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

[Haojian Wu via cfe-commits]

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@@ -1107,8 +1172,32 @@ BuildDeductionGuideForTypeAlias(Sema &SemaRef,
           Sema::CodeSynthesisContext::BuildingDeductionGuides)) {
     auto *GG = cast<CXXDeductionGuideDecl>(FPrime);
 
-    Expr *IsDeducible = buildIsDeducibleConstraint(
-        SemaRef, AliasTemplate, FPrime->getReturnType(), FPrimeTemplateParams);
+    TypeSourceInfo *TSI = GG->getTypeSourceInfo();
+    QualType ReturnType = FPrime->getReturnType();
+    TemplateDecl *DeducedTemplate =
+        FromInheritedCtor ? FromInheritedCtor->DerivedClassTemplate
+                          : AliasTemplate;
+    if (FromInheritedCtor) {
+      TSI = buildInheritedConstructorDeductionGuideType(
+          SemaRef, *FromInheritedCtor, TSI);
+      if (!TSI)
+        return nullptr;
+      ReturnType = TSI->getType()
+                       .getTypePtr()
+                       ->getAs<FunctionProtoType>()
+                       ->getReturnType();
+    }
+
+    // We omit the deducible constraint for inherited constructor deduction
+    // guides because they would take precedence over the derived class' own
+    // deduction guides due to [over.match.best.general]p2.5 and
+    // [temp.func.order]p6.4 If the alias were not deducible in this case, the
+    // deduction guide would already not be deducible due to the partial
+    // specialization `CC<>` failing substitution.
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hokein wrote:

Not yet, there's been no response from the CWG on the [issue](https://github.com/cplusplus/CWG/issues/607). The current implementation follows the suggestion outlined in that issue, and I think it's fine as is. However, we should probably reference this issue in a comment for clarity.

You can also check our related [discussion](https://github.com/llvm/llvm-project/pull/98788#discussion_r1686255752).

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