caf40ae419
Summary: The `[[nodiscard]]` attribute is intended to help users find bugs where function return values are ignored when they shouldn't be. After C++17 the C++ standard has started to declared such library functions as `[[nodiscard]]`. However, this application is limited and applies only to dialects after C++17. Users who want help diagnosing misuses of STL functions may desire a more liberal application of `[[nodiscard]]`. For this reason libc++ provides an extension that does just that! The extension must be enabled by defining `_LIBCPP_ENABLE_NODISCARD`. The extended applications of `[[nodiscard]]` takes two forms: 1. Backporting `[[nodiscard]]` to entities declared as such by the standard in newer dialects, but not in the present one. 2. Extended applications of `[[nodiscard]]`, at the libraries discretion, applied to entities never declared as such by the standard. Users may also opt-out of additional applications `[[nodiscard]]` using additional macros. Applications of the first form, which backport `[[nodiscard]]` from a newer dialect may be disabled using macros specific to the dialect it was added. For example `_LIBCPP_DISABLE_NODISCARD_AFTER_CXX17`. Applications of the second form, which are pure extensions, may be disabled by defining `_LIBCPP_DISABLE_NODISCARD_EXT`. This patch was originally written by me (Roman Lebedev), then but then reworked by Eric Fiselier. Reviewers: mclow.lists, thakis, EricWF Reviewed By: thakis, EricWF Subscribers: llvm-commits, mclow.lists, lebedev.ri, EricWF, rjmccall, Quuxplusone, cfe-commits, christof Differential Revision: https://reviews.llvm.org/D45179 git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@342808 91177308-0d34-0410-b5e6-96231b3b80d8
34 lines
1.2 KiB
C++
34 lines
1.2 KiB
C++
// -*- C++ -*-
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//===----------------------------------------------------------------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is dual licensed under the MIT and the University of Illinois Open
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// Source Licenses. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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// Test that _LIBCPP_NODISCARD_EXT and _LIBCPP_NODISCARD_AFTER_CXX17 are defined
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// to the appropriate warning-generating attribute when _LIBCPP_ENABLE_NODISCARD
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// is explicitly provided.
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// UNSUPPORTED: c++98, c++03
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// GCC 7 is the first version to introduce [[nodiscard]]
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// UNSUPPORTED: gcc-4.9, gcc-5, gcc-6
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// MODULES_DEFINES: _LIBCPP_ENABLE_NODISCARD
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#define _LIBCPP_ENABLE_NODISCARD
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#include <__config>
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_LIBCPP_NODISCARD_EXT int foo() { return 42; }
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_LIBCPP_NODISCARD_AFTER_CXX17 int bar() { return 42; }
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int main() {
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foo(); // expected-error-re {{ignoring return value of function declared with {{'nodiscard'|warn_unused_result}} attribute}}
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bar(); // expected-error-re {{ignoring return value of function declared with {{'nodiscard'|warn_unused_result}} attribute}}
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(void)foo(); // OK. void casts disable the diagnostic.
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(void)bar();
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}
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