[clang] [clang] CTAD: Generate deduction guides for alias templates from non-template explicit deduction guides (PR #96686)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Haojian Wu (hokein)

<details>
<summary>Changes</summary>

This patch addresses an issue where non-template explicit deduction guides were not considered when synthesized the deduction guides for alias templates.

Fixes #<!-- -->94927. 

---
Full diff: https://github.com/llvm/llvm-project/pull/96686.diff


2 Files Affected:

- (modified) clang/lib/Sema/SemaTemplate.cpp (+56-8) 
- (modified) clang/test/SemaCXX/cxx20-ctad-type-alias.cpp (+42) 


``````````diff
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index a032e3ec6f635..32879c0945bae 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -2262,8 +2262,12 @@ class ExtractTypeForDeductionGuide
   }
 };
 
-// Build a deduction guide with the specified parameter types.
-FunctionTemplateDecl *buildDeductionGuide(
+// Build a deduction guide using the provided information.
+//
+// A deduction guide can be either a template or a non-template function
+// declaration. If \p TemplateParams is null, a non-template function
+// declaration will be created.
+NamedDecl *buildDeductionGuide(
     Sema &SemaRef, TemplateDecl *OriginalTemplate,
     TemplateParameterList *TemplateParams, CXXConstructorDecl *Ctor,
     ExplicitSpecifier ES, TypeSourceInfo *TInfo, SourceLocation LocStart,
@@ -2289,16 +2293,21 @@ FunctionTemplateDecl *buildDeductionGuide(
     Param->setDeclContext(Guide);
   for (auto *TD : MaterializedTypedefs)
     TD->setDeclContext(Guide);
+  if (isa<CXXRecordDecl>(DC))
+    Guide->setAccess(AS_public);
+
+  if (!TemplateParams) {
+    DC->addDecl(Guide);
+    return Guide;
+  }
 
   auto *GuideTemplate = FunctionTemplateDecl::Create(
       SemaRef.Context, DC, Loc, DeductionGuideName, TemplateParams, Guide);
   GuideTemplate->setImplicit(IsImplicit);
   Guide->setDescribedFunctionTemplate(GuideTemplate);
 
-  if (isa<CXXRecordDecl>(DC)) {
-    Guide->setAccess(AS_public);
+  if (isa<CXXRecordDecl>(DC))
     GuideTemplate->setAccess(AS_public);
-  }
 
   DC->addDecl(GuideTemplate);
   return GuideTemplate;
@@ -2914,7 +2923,8 @@ Expr *buildIsDeducibleConstraint(Sema &SemaRef,
   ASTContext &Context = SemaRef.Context;
   // Constraint AST nodes must use uninstantiated depth.
   if (auto *PrimaryTemplate =
-          AliasTemplate->getInstantiatedFromMemberTemplate()) {
+          AliasTemplate->getInstantiatedFromMemberTemplate();
+      PrimaryTemplate && TemplateParams.size() > 0) {
     LocalInstantiationScope Scope(SemaRef);
 
     // Adjust the depth for TemplateParams.
@@ -3180,12 +3190,12 @@ BuildDeductionGuideForTypeAlias(Sema &SemaRef,
         FPrimeTemplateParams,
         AliasTemplate->getTemplateParameters()->getRAngleLoc(),
         /*RequiresClause=*/RequiresClause);
-    FunctionTemplateDecl *Result = buildDeductionGuide(
+    auto *Result = cast<FunctionTemplateDecl>(buildDeductionGuide(
         SemaRef, AliasTemplate, FPrimeTemplateParamList,
         GG->getCorrespondingConstructor(), GG->getExplicitSpecifier(),
         GG->getTypeSourceInfo(), AliasTemplate->getBeginLoc(),
         AliasTemplate->getLocation(), AliasTemplate->getEndLoc(),
-        F->isImplicit());
+        F->isImplicit()));
     cast<CXXDeductionGuideDecl>(Result->getTemplatedDecl())
         ->setDeductionCandidateKind(GG->getDeductionCandidateKind());
     return Result;
@@ -3216,6 +3226,44 @@ void DeclareImplicitDeductionGuidesForTypeAlias(
   Guides.suppressDiagnostics();
 
   for (auto *G : Guides) {
+    if (auto *DG = dyn_cast<CXXDeductionGuideDecl>(G)) {
+      // The deduction guide is a non-template function decl, we just clone it.
+      auto *FunctionType =
+          SemaRef.Context.getTrivialTypeSourceInfo(DG->getType());
+      FunctionProtoTypeLoc FPTL =
+          FunctionType->getTypeLoc().castAs<FunctionProtoTypeLoc>();
+
+      // Clone the parameters.
+      unsigned ProcessedParamIndex = 0;
+      for (auto *P : DG->parameters()) {
+        auto *TSI = SemaRef.Context.getTrivialTypeSourceInfo(P->getType());
+        ParmVarDecl *NewParam = ParmVarDecl::Create(
+            SemaRef.Context, G->getDeclContext(), P->getBeginLoc(),
+            P->getLocation(), nullptr, TSI->getType(), TSI, SC_None, nullptr);
+        NewParam->setScopeInfo(0, ProcessedParamIndex);
+        FPTL.setParam(ProcessedParamIndex, NewParam);
+        ProcessedParamIndex++;
+      }
+      auto *Transformed = cast<FunctionDecl>(buildDeductionGuide(
+          SemaRef, AliasTemplate, /*TemplateParams=*/nullptr,
+          /*Constructor=*/nullptr, DG->getExplicitSpecifier(), FunctionType,
+          AliasTemplate->getBeginLoc(), AliasTemplate->getLocation(),
+          AliasTemplate->getEndLoc(), DG->isImplicit()));
+
+      // FIXME: Here the synthesized deduction guide is not a templated function.
+      // Per [dcl.decl]p4, the requires-clause shall be present only if the
+      // declarator declares a templated function, a bug in standard?
+      auto *Constraint = buildIsDeducibleConstraint(
+          SemaRef, AliasTemplate, Transformed->getReturnType(), {});
+      if (auto *RC = DG->getTrailingRequiresClause()) {
+        auto Conjunction =
+            SemaRef.BuildBinOp(SemaRef.getCurScope(), SourceLocation{},
+                               BinaryOperatorKind::BO_LAnd, RC, Constraint);
+        if (!Conjunction.isInvalid())
+          Constraint = Conjunction.getAs<Expr>();
+      }
+      Transformed->setTrailingRequiresClause(Constraint);
+    }
     FunctionTemplateDecl *F = dyn_cast<FunctionTemplateDecl>(G);
     if (!F)
       continue;
diff --git a/clang/test/SemaCXX/cxx20-ctad-type-alias.cpp b/clang/test/SemaCXX/cxx20-ctad-type-alias.cpp
index b71dfc6ccaf4f..0881ce65631d7 100644
--- a/clang/test/SemaCXX/cxx20-ctad-type-alias.cpp
+++ b/clang/test/SemaCXX/cxx20-ctad-type-alias.cpp
@@ -402,3 +402,45 @@ template <typename U>
 using AFoo = A1<int>::A2<int>::Foo<U>;
 AFoo case3(1);
 } // namespace test24
+
+// GH94927
+namespace test25 {
+template <typename T>
+struct A {
+  A(T);
+};
+A(int) -> A<char>;
+
+template <typename U>
+using B1 = A<U>;
+B1 b1(100); // deduce to A<char>;
+static_assert(__is_same(decltype(b1), A<char>));
+
+template <typename U>
+requires (!__is_same(U, char)) // filter out the explicit deduction guide.
+using B2 = A<U>;
+template <typename V>
+using B3 = B2<V>;
+
+B2 b2(100); // deduced to A<int>;
+static_assert(__is_same(decltype(b2), A<int>));
+B3 b3(100); // decuded to A<int>;
+static_assert(__is_same(decltype(b3), A<int>));
+
+
+// the nested case
+template <typename T1>
+struct Out {
+  template <typename T2>
+  struct A {
+    A(T2);
+  };
+  A(int) -> A<T1>;
+  
+  template <typename T3>
+  using B = A<T3>;
+};
+
+Out<float>::B out(100); // deduced to Out<float>::A<float>;
+static_assert(__is_same(decltype(out), Out<float>::A<float>));
+} // namespace test25

``````````

</details>


https://github.com/llvm/llvm-project/pull/96686