Diagnose use of _Noreturn on a struct/union field

C99 6.7.4p2 clarifies that a function specifier can only be used in the
declaration of a function. _Noreturn is a function specifier, so it is
a constraint violation to write it on a structure or union field, but
we missed that case.

Fixes #56800

clang/docs/ReleaseNotes.rst

@@ -52,7 +52,9 @@ Bug Fixes
 - ``-Wtautological-compare`` missed warnings for tautological comparisons
   involving a negative integer literal. This fixes
   `Issue 42918 <https://github.com/llvm/llvm-project/issues/42918>`_.
-
+- Fixes an accepts-invalid bug in C when using a ``_Noreturn`` function
+  specifier on something other than a function declaration. This fixes
+  `Issue 56800 <https://github.com/llvm/llvm-project/issues/56800>`_.
 
 Improvements to Clang's diagnostics
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Parse/ParseDecl.cpp

@@ -2599,6 +2599,8 @@ void Parser::ParseSpecifierQualifierList(DeclSpec &DS, AccessSpecifier AS,
       Diag(DS.getVirtualSpecLoc(), diag::err_typename_invalid_functionspec);
     if (DS.hasExplicitSpecifier())
       Diag(DS.getExplicitSpecLoc(), diag::err_typename_invalid_functionspec);
+    if (DS.isNoreturnSpecified())
+      Diag(DS.getNoreturnSpecLoc(), diag::err_typename_invalid_functionspec);
     DS.ClearFunctionSpecs();
   }
 

clang/test/Parser/c11-noreturn.c

@@ -15,4 +15,15 @@ _Noreturn int; // expected-error {{'_Noreturn' can only appear on functions}} ex
 _Noreturn struct S; // expected-error {{'_Noreturn' can only appear on functions}}
 _Noreturn enum E { e }; // expected-error {{'_Noreturn' can only appear on functions}}
 
+struct GH56800 {
+  _Noreturn int f1; // expected-error {{type name does not allow function specifier to be specified}}
+};
+
+_Noreturn int AlsoBad; // expected-error {{'_Noreturn' can only appear on functions}}
+void func(_Noreturn int ThisToo) { // expected-error {{'_Noreturn' can only appear on functions}}
+  for (_Noreturn int i = 0; i < 10; ++i) // expected-error {{'_Noreturn' can only appear on functions}}
+    ;
+}
+
+
 // CHECK-EXT: '_Noreturn' is a C11 extension