Elaborate comments on why we skip explicit specializations

Add -Winvalid-noreturn diagnostic in LIT test

Author: snarang181

clang/lib/Sema/SemaDeclAttr.cpp

@@ -1970,8 +1970,12 @@ void clang::inferNoReturnAttr(Sema &S, const Decl *D) {
   if (!FD)
     return;
 
+  // Skip explicit specializations here as they may have
+  // a user-provided definition that may deliberately differ from the primary
+  // template. If an explicit specialization truly never returns, the user
+  // should explicitly mark it with [[noreturn]].
   if (FD->getTemplateSpecializationKind() == TSK_ExplicitSpecialization)
-    return; // Don't infer noreturn for explicit specializations.
+    return;
 
   auto *NonConstFD = const_cast<FunctionDecl *>(FD);
   DiagnosticsEngine &Diags = S.getDiagnostics();

clang/test/SemaCXX/wreturn-always-throws.cpp

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only -fcxx-exceptions -fexceptions -Wreturn-type -verify %s
+// RUN: %clang_cc1 -fsyntax-only -fcxx-exceptions -fexceptions -Wreturn-type -Winvalid-noreturn -verify %s
 // expected-no-diagnostics
 
 namespace std {
@@ -62,4 +62,4 @@ void S<int>::f() {}
 template<typename T> 
 void S<T>::g() {}  
 template<> 
-void S<int>::g() { throw 0; }
+void S<int>::g() { throw 0; }