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

[Matheus Izvekov via cfe-commits]

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@@ -11723,6 +11764,40 @@ static void DiagnoseBadDeduction(Sema &S, NamedDecl *Found, Decl *Templated,
       return;
     }
 
+    // Errors in deduction guides from inherited constructors
+    // will present as substitution failures in the mapping
+    // partial specialization, so we show a generic diagnostic
+    // in this case.
+    if (auto *DG = dyn_cast<CXXDeductionGuideDecl>(Templated);
+        DG && DG->getSourceKind() ==
+                  CXXDeductionGuideDecl::SourceKind::InheritedConstructor) {
+      CXXDeductionGuideDecl *Source = DG->getSourceDeductionGuide();
+      assert(Source &&
+             "Inherited constructor deduction guides must have a source");
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mizvekov wrote:

By calling `getSourceKind()`, you already assumed there is a source deduction guide, as otherwise the kind value is meaningless. See previous comment.

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