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

[via cfe-commits]

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@@ -11757,6 +11791,42 @@ static void DiagnoseBadDeduction(Sema &S, NamedDecl *Found, Decl *Templated,
       return;
     }
 
+    // Errors in deduction guides from inherited constructors
+    // will manifest as substitution failures in the return type
+    // partial specialization, so we show a generic diagnostic
+    // in this case.
+    if (auto *DG = dyn_cast<CXXDeductionGuideDecl>(Templated);
+        DG && DG->getSourceDeductionGuideKind() ==
+                  CXXDeductionGuideDecl::SourceDeductionGuideKind::
+                      InheritedConstructor) {
+      CXXDeductionGuideDecl *Source = DG->getSourceDeductionGuide();
+      assert(Source &&
+             "Inherited constructor deduction guides must have a source");
+
+      auto GetDGDeducedTemplateType =
+          [](CXXDeductionGuideDecl *DG) -> QualType {
+        return QualType(cast<ClassTemplateDecl>(DG->getDeducedTemplate())
+                            ->getTemplatedDecl()
+                            ->getTypeForDecl(),
+                        0);
+      };
+
+      QualType DeducedRecordType = GetDGDeducedTemplateType(DG);
+      QualType InheritedRecordType = GetDGDeducedTemplateType(Source);
+      S.Diag(Templated->getLocation(),
+             diag::note_ovl_candidate_inherited_constructor_deduction_failure)
+          << DeducedRecordType << InheritedRecordType << TemplateArgString;
+
+      CXXConstructorDecl *Ctor = DG->getCorrespondingConstructor();
+      if (Ctor)
+        S.Diag(
+            Ctor->getBeginLoc(),
+            diag::
+                note_ovl_candidate_inherited_constructor_deduction_failure_source)
+            << InheritedRecordType;
+      return;
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antangelo wrote:

I'm using the source location of the inherited constructor deduction guide to store the location of the using decl that inherits the base constructors (as it is otherwise difficult to pass this information around), so the notes for deduction guide failure will point there instead of at the source constructor (or deduction guide in the explicit case).

I have added additional diagnostics that print the source deduction guide declaration (with the source deduction guide's location) for both implicit and explicit guides when we note the deduction guide declarations for implicit guides as a workaround, which also removes the need for the above note by extension.

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