test/Preprocessor/warning_tests.c

// RUN: %clang_cc1 -fsyntax-only %s -verify
#ifndef __has_warning
#error Should have __has_warning
#endif

#if __has_warning("not valid") // expected-warning {{__has_warning expected option name}}
#endif

#if __has_warning("-Wparentheses")
#warning Should have -Wparentheses // expected-warning {{Should have -Wparentheses}}
#endif

#if __has_warning(-Wfoo) // expected-error {{builtin warning check macro requires a parenthesized string}}
#endif

#if __has_warning("-Wnot-a-valid-warning-flag-at-all")
#else
#warning Not a valid warning flag // expected-warning {{Not a valid warning flag}}
#endif