[PATCH] D129835: [clang] adds a discardable attribute

[Christopher Di Bella via Phabricator via cfe-commits]

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Although the default behaviour for C and C++ is to not diagnose ignored function
calls, we can make this the default by using `#pragma clang attribute
push([[nodiscard]],  apply_to = function)`. When we have a function that can
have its value discarded, we can use `[[clang::discardable]]` to indicate that
the ``[[nodiscard]]`` attribute should be ignored.

`[[clang::discardable]]` can be placed anywhere `[[nodiscard]]` is allowed,
but the presence of either is prioritised when it's applied to a callable.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D129835

Files:
  clang/docs/ReleaseNotes.rst
  clang/include/clang/AST/Expr.h
  clang/include/clang/Basic/Attr.td
  clang/include/clang/Basic/AttrDocs.td
  clang/lib/AST/Expr.cpp
  clang/lib/Sema/SemaDeclAttr.cpp
  clang/lib/Sema/SemaStmt.cpp
  clang/test/Misc/pragma-attribute-supported-attributes-list.test
  clang/test/SemaCXX/discardable.cpp

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