[llvm] r372761 - [Compiler] Fix LLVM_NODISCARD for GCC

[David Bolvansky via llvm-commits]

Author: xbolva00
Date: Tue Sep 24 07:01:14 2019
New Revision: 372761

URL: http://llvm.org/viewvc/llvm-project?rev=372761&view=rev
Log:
[Compiler] Fix LLVM_NODISCARD for GCC

Summary:
This branch is currently dead since we don't use C++17.
 #if __cplusplus > 201402L && LLVM_HAS_CPP_ATTRIBUTE(nodiscard)
 #define LLVM_NODISCARD [[nodiscard]]

This branch is Clang-only.
 #elif LLVM_HAS_CPP_ATTRIBUTE(clang::warn_unused_result)
 #define LLVM_NODISCARD [[clang::warn_unused_result]]


While we could use gnu variant  [[gnu::warn_unused_result]], it is not ideal because it works only on functions.
/home/xbolva00/LLVM/llvm/include/llvm/ADT/ArrayRef.h:41:24: warning: ‘warn_unused_result’ attribute only applies to function types [-Wattributes]

GCC (checked 5,6,7,8) seems to enable [[nodiscard]] even in C++14 mode and does not produce warnings that nodiscard is C++17 feature. but Clang does - but we do not reach it due the code above. So it affects only GCC and does what we want.

Reviewers: jfb, rsmith, echristo, aaron.ballman

Reviewed By: aaron.ballman

Subscribers: MaskRay, dexonsmith

Differential Revision: https://reviews.llvm.org/D67663

Modified:
    llvm/trunk/include/llvm/Support/Compiler.h

Modified: llvm/trunk/include/llvm/Support/Compiler.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Support/Compiler.h?rev=372761&r1=372760&r2=372761&view=diff
==============================================================================
--- llvm/trunk/include/llvm/Support/Compiler.h (original)
+++ llvm/trunk/include/llvm/Support/Compiler.h Tue Sep 24 07:01:14 2019
@@ -137,10 +137,18 @@
 #endif
 
 /// LLVM_NODISCARD - Warn if a type or return value is discarded.
+
+// Use the 'nodiscard' attribute in C++17 or newer mode.
 #if __cplusplus > 201402L && LLVM_HAS_CPP_ATTRIBUTE(nodiscard)
 #define LLVM_NODISCARD [[nodiscard]]
 #elif LLVM_HAS_CPP_ATTRIBUTE(clang::warn_unused_result)
 #define LLVM_NODISCARD [[clang::warn_unused_result]]
+// Clang in C++14 mode claims that it has the 'nodiscard' attribute, but also
+// warns in the pedantic mode that 'nodiscard' is a C++17 extension (PR33518).
+// Use the 'nodiscard' attribute in C++14 mode only with GCC.
+// TODO: remove this workaround when PR33518 is resolved.
+#elif defined(__GNUC__) && LLVM_HAS_CPP_ATTRIBUTE(nodiscard)
+#define LLVM_NODISCARD [[nodiscard]]
 #else
 #define LLVM_NODISCARD
 #endif