https://bugs.llvm.org/show_bug.cgi?id=50046
Bug ID: 50046
Summary: Pedantically warning on attributes used as extensions
Product: clang
Version: 11.0
Hardware: PC
OS: Windows NT
Status: NEW
Severity: enhancement
Priority: P
Component: C++
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected], [email protected],
[email protected]
Consider the following code, which tries to detect the ability to use
[[nodiscard]] and then uses it:
#ifndef __has_cpp_attribute
# define HAS_ATTRIBUTE(x) 0
#else
# define HAS_ATTRIBUTE(x) __has_cpp_attribute(x) > 0
#endif
#if HAS_ATTRIBUTE(nodiscard)
# define NODISCARD [[nodiscard]]
#else
# define NODISCARD
#endif
struct NODISCARD Result { };
With clang (on any version I've tested), if I compile this with -std=c++14
-pedantic, I get a warning:
<source>:13:8: warning: use of the 'nodiscard' attribute is a C++17 extension
[-Wc++17-extensions]
struct NODISCARD Result { };
^
<source>:8:23: note: expanded from macro 'NODISCARD'
# define NODISCARD [[nodiscard]]
^
But if __has_cpp_attribute(nodiscard) is not the right way to detect nodiscard,
what would be?
One alternative might be to use __cplusplus somehow to check to see if the
attribute is *really* available. That is, do this:
# define HAS_ATTRIBUTE(x) __has_cpp_attribute(x) > 0 && __has_cpp_attribute(x)
> 0 && __cplusplus >= __has_cpp_attribute(x)
This has the benefit of silencing the pedantic warning.
But it also has the consequence of actually not using [[nodiscard]] even when
it's available. For instance, on clang trunk 11.0.0, when compiling with
-std=c++17, __cplusplus is 201703 but __has_cpp_attribute(nodiscard) is 201907.
As a result, my Result here would not be [[nodiscard]] - a false negative.
clang 11.0 --std=c++20 would successfully mark Result as [[nodiscard]], but gcc
10.2 --std=c++20 would not (because that release does not yet increase
__cplusplus).
Is there a way to programmatically detect the availability of an attribute in a
way that would not be warned? Maybe that means __has_cpp_attribute(nodiscard)
should be 0 if compiling as --std=c++14 or --std=c++11 with -pedantic and
without -Wno-c++17-extensions? Otherwise, the only solution I think would be to
do what nlohmann/json did (https://github.com/nlohmann/json/issues/1535) and do
something like this:
#if __has_cpp_attribute(nodiscard)
#if defined(__clang__) && !defined(JSON_HAS_CPP_17) // issue #1535
#define JSON_NODISCARD
#else
#define JSON_NODISCARD [[nodiscard]]
#endif
#endif
But this kind of explicit compiler-based configuration was what we were hoping
to avoid with feature-test macros (and also means that clang users that are not
compiling with -pedantic still don't get nodiscard).
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