfo, Deduce, DeducedType) == DAR_Failed)
       return ExprError(Diag(TyBeginLoc, diag::err_auto_expr_deduction_failure)
                        << Ty << Deduce->getType() << FullRange
                        << Deduce->getSourceRange());
-    if (DeducedType.isNull()) {
-      assert(Result == TDK_AlreadyDiagnosed);
+    if (DeducedType.isNull())
       return ExprError();
-    }
 
     Ty = DeducedType;
     Entity = InitializedEntity::InitializeTemporary(TInfo, Ty);
@@ -2050,17 +2045,12 @@ Sema::BuildCXXNew(SourceRange Range, bool UseGlobal,
           Diag(Deduce->getBeginLoc(), diag::err_auto_expr_init_paren_braces)
           << Braced << AllocType << TypeRange);
     QualType DeducedType;
-    TemplateDeductionInfo Info(Deduce->getExprLoc());
-    TemplateDeductionResult Result =
-        DeduceAutoType(AllocTypeInfo->getTypeLoc(), Deduce, DeducedType, Info);
-    if (Result != TDK_Success && Result != TDK_AlreadyDiagnosed)
+    if (DeduceAutoType(AllocTypeInfo, Deduce, DeducedType) == DAR_Failed)
       return ExprError(Diag(StartLoc, diag::err_auto_new_deduction_failure)
-                       << AllocType << Deduce->getType() << TypeRange
-                       << Deduce->getSourceRange());
-    if (DeducedType.isNull()) {
-      assert(Result == TDK_AlreadyDiagnosed);
+                       << AllocType << Deduce->getType()
+                       << TypeRange << Deduce->getSourceRange());
+    if (DeducedType.isNull())
       return ExprError();
-    }
     AllocType = DeducedType;
   }
 
@@ -6698,6 +6688,79 @@ QualType Sema::CXXCheckConditionalOperands(ExprResult &Cond, ExprResult &LHS,
   return QualType();
 }
 
+static FunctionProtoType::ExceptionSpecInfo
+mergeExceptionSpecs(Sema &S, FunctionProtoType::ExceptionSpecInfo ESI1,
+                    FunctionProtoType::ExceptionSpecInfo ESI2,
+                    SmallVectorImpl<QualType> &ExceptionTypeStorage) {
+  ExceptionSpecificationType EST1 = ESI1.Type;
+  ExceptionSpecificationType EST2 = ESI2.Type;
+
+  // If either of them can throw anything, that is the result.
+  if (EST1 == EST_None) return ESI1;
+  if (EST2 == EST_None) return ESI2;
+  if (EST1 == EST_MSAny) return ESI1;
+  if (EST2 == EST_MSAny) return ESI2;
+  if (EST1 == EST_NoexceptFalse) return ESI1;
+  if (EST2 == EST_NoexceptFalse) return ESI2;
+
+  // If either of them is non-throwing, the result is the other.
+  if (EST1 == EST_NoThrow) return ESI2;
+  if (EST2 == EST_NoThrow) return ESI1;
+  if (EST1 == EST_DynamicNone) return ESI2;
+  if (EST2 == EST_DynamicNone) return ESI1;
+  if (EST1 == EST_BasicNoexcept) return ESI2;
+  if (EST2 == EST_BasicNoexcept) return ESI1;
+  if (EST1 == EST_NoexceptTrue) return ESI2;
+  if (EST2 == EST_NoexceptTrue) return ESI1;
+
+  // If we're left with value-dependent computed noexcept expressions, we're
+  // stuck. Before C++17, we can just drop the exception specification entirely,
+  // since it's not actually part of the canonical type. And this should never
+  // happen in C++17, because it would mean we were computing the composite
+  // pointer type of dependent types, which should never happen.
+  if (EST1 == EST_DependentNoexcept || EST2 == EST_DependentNoexcept) {
+    assert(!S.getLangOpts().CPlusPlus17 &&
+           "computing composite pointer type of dependent types");
+    return FunctionProtoType::ExceptionSpecInfo();
+  }
+
+  // Switch over the possibilities so that people adding new values know to
+  // update this function.
+  switch (EST1) {
+  case EST_None:
+  case EST_DynamicNone:
+  case EST_MSAny:
+  case EST_BasicNoexcept:
+  case EST_DependentNoexcept:
+  case EST_NoexceptFalse:
+  case EST_NoexceptTrue:
+  case EST_NoThrow:
+    llvm_unreachable("handled above");
+
+  case EST_Dynamic: {
+    // This is the fun case: both exception specifications are dynamic. Form
+    // the union of the two lists.
+    assert(EST2 == EST_Dynamic && "other cases should already be handled");
+    llvm::SmallPtrSet<QualType, 8> Found;
+    for (auto &Exceptions : {ESI1.Exceptions, ESI2.Exceptions})
+      for (QualType E : Exceptions)
+        if (Found.insert(S.Context.getCanonicalType(E)).second)
+          ExceptionTypeStorage.push_back(E);
+
+    FunctionProtoType::ExceptionSpecInfo Result(EST_Dynamic);
+    Result.Exceptions = ExceptionTypeStorage;
+    return Result;
+  }
+
+  case EST_Unevaluated:
+  case EST_Uninstantiated:
+  case EST_Unparsed:
+    llvm_unreachable("shouldn't see unresolved exception specifications here");
+  }
+
+  llvm_unreachable("invalid ExceptionSpecificationType");
+}
+
 /// Find a merged pointer type and convert the two expressions to it.
 ///
 /// This finds the composite pointer type for \p E1 and \p E2 according to
@@ -7001,9 +7064,9 @@ QualType Sema::FindCompositePointerType(SourceLocation Loc,
 
         // The result is nothrow if both operands are.
         SmallVector<QualType, 8> ExceptionTypeStorage;
-        EPI1.ExceptionSpec = EPI2.ExceptionSpec = Context.mergeExceptionSpecs(
-            EPI1.ExceptionSpec, EPI2.ExceptionSpec, ExceptionTypeStorage,
-            getLangOpts().CPlusPlus17);
+        EPI1.ExceptionSpec = EPI2.ExceptionSpec =
+            mergeExceptionSpecs(*this, EPI1.ExceptionSpec, EPI2.ExceptionSpec,
+                                ExceptionTypeStorage);
 
         Composite1 = Context.getFunctionType(FPT1->getReturnType(),
                                              FPT1->getParamTypes(), EPI1);

diff  --git a/clang/lib/Sema/SemaOverload.cpp b/clang/lib/Sema/SemaOverload.cpp
index 9aea1569f9b32..352738c850f23 100644
--- a/clang/lib/Sema/SemaOverload.cpp
+++ b/clang/lib/Sema/SemaOverload.cpp
@@ -684,7 +684,6 @@ clang::MakeDeductionFailureInfo(ASTContext &Context,
 
   case Sema::TDK_Success:
   case Sema::TDK_NonDependentConversionFailure:
-  case Sema::TDK_AlreadyDiagnosed:
     llvm_unreachable("not a deduction failure");
   }
 
@@ -734,7 +733,6 @@ void DeductionFailureInfo::Destroy() {
 
   // Unhandled
   case Sema::TDK_MiscellaneousDeductionFailure:
-  case Sema::TDK_AlreadyDiagnosed:
     break;
   }
 }
@@ -772,7 +770,6 @@ TemplateParameter DeductionFailureInfo::getTemplateParameter() {
 
   // Unhandled
   case Sema::TDK_MiscellaneousDeductionFailure:
-  case Sema::TDK_AlreadyDiagnosed:
     break;
   }
 
@@ -808,7 +805,6 @@ TemplateArgumentList *DeductionFailureInfo::getTemplateArgumentList() {
 
   // Unhandled
   case Sema::TDK_MiscellaneousDeductionFailure:
-  case Sema::TDK_AlreadyDiagnosed:
     break;
   }
 
@@ -840,7 +836,6 @@ const TemplateArgument *DeductionFailureInfo::getFirstArg() {
 
   // Unhandled
   case Sema::TDK_MiscellaneousDeductionFailure:
-  case Sema::TDK_AlreadyDiagnosed:
     break;
   }
 
@@ -872,7 +867,6 @@ const TemplateArgument *DeductionFailureInfo::getSecondArg() {
 
   // Unhandled
   case Sema::TDK_MiscellaneousDeductionFailure:
-  case Sema::TDK_AlreadyDiagnosed:
     break;
   }
 
@@ -11488,7 +11482,6 @@ static unsigned RankDeductionFailure(const DeductionFailureInfo &DFI) {
   switch ((Sema::TemplateDeductionResult)DFI.Result) {
   case Sema::TDK_Success:
   case Sema::TDK_NonDependentConversionFailure:
-  case Sema::TDK_AlreadyDiagnosed:
     llvm_unreachable("non-deduction failure while diagnosing bad deduction");
 
   case Sema::TDK_Invalid:

diff  --git a/clang/lib/Sema/SemaStmt.cpp b/clang/lib/Sema/SemaStmt.cpp
index 802e9cd8f364a..7d4edf98f3a75 100644
--- a/clang/lib/Sema/SemaStmt.cpp
+++ b/clang/lib/Sema/SemaStmt.cpp
@@ -2308,14 +2308,11 @@ Sema::ActOnObjCForCollectionStmt(SourceLocation ForLoc,
 
       // If the type contained 'auto', deduce the 'auto' to 'id'.
       if (FirstType->getContainedAutoType()) {
-        SourceLocation Loc = D->getLocation();
-        OpaqueValueExpr OpaqueId(Loc, Context.getObjCIdType(), VK_PRValue);
+        OpaqueValueExpr OpaqueId(D->getLocation(), Context.getObjCIdType(),
+                                 VK_PRValue);
         Expr *DeducedInit = &OpaqueId;
-        TemplateDeductionInfo Info(Loc);
-        FirstType = QualType();
-        TemplateDeductionResult Result = DeduceAutoType(
-            D->getTypeSourceInfo()->getTypeLoc(), DeducedInit, FirstType, Info);
-        if (Result != TDK_Success && Result != TDK_AlreadyDiagnosed)
+        if (DeduceAutoType(D->getTypeSourceInfo(), DeducedInit, FirstType) ==
+                DAR_Failed)
           DiagnoseAutoDeductionFailure(D, DeducedInit);
         if (FirstType.isNull()) {
           D->setInvalidDecl();
@@ -2379,16 +2376,10 @@ static bool FinishForRangeVarDecl(Sema &SemaRef, VarDecl *Decl, Expr *Init,
   // Deduce the type for the iterator variable now rather than leaving it to
   // AddInitializerToDecl, so we can produce a more suitable diagnostic.
   QualType InitType;
-  if (!isa<InitListExpr>(Init) && Init->getType()->isVoidType()) {
+  if ((!isa<InitListExpr>(Init) && Init->getType()->isVoidType()) ||
+      SemaRef.DeduceAutoType(Decl->getTypeSourceInfo(), Init, InitType) ==
+          Sema::DAR_Failed)
     SemaRef.Diag(Loc, DiagID) << Init->getType();
-  } else {
-    TemplateDeductionInfo Info(Init->getExprLoc());
-    Sema::TemplateDeductionResult Result = SemaRef.DeduceAutoType(
-        Decl->getTypeSourceInfo()->getTypeLoc(), Init, InitType, Info);
-    if (Result != Sema::TDK_Success && Result != Sema::TDK_AlreadyDiagnosed)
-      SemaRef.Diag(Loc, DiagID) << Init->getType();
-  }
-
   if (InitType.isNull()) {
     Decl->setInvalidDecl();
     return true;
@@ -3778,13 +3769,17 @@ TypeLoc Sema::getReturnTypeLoc(FunctionDecl *FD) const {
 /// C++1y [dcl.spec.auto]p6.
 bool Sema::DeduceFunctionTypeFromReturnExpr(FunctionDecl *FD,
                                             SourceLocation ReturnLoc,
-                                            Expr *RetExpr, const AutoType *AT) {
+                                            Expr *&RetExpr,
+                                            const AutoType *AT) {
   // If this is the conversion function for a lambda, we choose to deduce its
   // type from the corresponding call operator, not from the synthesized return
   // statement within it. See Sema::DeduceReturnType.
   if (isLambdaConversionOperator(FD))
     return false;
 
+  TypeLoc OrigResultType = getReturnTypeLoc(FD);
+  QualType Deduced;
+
   if (RetExpr && isa<InitListExpr>(RetExpr)) {
     //  If the deduction is for a return statement and the initializer is
     //  a braced-init-list, the program is ill-formed.
@@ -3804,74 +3799,87 @@ bool Sema::DeduceFunctionTypeFromReturnExpr(FunctionDecl *FD,
     return false;
   }
 
-  TypeLoc OrigResultType = getReturnTypeLoc(FD);
-  //  In the case of a return with no operand, the initializer is considered
-  //  to be void().
-  CXXScalarValueInitExpr VoidVal(Context.VoidTy, nullptr, SourceLocation());
-  if (!RetExpr) {
-    // For a function with a deduced result type to return with omitted
-    // expression, the result type as written must be 'auto' or
-    // 'decltype(auto)', possibly cv-qualified or constrained, but not
-    // ref-qualified.
+  if (RetExpr) {
+    //  Otherwise, [...] deduce a value for U using the rules of template
+    //  argument deduction.
+    DeduceAutoResult DAR = DeduceAutoType(OrigResultType, RetExpr, Deduced);
+
+    if (DAR == DAR_Failed && !FD->isInvalidDecl())
+      Diag(RetExpr->getExprLoc(), diag::err_auto_fn_deduction_failure)
+        << OrigResultType.getType() << RetExpr->getType();
+
+    if (DAR != DAR_Succeeded)
+      return true;
+
+    // If a local type is part of the returned type, mark its fields as
+    // referenced.
+    LocalTypedefNameReferencer Referencer(*this);
+    Referencer.TraverseType(RetExpr->getType());
+  } else {
+    // For a function with a deduced result type to return void,
+    // the result type as written must be 'auto' or 'decltype(auto)',
+    // possibly cv-qualified or constrained, but not ref-qualified.
     if (!OrigResultType.getType()->getAs<AutoType>()) {
       Diag(ReturnLoc, diag::err_auto_fn_return_void_but_not_auto)
-          << OrigResultType.getType();
+        << OrigResultType.getType();
       return true;
     }
-    RetExpr = &VoidVal;
+    // In the case of a return with no operand, the initializer is considered
+    // to be 'void()'.
+    Expr *Dummy = new (Context) CXXScalarValueInitExpr(
+        Context.VoidTy,
+        Context.getTrivialTypeSourceInfo(Context.VoidTy, ReturnLoc), ReturnLoc);
+    DeduceAutoResult DAR = DeduceAutoType(OrigResultType, Dummy, Deduced);
+
+    if (DAR == DAR_Failed && !FD->isInvalidDecl())
+      Diag(ReturnLoc, diag::err_auto_fn_deduction_failure)
+          << OrigResultType.getType() << Dummy->getType();
+
+    if (DAR != DAR_Succeeded)
+      return true;
   }
 
-  QualType Deduced = AT->getDeducedType();
-  {
-    //  Otherwise, [...] deduce a value for U using the rules of template
-    //  argument deduction.
-    TemplateDeductionInfo Info(RetExpr->getExprLoc());
-    TemplateDeductionResult Res =
-        DeduceAutoType(OrigResultType, RetExpr, Deduced, Info);
-    if (Res != TDK_Success && FD->isInvalidDecl())
-      return true;
-    switch (Res) {
-    case TDK_Success:
-      break;
-    case TDK_AlreadyDiagnosed:
+  // CUDA: Kernel function must have 'void' return type.
+  if (getLangOpts().CUDA)
+    if (FD->hasAttr<CUDAGlobalAttr>() && !Deduced->isVoidType()) {
+      Diag(FD->getLocation(), diag::err_kern_type_not_void_return)
+          << FD->getType() << FD->getSourceRange();
       return true;
-    case TDK_Inconsistent: {
-      //  If a function with a declared return type that contains a placeholder
-      //  type has multiple return statements, the return type is deduced for
-      //  each return statement. [...] if the type deduced is not the same in
-      //  each deduction, the program is ill-formed.
+    }
+
+  //  If a function with a declared return type that contains a placeholder type
+  //  has multiple return statements, the return type is deduced for each return
+  //  statement. [...] if the type deduced is not the same in each deduction,
+  //  the program is ill-formed.
+  QualType DeducedT = AT->getDeducedType();
+  if (!DeducedT.isNull() && !FD->isInvalidDecl()) {
+    AutoType *NewAT = Deduced->getContainedAutoType();
+    // It is possible that NewAT->getDeducedType() is null. When that happens,
+    // we should not crash, instead we ignore this deduction.
+    if (NewAT->getDeducedType().isNull())
+      return false;
+
+    CanQualType OldDeducedType = Context.getCanonicalFunctionResultType(
+                                   DeducedT);
+    CanQualType NewDeducedType = Context.getCanonicalFunctionResultType(
+                                   NewAT->getDeducedType());
+    if (!FD->isDependentContext() && OldDeducedType != NewDeducedType) {
       const LambdaScopeInfo *LambdaSI = getCurLambda();
-      if (LambdaSI && LambdaSI->HasImplicitReturnType)
+      if (LambdaSI && LambdaSI->HasImplicitReturnType) {
         Diag(ReturnLoc, diag::err_typecheck_missing_return_type_incompatible)
-            << Info.SecondArg << Info.FirstArg << true /*IsLambda*/;
-      else
+          << NewAT->getDeducedType() << DeducedT
+          << true /*IsLambda*/;
+      } else {
         Diag(ReturnLoc, diag::err_auto_fn_
diff erent_deductions)
-            << (AT->isDecltypeAuto() ? 1 : 0) << Info.SecondArg
-            << Info.FirstArg;
-      return true;
-    }
-    default:
-      Diag(RetExpr->getExprLoc(), diag::err_auto_fn_deduction_failure)
-          << OrigResultType.getType() << RetExpr->getType();
+          << (AT->isDecltypeAuto() ? 1 : 0)
+          << NewAT->getDeducedType() << DeducedT;
+      }
       return true;
     }
-  }
-
-  // If a local type is part of the returned type, mark its fields as
-  // referenced.
-  LocalTypedefNameReferencer(*this).TraverseType(RetExpr->getType());
-
-  // CUDA: Kernel function must have 'void' return type.
-  if (getLangOpts().CUDA && FD->hasAttr<CUDAGlobalAttr>() &&
-      !Deduced->isVoidType()) {
-    Diag(FD->getLocation(), diag::err_kern_type_not_void_return)
-        << FD->getType() << FD->getSourceRange();
-    return true;
-  }
-
-  if (!FD->isInvalidDecl() && AT->getDeducedType() != Deduced)
+  } else if (!FD->isInvalidDecl()) {
     // Update all declarations of the function to have the deduced return type.
     Context.adjustDeducedFunctionResultType(FD, Deduced);
+  }
 
   return false;
 }

diff  --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index e0a154b0d9630..e9076bb6633af 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -6884,6 +6884,7 @@ ExprResult Sema::CheckTemplateArgument(NonTypeTemplateParmDecl *Param,
     // When checking a deduced template argument, deduce from its type even if
     // the type is dependent, in order to check the types of non-type template
     // arguments line up properly in partial ordering.
+    Optional<unsigned> Depth = Param->getDepth() + 1;
     Expr *DeductionArg = Arg;
     if (auto *PE = dyn_cast<PackExpansionExpr>(DeductionArg))
       DeductionArg = PE->getPattern();
@@ -6899,27 +6900,20 @@ ExprResult Sema::CheckTemplateArgument(NonTypeTemplateParmDecl *Param,
           DeduceTemplateSpecializationFromInitializer(TSI, Entity, Kind, Inits);
       if (ParamType.isNull())
         return ExprError();
-    } else {
-      TemplateDeductionInfo Info(DeductionArg->getExprLoc(),
-                                 Param->getDepth() + 1);
-      ParamType = QualType();
-      TemplateDeductionResult Result =
-          DeduceAutoType(TSI->getTypeLoc(), DeductionArg, ParamType, Info,
-                         /*DependentDeduction=*/true,
-                         // We do not check constraints right now because the
-                         // immediately-declared constraint of the auto type is
-                         // also an associated constraint, and will be checked
-                         // along with the other associated constraints after
-                         // checking the template argument list.
-                         /*IgnoreConstraints=*/true);
-      if (Result != TDK_Success && Result != TDK_AlreadyDiagnosed) {
-        Diag(Arg->getExprLoc(),
-             diag::err_non_type_template_parm_type_deduction_failure)
-            << Param->getDeclName() << Param->getType() << Arg->getType()
-            << Arg->getSourceRange();
-        Diag(Param->getLocation(), diag::note_template_param_here);
-        return ExprError();
-      }
+    } else if (DeduceAutoType(
+                   TSI, DeductionArg, ParamType, Depth,
+                   // We do not check constraints right now because the
+                   // immediately-declared constraint of the auto type is also
+                   // an associated constraint, and will be checked along with
+                   // the other associated constraints after checking the
+                   // template argument list.
+                   /*IgnoreConstraints=*/true) == DAR_Failed) {
+      Diag(Arg->getExprLoc(),
+           diag::err_non_type_template_parm_type_deduction_failure)
+        << Param->getDeclName() << Param->getType() << Arg->getType()
+        << Arg->getSourceRange();
+      Diag(Param->getLocation(), diag::note_template_param_here);
+      return ExprError();
     }
     // CheckNonTypeTemplateParameterType will produce a diagnostic if there's
     // an error. The error message normally references the parameter

diff  --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
index 0e07a150a8b28..3419eb1eec6b4 100644
--- a/clang/lib/Sema/SemaTemplateDeduction.cpp
+++ b/clang/lib/Sema/SemaTemplateDeduction.cpp
@@ -236,13 +236,11 @@ checkDeducedTemplateArguments(ASTContext &Context,
   case TemplateArgument::Null:
     llvm_unreachable("Non-deduced template arguments handled above");
 
-  case TemplateArgument::Type: {
+  case TemplateArgument::Type:
     // If two template type arguments have the same type, they're compatible.
-    QualType TX = X.getAsType(), TY = Y.getAsType();
-    if (Y.getKind() == TemplateArgument::Type && Context.hasSameType(TX, TY))
-      return DeducedTemplateArgument(Context.getCommonSugaredType(TX, TY),
-                                     X.wasDeducedFromArrayBound() ||
-                                         Y.wasDeducedFromArrayBound());
+    if (Y.getKind() == TemplateArgument::Type &&
+        Context.hasSameType(X.getAsType(), Y.getAsType()))
+      return X;
 
     // If one of the two arguments was deduced from an array bound, the other
     // supersedes it.
@@ -251,7 +249,6 @@ checkDeducedTemplateArguments(ASTContext &Context,
 
     // The arguments are not compatible.
     return DeducedTemplateArgument();
-  }
 
   case TemplateArgument::Integral:
     // If we deduced a constant in one case and either a dependent expression or
@@ -329,9 +326,7 @@ checkDeducedTemplateArguments(ASTContext &Context,
     // If we deduced a null pointer and a dependent expression, keep the
     // null pointer.
     if (Y.getKind() == TemplateArgument::Expression)
-      return TemplateArgument(Context.getCommonSugaredType(
-                                  X.getNullPtrType(), Y.getAsExpr()->getType()),
-                              true);
+      return X;
 
     // If we deduced a null pointer and an integral constant, keep the
     // integral constant.
@@ -340,9 +335,7 @@ checkDeducedTemplateArguments(ASTContext &Context,
 
     // If we deduced two null pointers, they are the same.
     if (Y.getKind() == TemplateArgument::NullPtr)
-      return TemplateArgument(
-          Context.getCommonSugaredType(X.getNullPtrType(), Y.getNullPtrType()),
-          true);
+      return X;
 
     // All other combinations are incompatible.
     return DeducedTemplateArgument();
@@ -4581,9 +4574,42 @@ namespace {
 
 } // namespace
 
-static bool CheckDeducedPlaceholderConstraints(Sema &S, const AutoType &Type,
-                                               AutoTypeLoc TypeLoc,
-                                               QualType Deduced) {
+Sema::DeduceAutoResult
+Sema::DeduceAutoType(TypeSourceInfo *Type, Expr *&Init, QualType &Result,
+                     Optional<unsigned> DependentDeductionDepth,
+                     bool IgnoreConstraints) {
+  return DeduceAutoType(Type->getTypeLoc(), Init, Result,
+                        DependentDeductionDepth, IgnoreConstraints);
+}
+
+/// Attempt to produce an informative diagostic explaining why auto deduction
+/// failed.
+/// \return \c true if diagnosed, \c false if not.
+static bool diagnoseAutoDeductionFailure(Sema &S,
+                                         Sema::TemplateDeductionResult TDK,
+                                         TemplateDeductionInfo &Info,
+                                         ArrayRef<SourceRange> Ranges) {
+  switch (TDK) {
+  case Sema::TDK_Inconsistent: {
+    // Inconsistent deduction means we were deducing from an initializer list.
+    auto D = S.Diag(Info.getLocation(), diag::err_auto_inconsistent_deduction);
+    D << Info.FirstArg << Info.SecondArg;
+    for (auto R : Ranges)
+      D << R;
+    return true;
+  }
+
+  // FIXME: Are there other cases for which a custom diagnostic is more useful
+  // than the basic "types don't match" diagnostic?
+
+  default:
+    return false;
+  }
+}
+
+static Sema::DeduceAutoResult
+CheckDeducedPlaceholderConstraints(Sema &S, const AutoType &Type,
+                                   AutoTypeLoc TypeLoc, QualType Deduced) {
   ConstraintSatisfaction Satisfaction;
   ConceptDecl *Concept = Type.getTypeConstraintConcept();
   TemplateArgumentListInfo TemplateArgs(TypeLoc.getLAngleLoc(),
@@ -4598,13 +4624,13 @@ static bool CheckDeducedPlaceholderConstraints(Sema &S, const AutoType &Type,
   llvm::SmallVector<TemplateArgument, 4> Converted;
   if (S.CheckTemplateArgumentList(Concept, SourceLocation(), TemplateArgs,
                                   /*PartialTemplateArgs=*/false, Converted))
-    return true;
+    return Sema::DAR_FailedAlreadyDiagnosed;
   MultiLevelTemplateArgumentList MLTAL;
   MLTAL.addOuterTemplateArguments(Converted);
   if (S.CheckConstraintSatisfaction(Concept, {Concept->getConstraintExpr()},
                                     MLTAL, TypeLoc.getLocalSourceRange(),
                                     Satisfaction))
-    return true;
+    return Sema::DAR_FailedAlreadyDiagnosed;
   if (!Satisfaction.IsSatisfied) {
     std::string Buf;
     llvm::raw_string_ostream OS(Buf);
@@ -4618,11 +4644,11 @@ static bool CheckDeducedPlaceholderConstraints(Sema &S, const AutoType &Type,
     OS.flush();
     S.Diag(TypeLoc.getConceptNameLoc(),
            diag::err_placeholder_constraints_not_satisfied)
-        << Deduced << Buf << TypeLoc.getLocalSourceRange();
+         << Deduced << Buf << TypeLoc.getLocalSourceRange();
     S.DiagnoseUnsatisfiedConstraint(Satisfaction);
-    return true;
+    return Sema::DAR_FailedAlreadyDiagnosed;
   }
-  return false;
+  return Sema::DAR_Succeeded;
 }
 
 /// Deduce the type for an auto type-specifier (C++11 [dcl.spec.auto]p6)
@@ -4635,165 +4661,184 @@ static bool CheckDeducedPlaceholderConstraints(Sema &S, const AutoType &Type,
 /// \param Init the initializer for the variable whose type is to be deduced.
 /// \param Result if type deduction was successful, this will be set to the
 ///        deduced type.
-/// \param Info the argument will be updated to provide additional information
-///        about template argument deduction.
-/// \param DependentDeduction Set if we should permit deduction in
+/// \param DependentDeductionDepth Set if we should permit deduction in
 ///        dependent cases. This is necessary for template partial ordering with
-///        'auto' template parameters. The template parameter depth to be used
-///        should be specified in the 'Info' parameter.
+///        'auto' template parameters. The value specified is the template
+///        parameter depth at which we should perform 'auto' deduction.
 /// \param IgnoreConstraints Set if we should not fail if the deduced type does
 ///                          not satisfy the type-constraint in the auto type.
-Sema::TemplateDeductionResult Sema::DeduceAutoType(TypeLoc Type, Expr *Init,
-                                                   QualType &Result,
-                                                   TemplateDeductionInfo &Info,
-                                                   bool DependentDeduction,
-                                                   bool IgnoreConstraints) {
-  assert(DependentDeduction || Info.getDeducedDepth() == 0);
+Sema::DeduceAutoResult
+Sema::DeduceAutoType(TypeLoc Type, Expr *&Init, QualType &Result,
+                     Optional<unsigned> DependentDeductionDepth,
+                     bool IgnoreConstraints) {
   if (Init->containsErrors())
-    return TDK_AlreadyDiagnosed;
-
-  const AutoType *AT = Type.getType()->getContainedAutoType();
-  assert(AT);
-
-  if (Init->getType()->isNonOverloadPlaceholderType() || AT->isDecltypeAuto()) {
+    return DAR_FailedAlreadyDiagnosed;
+  if (Init->getType()->isNonOverloadPlaceholderType()) {
     ExprResult NonPlaceholder = CheckPlaceholderExpr(Init);
     if (NonPlaceholder.isInvalid())
-      return TDK_AlreadyDiagnosed;
+      return DAR_FailedAlreadyDiagnosed;
     Init = NonPlaceholder.get();
   }
 
   DependentAuto DependentResult = {
       /*.IsPack = */ (bool)Type.getAs<PackExpansionTypeLoc>()};
 
-  if (!DependentDeduction &&
+  if (!DependentDeductionDepth &&
       (Type.getType()->isDependentType() || Init->isTypeDependent() ||
        Init->containsUnexpandedParameterPack())) {
     Result = SubstituteDeducedTypeTransform(*this, DependentResult).Apply(Type);
     assert(!Result.isNull() && "substituting DependentTy can't fail");
-    return TDK_Success;
+    return DAR_Succeeded;
   }
 
-  auto *InitList = dyn_cast<InitListExpr>(Init);
-  if (!getLangOpts().CPlusPlus && InitList) {
-    Diag(Init->getBeginLoc(), diag::err_auto_init_list_from_c);
-    return TDK_AlreadyDiagnosed;
+  // Find the depth of template parameter to synthesize.
+  unsigned Depth = DependentDeductionDepth.value_or(0);
+
+  // If this is a 'decltype(auto)' specifier, do the decltype dance.
+  // Since 'decltype(auto)' can only occur at the top of the type, we
+  // don't need to go digging for it.
+  if (const AutoType *AT = Type.getType()->getAs<AutoType>()) {
+    if (AT->isDecltypeAuto()) {
+      if (isa<InitListExpr>(Init)) {
+        Diag(Init->getBeginLoc(), diag::err_decltype_auto_initializer_list);
+        return DAR_FailedAlreadyDiagnosed;
+      }
+
+      ExprResult ER = CheckPlaceholderExpr(Init);
+      if (ER.isInvalid())
+        return DAR_FailedAlreadyDiagnosed;
+      QualType Deduced = getDecltypeForExpr(ER.get());
+      assert(!Deduced.isNull());
+      if (AT->isConstrained() && !IgnoreConstraints) {
+        auto ConstraintsResult =
+            CheckDeducedPlaceholderConstraints(*this, *AT,
+                                               Type.getContainedAutoTypeLoc(),
+                                               Deduced);
+        if (ConstraintsResult != DAR_Succeeded)
+          return ConstraintsResult;
+      }
+      Result = SubstituteDeducedTypeTransform(*this, Deduced).Apply(Type);
+      if (Result.isNull())
+        return DAR_FailedAlreadyDiagnosed;
+      return DAR_Succeeded;
+    } else if (!getLangOpts().CPlusPlus) {
+      if (isa<InitListExpr>(Init)) {
+        Diag(Init->getBeginLoc(), diag::err_auto_init_list_from_c);
+        return DAR_FailedAlreadyDiagnosed;
+      }
+    }
   }
 
+  SourceLocation Loc = Init->getExprLoc();
+
+  LocalInstantiationScope InstScope(*this);
+
+  // Build template<class TemplParam> void Func(FuncParam);
+  TemplateTypeParmDecl *TemplParam = TemplateTypeParmDecl::Create(
+      Context, nullptr, SourceLocation(), Loc, Depth, 0, nullptr, false, false,
+      false);
+  QualType TemplArg = QualType(TemplParam->getTypeForDecl(), 0);
+  NamedDecl *TemplParamPtr = TemplParam;
+  FixedSizeTemplateParameterListStorage<1, false> TemplateParamsSt(
+      Context, Loc, Loc, TemplParamPtr, Loc, nullptr);
+
+  QualType FuncParam =
+      SubstituteDeducedTypeTransform(*this, TemplArg, /*UseTypeSugar*/ true)
+          .Apply(Type);
+  assert(!FuncParam.isNull() &&
+         "substituting template parameter for 'auto' failed");
+
   // Deduce type of TemplParam in Func(Init)
   SmallVector<DeducedTemplateArgument, 1> Deduced;
   Deduced.resize(1);
 
+  TemplateDeductionInfo Info(Loc, Depth);
+
   // If deduction failed, don't diagnose if the initializer is dependent; it
   // might acquire a matching type in the instantiation.
-  auto DeductionFailed = [&](TemplateDeductionResult TDK) {
+  auto DeductionFailed = [&](TemplateDeductionResult TDK,
+                             ArrayRef<SourceRange> Ranges) -> DeduceAutoResult {
     if (Init->isTypeDependent()) {
       Result =
           SubstituteDeducedTypeTransform(*this, DependentResult).Apply(Type);
       assert(!Result.isNull() && "substituting DependentTy can't fail");
-      return TDK_Success;
+      return DAR_Succeeded;
     }
-    return TDK;
+    if (diagnoseAutoDeductionFailure(*this, TDK, Info, Ranges))
+      return DAR_FailedAlreadyDiagnosed;
+    return DAR_Failed;
   };
 
   SmallVector<OriginalCallArg, 4> OriginalCallArgs;
 
-  QualType DeducedType;
-  // If this is a 'decltype(auto)' specifier, do the decltype dance.
-  if (AT->isDecltypeAuto()) {
-    if (InitList) {
-      Diag(Init->getBeginLoc(), diag::err_decltype_auto_initializer_list);
-      return TDK_AlreadyDiagnosed;
-    }
+  InitListExpr *InitList = dyn_cast<InitListExpr>(Init);
+  if (InitList) {
+    // Notionally, we substitute std::initializer_list<T> for 'auto' and deduce
+    // against that. Such deduction only succeeds if removing cv-qualifiers and
+    // references results in std::initializer_list<T>.
+    if (!Type.getType().getNonReferenceType()->getAs<AutoType>())
+      return DAR_Failed;
 
-    DeducedType = getDecltypeForExpr(Init);
-    assert(!DeducedType.isNull());
-  } else {
-    LocalInstantiationScope InstScope(*this);
-
-    // Build template<class TemplParam> void Func(FuncParam);
-    SourceLocation Loc = Init->getExprLoc();
-    TemplateTypeParmDecl *TemplParam = TemplateTypeParmDecl::Create(
-        Context, nullptr, SourceLocation(), Loc, Info.getDeducedDepth(), 0,
-        nullptr, false, false, false);
-    QualType TemplArg = QualType(TemplParam->getTypeForDecl(), 0);
-    NamedDecl *TemplParamPtr = TemplParam;
-    FixedSizeTemplateParameterListStorage<1, false> TemplateParamsSt(
-        Context, Loc, Loc, TemplParamPtr, Loc, nullptr);
-
-    if (InitList) {
-      // Notionally, we substitute std::initializer_list<T> for 'auto' and
-      // deduce against that. Such deduction only succeeds if removing
-      // cv-qualifiers and references results in std::initializer_list<T>.
-      if (!Type.getType().getNonReferenceType()->getAs<AutoType>())
-        return TDK_Invalid;
-
-      SourceRange DeducedFromInitRange;
-      for (Expr *Init : InitList->inits()) {
-        // Resolving a core issue: a braced-init-list containing any designators
-        // is a non-deduced context.
-        if (isa<DesignatedInitExpr>(Init))
-          return TDK_Invalid;
-        if (auto TDK = DeduceTemplateArgumentsFromCallArgument(
-                *this, TemplateParamsSt.get(), 0, TemplArg, Init, Info, Deduced,
-                OriginalCallArgs, /*Decomposed=*/true,
-                /*ArgIdx=*/0, /*TDF=*/0)) {
-          if (TDK == TDK_Inconsistent) {
-            Diag(Info.getLocation(), diag::err_auto_inconsistent_deduction)
-                << Info.FirstArg << Info.SecondArg << DeducedFromInitRange
-                << Init->getSourceRange();
-            return DeductionFailed(TDK_AlreadyDiagnosed);
-          }
-          return DeductionFailed(TDK);
-        }
+    // Resolving a core issue: a braced-init-list containing any designators is
+    // a non-deduced context.
+    for (Expr *E : InitList->inits())
+      if (isa<DesignatedInitExpr>(E))
+        return DAR_Failed;
+
+    SourceRange DeducedFromInitRange;
+    for (unsigned i = 0, e = InitList->getNumInits(); i < e; ++i) {
+      Expr *Init = InitList->getInit(i);
 
-        if (DeducedFromInitRange.isInvalid() &&
-            Deduced[0].getKind() != TemplateArgument::Null)
-          DeducedFromInitRange = Init->getSourceRange();
-      }
-    } else {
-      if (!getLangOpts().CPlusPlus && Init->refersToBitField()) {
-        Diag(Loc, diag::err_auto_bitfield);
-        return TDK_AlreadyDiagnosed;
-      }
-      QualType FuncParam =
-          SubstituteDeducedTypeTransform(*this, TemplArg).Apply(Type);
-      assert(!FuncParam.isNull() &&
-             "substituting template parameter for 'auto' failed");
       if (auto TDK = DeduceTemplateArgumentsFromCallArgument(
-              *this, TemplateParamsSt.get(), 0, FuncParam, Init, Info, Deduced,
-              OriginalCallArgs, /*Decomposed=*/false, /*ArgIdx=*/0, /*TDF=*/0))
-        return DeductionFailed(TDK);
+              *this, TemplateParamsSt.get(), 0, TemplArg, Init,
+              Info, Deduced, OriginalCallArgs, /*Decomposed*/ true,
+              /*ArgIdx*/ 0, /*TDF*/ 0))
+        return DeductionFailed(TDK, {DeducedFromInitRange,
+                                     Init->getSourceRange()});
+
+      if (DeducedFromInitRange.isInvalid() &&
+          Deduced[0].getKind() != TemplateArgument::Null)
+        DeducedFromInitRange = Init->getSourceRange();
+    }
+  } else {
+    if (!getLangOpts().CPlusPlus && Init->refersToBitField()) {
+      Diag(Loc, diag::err_auto_bitfield);
+      return DAR_FailedAlreadyDiagnosed;
     }
 
-    // Could be null if somehow 'auto' appears in a non-deduced context.
-    if (Deduced[0].getKind() != TemplateArgument::Type)
-      return DeductionFailed(TDK_Incomplete);
-    DeducedType = Deduced[0].getAsType();
+    if (auto TDK = DeduceTemplateArgumentsFromCallArgument(
+            *this, TemplateParamsSt.get(), 0, FuncParam, Init, Info, Deduced,
+            OriginalCallArgs, /*Decomposed*/ false, /*ArgIdx*/ 0, /*TDF*/ 0))
+      return DeductionFailed(TDK, {});
+  }
 
-    if (InitList) {
-      DeducedType = BuildStdInitializerList(DeducedType, Loc);
-      if (DeducedType.isNull())
-        return TDK_AlreadyDiagnosed;
-    }
+  // Could be null if somehow 'auto' appears in a non-deduced context.
+  if (Deduced[0].getKind() != TemplateArgument::Type)
+    return DeductionFailed(TDK_Incomplete, {});
+
+  QualType DeducedType = Deduced[0].getAsType();
+
+  if (InitList) {
+    DeducedType = BuildStdInitializerList(DeducedType, Loc);
+    if (DeducedType.isNull())
+      return DAR_FailedAlreadyDiagnosed;
   }
 
-  if (!Result.isNull()) {
-    if (!Context.hasSameType(DeducedType, Result)) {
-      Info.FirstArg = Result;
-      Info.SecondArg = DeducedType;
-      return DeductionFailed(TDK_Inconsistent);
+  QualType MaybeAuto = Type.getType().getNonReferenceType();
+  while (MaybeAuto->isPointerType())
+    MaybeAuto = MaybeAuto->getPointeeType();
+  if (const auto *AT = MaybeAuto->getAs<AutoType>()) {
+    if (AT->isConstrained() && !IgnoreConstraints) {
+      auto ConstraintsResult = CheckDeducedPlaceholderConstraints(
+          *this, *AT, Type.getContainedAutoTypeLoc(), DeducedType);
+      if (ConstraintsResult != DAR_Succeeded)
+        return ConstraintsResult;
     }
-    DeducedType = Context.getCommonSugaredType(Result, DeducedType);
   }
 
-  if (AT->isConstrained() && !IgnoreConstraints &&
-      CheckDeducedPlaceholderConstraints(
-          *this, *AT, Type.getContainedAutoTypeLoc(), DeducedType))
-    return TDK_AlreadyDiagnosed;
-
   Result = SubstituteDeducedTypeTransform(*this, DeducedType).Apply(Type);
   if (Result.isNull())
-    return TDK_AlreadyDiagnosed;
+    return DAR_FailedAlreadyDiagnosed;
 
   // Check that the deduced argument type is compatible with the original
   // argument type per C++ [temp.deduct.call]p4.
@@ -4804,11 +4849,11 @@ Sema::TemplateDeductionResult Sema::DeduceAutoType(TypeLoc Type, Expr *Init,
     if (auto TDK =
             CheckOriginalCallArgDeduction(*this, Info, OriginalArg, DeducedA)) {
       Result = QualType();
-      return DeductionFailed(TDK);
+      return DeductionFailed(TDK, {});
     }
   }
 
-  return TDK_Success;
+  return DAR_Succeeded;
 }
 
 QualType Sema::SubstAutoType(QualType TypeWithAuto,

diff  --git a/clang/test/SemaCXX/cxx0x-initializer-stdinitializerlist.cpp b/clang/test/SemaCXX/cxx0x-initializer-stdinitializerlist.cpp
index 143d93cefc7c4..80baefa7c8b8e 100644
--- a/clang/test/SemaCXX/cxx0x-initializer-stdinitializerlist.cpp
+++ b/clang/test/SemaCXX/cxx0x-initializer-stdinitializerlist.cpp
@@ -374,7 +374,7 @@ namespace weird_initlist {
   // We don't check the struct layout in Sema.
   auto x = {weird{}, weird{}, weird{}, weird{}, weird{}};
   // ... but we do in constant evaluation.
-  constexpr auto y = {weird{}, weird{}, weird{}, weird{}, weird{}}; // expected-error {{constant}} expected-note {{type 'const std::initializer_list<weird>' has unexpected layout}}
+  constexpr auto y = {weird{}, weird{}, weird{}, weird{}, weird{}}; // expected-error {{constant}} expected-note {{type 'const std::initializer_list<weird_initlist::weird>' has unexpected layout}}
 }
 
 auto v = std::initializer_list<int>{1,2,3}; // expected-warning {{array backing local initializer list 'v' will be destroyed at the end of the full-expression}}

diff  --git a/clang/test/SemaCXX/deduced-return-void.cpp b/clang/test/SemaCXX/deduced-return-void.cpp
index 84d5936c830ae..2cd518d426e5c 100644
--- a/clang/test/SemaCXX/deduced-return-void.cpp
+++ b/clang/test/SemaCXX/deduced-return-void.cpp
@@ -115,7 +115,7 @@ auto& f4() {
 }
 auto& f5() {
   return i;
-  return void(); // expected-error {{deduced as 'int' in earlier return statement}}
+  return void(); // expected-error at -2 {{cannot form a reference to 'void'}}
 }
 auto& f6() { return 42; } // expected-error {{non-const lvalue reference to type 'int' cannot bind to a temporary of type 'int'}}
 
@@ -130,9 +130,9 @@ auto l4 = []() -> auto& {
   return i;
   return; // expected-error {{cannot deduce return type 'auto &' from omitted return expression}}
 };
-auto l5 = []() -> auto & {
+auto l5 = []() -> auto& { // expected-error {{cannot form a reference to 'void'}}
   return i;
-  return void(); // expected-error {{deduced as 'int' in earlier return statement}}
+  return void();
 };
 auto l6 = []() -> auto& {
   return 42; // expected-error {{non-const lvalue reference to type 'int' cannot bind to a temporary of type 'int'}}

diff  --git a/clang/test/SemaCXX/sugared-auto.cpp b/clang/test/SemaCXX/sugared-auto.cpp
index 15bcf3616464a..d63b0bccb2ed1 100644
--- a/clang/test/SemaCXX/sugared-auto.cpp
+++ b/clang/test/SemaCXX/sugared-auto.cpp
@@ -1,12 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only -verify -xobjective-c++ %s -std=c++20 -fms-extensions -fblocks -fobjc-arc -fobjc-runtime-has-weak -fenable-matrix -Wno-dynamic-exception-spec -Wno-c++17-compat-mangling
-// RUN: %clang_cc1 -fsyntax-only -verify -xobjective-c++ %s -std=c++14 -fms-extensions -fblocks -fobjc-arc -fobjc-runtime-has-weak -fenable-matrix -Wno-dynamic-exception-spec -Wno-c++17-compat-mangling
-
-namespace std {
-template<typename T> struct initializer_list {
-  const T *begin, *end;
-  initializer_list();
-};
-} // namespace std
+// RUN: %clang_cc1 -fsyntax-only -verify %s -std=c++20
 
 enum class N {};
 
@@ -17,26 +9,6 @@ using AnimalPtr = Animal *;
 using Man = Animal;
 using Dog = Animal;
 
-using ManPtr = Man *;
-using DogPtr = Dog *;
-
-using SocratesPtr = ManPtr;
-
-using ConstMan = const Man;
-using ConstDog = const Dog;
-
-using Virus = void;
-using SARS = Virus;
-using Ebola = Virus;
-
-using Bacteria = float;
-using Bacilli = Bacteria;
-using Vibrio = Bacteria;
-
-struct Plant;
-using Gymnosperm = Plant;
-using Angiosperm = Plant;
-
 namespace variable {
 
 auto x1 = Animal();
@@ -53,9 +25,6 @@ auto x4 = Man(), x5 = Dog();
 N t4 = x4; // expected-error {{lvalue of type 'Man' (aka 'int')}}
 N t5 = x5; // expected-error {{lvalue of type 'Dog' (aka 'int')}}
 
-auto x6 = { Man(), Dog() };
-N t6 = x6; // expected-error {{from 'std::initializer_list<Animal>' (aka 'initializer_list<int>')}}
-
 } // namespace variable
 
 namespace function_basic {
@@ -72,160 +41,3 @@ auto x3 = [a = Animal()] { return a; }();
 N t3 = x3; // expected-error {{lvalue of type 'Animal' (aka 'int')}}
 
 } // namespace function_basic
-
-namespace function_multiple_basic {
-
-N t1 = [] { // expected-error {{rvalue of type 'Animal' (aka 'int')}}
-  if (true)
-    return Man();
-  return Dog();
-}();
-
-N t2 = []() -> decltype(auto) { // expected-error {{rvalue of type 'Animal' (aka 'int')}}
-  if (true)
-    return Man();
-  return Dog();
-}();
-
-N t3 = [] { // expected-error {{rvalue of type 'Animal' (aka 'int')}}
-  if (true)
-    return Dog();
-  auto x = Man();
-  return x;
-}();
-
-N t4 = [] { // expected-error {{rvalue of type 'int'}}
-  if (true)
-    return Dog();
-  return 1;
-}();
-
-N t5 = [] { // expected-error {{rvalue of type 'Virus' (aka 'void')}}
-  if (true)
-    return Ebola();
-  return SARS();
-}();
-
-N t6 = [] { // expected-error {{rvalue of type 'void'}}
-  if (true)
-    return SARS();
-  return;
-}();
-
-} // namespace function_multiple_basic
-
-#define TEST_AUTO(X, A, B) \
-  static_assert(__is_same(A, B), ""); \
-  auto X(A a, B b) {       \
-    if (0)                 \
-      return a;            \
-    if (0)                 \
-      return b;            \
-    return N();            \
-  }
-#define TEST_DAUTO(X, A, B)     \
-  static_assert(__is_same(A, B), ""); \
-  decltype(auto) X(A a, B b) {  \
-    if (0)                      \
-      return static_cast<A>(a); \
-    if (0)                      \
-      return static_cast<B>(b); \
-    return N();                 \
-  }
-
-namespace misc {
-
-TEST_AUTO(t1, ManPtr, DogPtr)      // expected-error {{but deduced as 'Animal *' (aka 'int *')}}
-TEST_AUTO(t2, ManPtr, int *)       // expected-error {{but deduced as 'int *'}}
-TEST_AUTO(t3, SocratesPtr, ManPtr) // expected-error {{but deduced as 'ManPtr' (aka 'int *')}}
-
-TEST_AUTO(t4, _Atomic(Man), _Atomic(Dog)) // expected-error {{but deduced as '_Atomic(Animal)'}}
-
-using block_man = void (^)(Man);
-using block_dog = void (^)(Dog);
-TEST_AUTO(t5, block_man, block_dog) // expected-error {{but deduced as 'void (^__strong)(Animal)'}}
-
-#if __cplusplus >= 201500
-using fp1 = SARS (*)(Man, DogPtr) throw(Vibrio);
-using fp2 = Ebola (*)(Dog, ManPtr) throw(Bacilli);
-TEST_AUTO(t6, fp1, fp2); // expected-error {{but deduced as 'Virus (*)(Animal, Animal *) throw(Bacteria)' (aka 'void (*)(int, int *) throw(Bacteria)')}}
-
-using fp3 = SARS (*)() throw(Man);
-using fp4 = Ebola (*)() throw(Vibrio);
-auto t7(fp3 a, fp4 b) {
-  if (false)
-    return true ? a : b;
-  if (false)
-    return a;
-  return N(); // expected-error {{but deduced as 'SARS (*)() throw(Man, Vibrio)' (aka 'void (*)() throw(Man, Vibrio)')}}
-}
-#endif
-
-using fp5 = void (*)(const Man);
-using fp6 = void (*)(Dog);
-TEST_AUTO(t8, fp5, fp6); // expected-error {{but deduced as 'void (*)(Animal)' (aka 'void (*)(int)')}}
-
-using fp7 = void (*)(ConstMan);
-using fp8 = void (*)(ConstDog);
-TEST_AUTO(t9, fp7, fp8); // expected-error {{but deduced as 'void (*)(const Animal)' (aka 'void (*)(const int)')}}
-
-using fp9 = void (*)(ConstMan);
-using fp10 = void (*)(const Dog);
-TEST_AUTO(t10, fp9, fp10); // expected-error {{but deduced as 'void (*)(const Animal)' (aka 'void (*)(const int)')}}
-
-using fp11 = void (*)(__strong block_man);
-using fp12 = void (*)(__weak block_dog);
-TEST_AUTO(t11, fp11, fp12); // expected-error {{but deduced as 'void (*)(void (^)(Animal))'}}
-
-TEST_AUTO(t12, Man Angiosperm::*, Dog Gymnosperm::*) // expected-error {{but deduced as 'Animal Plant::*'}}
-
-TEST_DAUTO(t13, const Man &, const Dog &) // expected-error {{but deduced as 'const Animal &' (aka 'const int &')}}
-
-TEST_DAUTO(t14, Man &&, Dog &&) // expected-error {{but deduced as 'Animal &&' (aka 'int &&')}}
-
-using matrix_man = Man __attribute__((matrix_type(4, 4)));
-using matrix_dog = Dog __attribute__((matrix_type(4, 4)));
-TEST_AUTO(t15, matrix_man, matrix_dog) // expected-error {{but deduced as 'Animal __attribute__((matrix_type(4, 4)))'}}
-
-using vector_man = Man __attribute__((vector_size(4)));
-using vector_dog = Dog __attribute__((vector_size(4)));
-TEST_AUTO(t16, vector_man, vector_dog) // expected-error {{but deduced as '__attribute__((__vector_size__(1 * sizeof(Animal)))) Animal' (vector of 1 'Animal' value)}}
-
-using ext_vector_man = Man __attribute__((ext_vector_type(4)));
-using ext_vector_dog = Dog __attribute__((ext_vector_type(4)));
-TEST_AUTO(t17, ext_vector_man, ext_vector_dog) // expected-error {{but deduced as 'Animal __attribute__((ext_vector_type(4)))' (vector of 4 'Animal' values)}}
-
-} // namespace misc
-
-namespace exception_spec {
-
-void none();
-void dyn_none() throw();
-void dyn() throw(int);
-void ms_any() throw(...);
-void __declspec(nothrow) nothrow();
-void noexcept_basic() noexcept;
-void noexcept_true() noexcept(true);
-void noexcept_false() noexcept(false);
-
-#if __cplusplus < 201500
-TEST_AUTO(t1, decltype(&noexcept_false), decltype(&noexcept_true)) // expected-error {{but deduced as 'void (*)() noexcept(false)'}}
-TEST_AUTO(t2, decltype(&noexcept_basic), decltype(&noexcept_true)) // expected-error {{but deduced as 'void (*)() noexcept(true)'}}
-TEST_AUTO(t3, decltype(&none), decltype(&ms_any)) // expected-error {{but deduced as 'void (*)()'}}
-TEST_AUTO(t4, decltype(&noexcept_false), decltype(&ms_any)) // expected-error {{but deduced as 'void (*)() throw(...)'}}
-TEST_AUTO(t5, decltype(&nothrow), decltype(&noexcept_false)) // expected-error {{but deduced as 'void (*)() noexcept(false)'}}
-TEST_AUTO(t6, decltype(&dyn_none), decltype(&nothrow)) // expected-error {{but deduced as 'void (*)() throw()'}}
-TEST_AUTO(t7, decltype(&noexcept_true), decltype(&dyn)) // expected-error {{but deduced as 'void (*)() throw(int)'}}
-#endif
-} // namespace exception_spec
-
-namespace non_deduced {
-  void f();
-  void g();
-  void g(int);
-  auto h() {
-    if (false) return f;
-    return g;
-    // expected-error at -1 {{returned value of type '<overloaded function type>'}}
-  }
-} // namespace non_deduced

diff  --git a/clang/test/SemaTemplate/deduction.cpp b/clang/test/SemaTemplate/deduction.cpp
index 1414f7d906301..17ccc9c1dc458 100644
--- a/clang/test/SemaTemplate/deduction.cpp
+++ b/clang/test/SemaTemplate/deduction.cpp
@@ -162,15 +162,6 @@ template void e(const float *, int);
 
 } // namespace test4
 
-namespace test5 {
-
-template <bool, int = 0> class a {};
-template <class b> void c(b, b);
-template <bool b> void c(a<b>, a<b>);
-void d() { c(a<true>(), a<true>()); }
-
-} // namespace test5
-
 // Verify that we can deduce enum-typed arguments correctly.
 namespace test14 {
   enum E { E0, E1 };


        


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