[llvm-bugs] [Bug 51582] New: /permissive- and Implicit conversion of integral constant expressions to null pointer.

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=51582

            Bug ID: 51582
           Summary: /permissive- and Implicit conversion of integral
                    constant expressions to null pointer.
           Product: clang
           Version: 12.0
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++17
          Assignee: unassignedclangbugs at nondot.org
          Reporter: tormardi at gmail.com
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

Created attachment 25182
  --> https://bugs.llvm.org/attachment.cgi?id=25182&action=edit
Bad overload chosen, -Wnon-literal-null-conversion

From

https://docs.microsoft.com/en-us/cpp/overview/cpp-conformance-improvements?view=msvc-160&viewFallbackFrom=vs-2019#implicit\
\-conversion-of-integral-constant-expressions-to-null-pointer

"The MSVC compiler now implements CWG Issue 903 in conformance mode
 (/permissive-). This rule disallows implicit conversion of integral
 constant expressions (except for the integer literal '0') to null
 pointer constants."

The attached snippet, built with clang 12.0.1 for windows outputs
  CTOR two
  CTOR three

And so does Visual Studio with "/permissive"
With "/permissive-" Visual Studio gets it right.

clang says:
warning: expression which evaluates to zero treated as a null pointer
constant of type 'int *const'
[-Wnon-literal-null-conversion]

If I remove 'constexpr' the warning goes away, and we get correct behaviour.

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