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

[Erich Keane via cfe-commits]

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@@ -942,34 +946,145 @@ 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 function type and return 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` ...
+std::pair<TypeSourceInfo *, QualType>
+buildInheritedConstructorDeductionGuideType(
+    Sema &SemaRef, const InheritedConstructorDeductionInfo &Info,
+    TypeSourceInfo *SourceGuideTSI) {
+  ASTContext &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
+  // FIXME: There is currently no diagnostic emitted in this case,
+  // as it is nontrivial to propagate substitution failure messages up
+  // to the point where deduction guides are used-- we do not have a type
+  // with which we can create a deduction guide AST node and must encode the
+  // SFINAE message.
+  Sema::SFINAETrap Trap(SemaRef);
+
+  MultiLevelTemplateArgumentList Args;
+  Args.addOuterTemplateArguments(Info.DerivedClassTemplate,
+                                 TemplateArgument(FPT->getReturnType()),
+                                 /*Final=*/false);
+  Args.addOuterRetainedLevels(Info.DerivedClassTemplate->getTemplateDepth());
----------------
erichkeane wrote:

As we sure this is sufficient and shouldn't be calling the `getInstantiationArgs`?  It might need help with Deduction guides, but is meant to get the full depth of the deduction guide.  But `OuterRetainedLevels` means it won't fully instantiate (and those levels will just not fill anything in).  Though is that perhaps what is intended here?

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