Enable support for the [[nodiscard]] attribute from WG14 N2050 when enabling double square bracket attributes in C code.

llvm-svn: 316026

clang/include/clang/Basic/Attr.td

@@ -2004,10 +2004,10 @@ def WarnUnused : InheritableAttr {
 }
 
 def WarnUnusedResult : InheritableAttr {
-  let Spellings = [CXX11<"", "nodiscard", 201603>,
+  let Spellings = [CXX11<"", "nodiscard", 201603>, C2x<"", "nodiscard">,
                    CXX11<"clang", "warn_unused_result">,
                    GCC<"warn_unused_result">];
-  let Subjects = SubjectList<[ObjCMethod, Enum, CXXRecord, FunctionLike],
+  let Subjects = SubjectList<[ObjCMethod, Enum, Record, FunctionLike],
                              WarnDiag, "ExpectedFunctionMethodEnumOrClass">;
   let Documentation = [WarnUnusedResultsDocs];
 }

clang/lib/AST/Decl.cpp

@@ -3056,7 +3056,8 @@ SourceRange FunctionDecl::getExceptionSpecSourceRange() const {
 const Attr *FunctionDecl::getUnusedResultAttr() const {
   QualType RetType = getReturnType();
   if (RetType->isRecordType()) {
-    if (const CXXRecordDecl *Ret = RetType->getAsCXXRecordDecl()) {
+    if (const auto *Ret =
+            dyn_cast_or_null<RecordDecl>(RetType->getAsTagDecl())) {
       if (const auto *R = Ret->getAttr<WarnUnusedResultAttr>())
         return R;
     }

clang/test/Sema/c2x-nodiscard.c

@@ -0,0 +1,49 @@
+// RUN: %clang_cc1 -fsyntax-only -fdouble-square-bracket-attributes -verify %s
+
+struct [[nodiscard]] S1 { // ok
+  int i;
+};
+struct [[nodiscard nodiscard]] S2 { // expected-error {{attribute 'nodiscard' cannot appear multiple times in an attribute specifier}}
+  int i;
+};
+struct [[nodiscard("Wrong")]] S3 { // expected-error {{'nodiscard' cannot have an argument list}}
+  int i;
+};
+
+[[nodiscard]] int f1(void);
+enum [[nodiscard]] E1 { One };
+
+[[nodiscard]] int i; // expected-warning {{'nodiscard' attribute only applies to functions, methods, enums, and classes}}
+
+struct [[nodiscard]] S4 {
+  int i;
+};
+struct S4 get_s(void);
+
+enum [[nodiscard]] E2 { Two };
+enum E2 get_e(void);
+
+[[nodiscard]] int get_i();
+
+void f2(void) {
+  get_s(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  get_i(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  get_e(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+  // Okay, warnings are not encouraged
+  (void)get_s();
+  (void)get_i();
+  (void)get_e();
+}
+
+struct [[nodiscard]] error_info{
+  int i;
+};
+
+struct error_info enable_missile_safety_mode(void);
+void launch_missiles(void);
+void test_missiles(void) {
+  enable_missile_safety_mode(); // expected-warning {{ignoring return value of function declared with 'nodiscard'}}
+  launch_missiles();
+}
+