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

[Matheus Izvekov via cfe-commits]

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@@ -980,19 +986,69 @@ getRHSTemplateDeclAndArgs(Sema &SemaRef, TypeAliasTemplateDecl *AliasTemplate) {
   return {Template, AliasRhsTemplateArgs};
 }
 
+// Build the type for a deduction guide generated from an inherited constructor
+// [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` ...
+std::pair<TypeSourceInfo *, QualType>
+buildInheritedConstructorDeductionGuideType(
+    Sema &SemaRef, TypeSourceInfo *DerivedClassMapperType,
+    TemplateDecl *DeducingTemplate, 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(DeducingTemplate,
+                                 TemplateArgument(FPT->getReturnType()), false);
+  Args.addOuterRetainedLevels(DeducingTemplate->getTemplateDepth());
+  TypeSourceInfo *ReturnTypeTSI =
+      SemaRef.SubstType(DerivedClassMapperType, Args,
+                        DeducingTemplate->getBeginLoc(), DeclarationName());
+  if (!ReturnTypeTSI || Trap.hasErrorOccurred())
+    return {nullptr, QualType()};
+  const QualType &ReturnType = ReturnTypeTSI->getType();
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mizvekov wrote:

A QualType is pointer sized, and designed to be copied cheaply, so we never take references to these.

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