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
