test/Sema/c2x-nodiscard.c

// RUN: %clang_cc1 -fsyntax-only -std=c2x -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 { // FIXME: may need an extension warning.
  int i;
};

struct S3 get_s3(void);

[[nodiscard]] int f1(void);
enum [[nodiscard]] E1 { One };

[[nodiscard]] int i; // expected-warning {{'nodiscard' attribute only applies to Objective-C methods, enums, structs, unions, classes, functions, and function pointers}}

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_s3(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute: Wrong}}
  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_s3();
  (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();
}