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

[Matheus Izvekov via cfe-commits]

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@@ -1173,25 +1276,52 @@ void DeclareImplicitDeductionGuidesForTypeAlias(
         NewParam->setScopeInfo(0, I);
         FPTL.setParam(I, NewParam);
       }
-      auto *Transformed = cast<FunctionDecl>(buildDeductionGuide(
-          SemaRef, AliasTemplate, /*TemplateParams=*/nullptr,
+
+      QualType ReturnType =
+          cast<FunctionProtoType>(FunctionType->getType())->getReturnType();
+      if (FromInheritedCtor) {
+        FunctionType = buildInheritedConstructorDeductionGuideType(
+            SemaRef, *FromInheritedCtor, FunctionType);
+        if (!FunctionType)
+          continue;
+        ReturnType = FunctionType->getType()
+                         .getTypePtr()
+                         ->getAs<FunctionProtoType>()
+                         ->getReturnType();
+      }
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mizvekov wrote:

There is a lot of commonality with this block here and another that appears in `BuildDeductionGuideForTypeAlias`, can we factor it out?

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