[all-commits] [llvm/llvm-project] aa0884: [libc++] Avoid -Wzero-as-null-pointer-constant in ...

[Louis Dionne via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: aa088438784dd76a859eee229ddaec17e0cb0651
      https://github.com/llvm/llvm-project/commit/aa088438784dd76a859eee229ddaec17e0cb0651
  Author: Louis Dionne <ldionne.2 at gmail.com>
  Date:   2024-08-21 (Wed, 21 Aug 2024)

  Changed paths:
    M libcxx/include/__compare/ordering.h
    A libcxx/test/std/language.support/cmp/cmp.categories.pre/reject-other-than-literal-zero.verify.cpp
    R libcxx/test/std/language.support/cmp/cmp.categories.pre/zero_type.verify.cpp

  Log Message:
  -----------
  [libc++] Avoid -Wzero-as-null-pointer-constant in operator<=> (#79465)

Issue #43670 describes a situation where the following comparison will
issue a warning when -Wzero-as-null-pointer-constant is enabled:

    #include <compare>
    auto b = (1 <=> 2) < 0;

This code uses operator<(strong_ordering, Unspecified), which is
specified by the Standard to only work with a literal 0. In the library,
this is achieved by constructing Unspecified from a pointer, which works
but has the downside of triggering the warning.

This patch uses an alternative implementation where we require that the
operator is used exactly with an int of value 0 (known at compile-time),
however that value can technically be an expression like `1 - 1`, which
makes us a bit less strict than what's specified in the Standard.

Fixes #43670



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