[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 &&
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erichkeane wrote:

The old-decl should be a primary template, right?  What sort of thigns are we looking to avoid here?

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