[clang] [Clang] Allow explicit member specialization to differ from template declaration wrt constexpr (PR #145272)

[Erich Keane via cfe-commits]

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@@ -12163,6 +12163,23 @@ bool Sema::CheckFunctionDeclaration(Scope *S, FunctionDecl *NewFD,
     }
   }
 
+  // C++11 [dcl.constexpr]p1: An explicit specialization of a constexpr
+  // function can differ from the template declaration with respect to
+  // the constexpr specifier.
+  if (IsMemberSpecialization) {
+    FunctionDecl *InstantiationFunction =
+        OldDecl ? OldDecl->getAsFunction() : nullptr;
+    if (InstantiationFunction &&
+        InstantiationFunction->getTemplateSpecializationKind() ==
+            TSK_ImplicitInstantiation &&
+        (NewFD->getTemplateSpecializationKind() == TSK_ExplicitSpecialization ||
+         NewFD->getTemplateSpecializationKind() == TSK_Undeclared)) {
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erichkeane wrote:

What is going on here?  Why do you not need to handle `TSK_ExplicitInstantiationDeclaration` and `TSK_ExplicitInstantiationDefinition`?  Can you play with examples a bit more to see if/when those should be applied?


https://github.com/llvm/llvm-project/pull/145272