[clang] [clang] Implement P2582R1: CTAD from inherited constructors (PR #98788)

[via cfe-commits]

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@@ -931,12 +935,73 @@ getRHSTemplateDeclAndArgs(Sema &SemaRef, TypeAliasTemplateDecl *AliasTemplate) {
   return {Template, AliasRhsTemplateArgs};
 }
 
+struct InheritedConstructorDeductionInfo {
+  // Class template for which we are declaring deduction guides
+  // This is `C` in the standard wording
+  TemplateDecl *DerivedClassTemplate;
+
+  // `template<typename> CC` in the standard wording
+  // This is the type of template that is substituted in the deduction guide
+  // return type `CC<R>`
+  TypeSourceInfo *CCType;
+};
+
+// Build the type for a deduction guide generated from an inherited constructor
+// C++23 [over.match.class.deduct]p1.10:
+// ... the set contains the guides of A with the return type R
+// of each guide replaced with `typename CC<R>::type` ...
+TypeSourceInfo *buildInheritedConstructorDeductionGuideType(
+    Sema &SemaRef, const InheritedConstructorDeductionInfo &Info,
+    TypeSourceInfo *SourceGuideTSI) {
+  auto &Context = SemaRef.Context;
+  const auto *FPT = SourceGuideTSI->getType()->getAs<FunctionProtoType>();
+  assert(FPT && "Source Guide type should be a FunctionProtoType");
+
+  // This substitution can fail in cases where the source return type
+  // is not dependent and the derived class is not deducible
+  Sema::SFINAETrap Trap(SemaRef);
+
+  MultiLevelTemplateArgumentList Args;
+  Args.addOuterTemplateArguments(Info.DerivedClassTemplate,
+                                 TemplateArgument(FPT->getReturnType()),
+                                 /*Final=*/false);
+  Args.addOuterRetainedLevels(Info.DerivedClassTemplate->getTemplateDepth());
+  TypeSourceInfo *ReturnTypeTSI = SemaRef.SubstType(
+      Info.CCType, Args, Info.DerivedClassTemplate->getBeginLoc(),
+      DeclarationName());
+  if (!ReturnTypeTSI || Trap.hasErrorOccurred())
+    return nullptr;
+  QualType ReturnType = ReturnTypeTSI->getType();
+
+  TypeLocBuilder TLB;
+  TLB.pushFullCopy(ReturnTypeTSI->getTypeLoc());
+
+  QualType FT = Context.getFunctionType(ReturnType, FPT->getParamTypes(),
+                                        FPT->getExtProtoInfo());
+  FunctionProtoTypeLoc NewTL = TLB.push<FunctionProtoTypeLoc>(FT);
+  const auto &TL = SourceGuideTSI->getTypeLoc().getAs<FunctionProtoTypeLoc>();
+  NewTL.setLocalRangeBegin(TL.getLocalRangeBegin());
+  NewTL.setLParenLoc(TL.getLParenLoc());
+  NewTL.setRParenLoc(TL.getRParenLoc());
+  NewTL.setExceptionSpecRange(TL.getExceptionSpecRange());
+  NewTL.setLocalRangeEnd(TL.getLocalRangeEnd());
+  for (unsigned I = 0, E = NewTL.getNumParams(); I != E; ++I)
----------------
antangelo wrote:

I don't believe so, from the comment on `FunctionProtoType` it seems the source location for ellipses is stored in the type itself, and there isn't any mention of them in `FunctionProtoTypeLoc`
https://github.com/llvm/llvm-project/blob/476b208e0115e766605e9f850982996a1d51c287/clang/include/clang/AST/Type.h#L5111-L5112

I have added a C-style variadic test case as well to verify that they work as expected.

https://github.com/llvm/llvm-project/pull/98788