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From: "gabravier at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/102623] New: Failure to detect destructed scalar objects in
 consteval function
Date: Tue, 05 Oct 2021 19:41:02 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102623

            Bug ID: 102623
           Summary: Failure to detect destructed scalar objects in
                    consteval function
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gabravier at gmail dot com
  Target Milestone: ---

using T =3D int;
consteval bool f()
{
  T t =3D 42;
  t.~T();
  return (t =3D=3D 42);
}

bool x =3D f();

This code should not compile, as `f` invokes undefined behavior during cons=
tant
evaluation, as Clang diagnoses:

<source>:9:10: error: call to consteval function 'f' is not a constant
expression
bool x =3D f();
         ^
<source>:6:11: note: read of object outside its lifetime is not allowed in a
constant expression
  return (t =3D=3D 42);
          ^

The specific usage with `int` here was not UB until C++20 (although
pseudo-destructors weren't allowed in constant expressions until C++20 anyw=
ay),
but as of
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p0593r6.html#pseudo=
-destructor-calls
pseudo-destructors end the lifetime of the operand.=