assert: Support assert as variadic macro for C++26 [PR27276]HEADmaster

C++26 changes assert into a variadic macro to support using
assignment-expressions that would be interpreted as multiple macro
arguments, in particular one containing:
* template parameter lists: func<int, float>()
* calls to overloaded operator[] that accepts multiple arguments: arr[1, 2]
  this is C++23 feature, see libstdc++ PR/119855 [1]
* lambdas with explicit captures: [x, y] { ... }

The new expansion in form:
  (__VA_ARGS__) ? void (1 ? 1 : bool (__VA_ARGS__))
                : __assert_fail (...)
Has the following properties:
* Use of (__VA_ARGS__) ? ... : ..., requires that __VA_ARGS__
  is contextually convertible to bool. This means that enumerators
  of scoped enumeration are no longer accepted (they are only
  explicitly convertible). Thus this patch address the glibc PR/27276 [2].
* Nested ternary 1 ? 1 : bool (__VA_ARGS__) guarantees that
  expression expanded from __VA_ARGS__ is not evaluated twice.
  This is used instead of unevaluated context (like sizeof...)
  to support C++ expressions that are not allowed in unevaluated
  context (lambdas until C++20, co_await, co_yield).
* bool (__VA_ARGS__) is ill-formed if __VA_ARGS__ expands to
  multiple arguments: assert(1, 2)
* bool (__VA_ARGS__) also triggers warnings when __VA_ARGS__
  expands to x = 1: assert(x = 1)

To guarantee that the code snippets from assert/test-assert-c++-variadic.cc,
are actually checked for validity, we need to compile this test in C++26
(-std=c++26) mode. To achieve that, this patch compiles the file with
test-config-cxxflags-stdcxx26 variable as additional flag, that is set to
-std=c++26 if $(TEST_CXX) executable supports that flag, and empty otherwise.

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119855
[2] https://sourceware.org/bugzilla/show_bug.cgi?id=27276

Co-authored-by: Tomasz Kamiński <tkaminsk@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>
Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

0 files changed, 0 insertions, 0 deletions