Disallow [[nodiscard]] on a function pointer declaration.

This is not allowed by [dcl.attr.nodiscard]p1 for the standard attribute, but
is still supported for the [[clang::warn_unused_result]] spelling.

clang/lib/Sema/SemaDeclAttr.cpp

@@ -2826,6 +2826,12 @@ static void handleWarnUnusedResult(Sema &S, Decl *D, const ParsedAttr &AL) {
 
   StringRef Str;
   if ((AL.isCXX11Attribute() || AL.isC2xAttribute()) && !AL.getScopeName()) {
+    // The standard attribute cannot be applied to variable declarations such
+    // as a function pointer.
+    if (isa<VarDecl>(D))
+      S.Diag(AL.getLoc(), diag::warn_attribute_wrong_decl_type_str)
+          << AL << "functions, classes, or enumerations";
+
     // If this is spelled as the standard C++17 attribute, but not in C++17,
     // warn about using it as an extension. If there are attribute arguments,
     // then claim it's a C++2a extension instead.

clang/test/CXX/dcl.dcl/dcl.attr/dcl.attr.nodiscard/p2.cpp

@@ -26,7 +26,7 @@ void f() {
   (void)get_e();
 }
 
-[[nodiscard]] volatile char &(*fp)();
+[[nodiscard]] volatile char &(*fp)(); // expected-warning {{'nodiscard' attribute only applies to functions, classes, or enumerations}}
 void g() {
   fp(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
 

clang/test/SemaCXX/warn-unused-result.cpp

@@ -246,3 +246,11 @@ void g() {
     f(b); // expected-warning {{ignoring return value}}
 }
 } // namespace PR39837
+
+namespace PR45520 {
+[[nodiscard]] bool (*f)(); // expected-warning {{'nodiscard' attribute only applies to functions, classes, or enumerations}}
+[[clang::warn_unused_result]] bool (*g)();
+__attribute__((warn_unused_result)) bool (*h)();
+
+void i([[nodiscard]] bool (*fp)()); // expected-warning {{'nodiscard' attribute only applies to functions, classes, or enumerations}}
+}