[Bug c++/110570] New: Error reading mutable subobject in constexpr when object lifetime began within the evaluation of E

[Bug c++/110570] New: Error reading mutable subobject in constexpr when object lifetime began within the evaluation of E

[fchelnokov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110570

            Bug ID: 110570
           Summary: Error reading mutable subobject in constexpr when
                    object lifetime began within the evaluation of E
           Product: gcc
           Version: 13.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fchelnokov at gmail dot com
  Target Milestone: ---

This program

struct S {
    mutable int i = 2;
};

constexpr auto f = []{
    constexpr S s;
    s.i = 3;
    return s.i;
};

static_assert( f() == 3 );

is accepted in Clang, MSVC and ICC.

But GCC rejects the code with the error

  in 'constexpr' expansion of 'f.<lambda()>()'
error: mutable 'S::i' is not usable in a constant expression

Online demo: https://gcc.godbolt.org/z/Wsfeah3hq

It seems wrong, since lifetime of object s begins within the evaluation of
constant expression, see
https://timsong-cpp.github.io/cppwp/n4861/expr.const#5.16

Clang developers think that GCC is wrong here:
https://github.com/llvm/llvm-project/issues/63695

SO question: https://stackoverflow.com/q/76608087/7325599