[Bug c++/102921] New: error: modification of '<temporary>' is not a constant expression

[Bug c++/102921] New: error: modification of '<temporary>' is not a constant expression

[f.heckenbach@fh-soft.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102921

            Bug ID: 102921
           Summary: error: modification of '<temporary>' is not a constant
                    expression
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: f.heckenbach@fh-soft.de
  Target Milestone: ---

[Bug c++/102921] error: modification of '<temporary>' is not a constant expression

[f.heckenbach@fh-soft.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102921

--- Comment #1 from Frank Heckenbach <f.heckenbach@fh-soft.de> ---
The following program, compiled with "-std=c++20" gives this error message; I
don't even understand what it's trying to tell me:

  error: modification of '<temporary>' is not a constant expression

#include <initializer_list>
#include <string_view>

constexpr std::initializer_list <std::string_view> a { "" };

Other compilers seem to have no problem with it.

Digging through the headers, I could trace it (via char_traits and
__constant_string_p) to __builtin_is_constant_evaluated. The following version
gives the same error (but I'm still not sure if the problem is in the compiler
or the headers):

#include <initializer_list>
#include <type_traits>

struct S
{
  constexpr S () noexcept { }
  int a = std::is_constant_evaluated ();
};

constexpr std::initializer_list <S> a { { } };

[Bug c++/102921] error: modification of '<temporary>' is not a constant expression

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102921

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2021-10-25
                 CC|                            |jakub at gcc dot gnu.org,
                   |                            |jason at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Testcase without headers:
namespace std
{
  typedef decltype (sizeof 0) size_t;
  template<class T>
  class initializer_list
  {
    const T *_M_array;
    size_t _M_len;
    constexpr initializer_list (const T &a, size_t l) : _M_array(a), _M_len(l)
{ }
  };
}

struct S
{
  constexpr S () noexcept { }
  int a = __builtin_is_constant_evaluated ();
};

constexpr std::initializer_list <S> a { { } };

The <temporary> is _ZGRL1a_[0] demangled reference temporary #0 for a, and the
expression that is being constant evaluated is (without cleanup points and
expr_stmt):
TARGET_EXPR <D.2197, {._M_array= (_ZGRL1a_[0] = TARGET_EXPR <D.2186, S::S
(&D.2186)>;, (const struct S *) &_ZGRL1a_;, ._M_len=1}>;
No idea how to fix that though...
Doesn't seem to be a regression, behaved this way since 
r9-2311-ge408261123697a82b5965c700fa2465999f0fd62 when
__builtin_is_constant_evaluated has been introduced.
Without the builtin, bet the ctor is folded into a CONSTRUCTOR already when
performing non-manifestly constant evaluation before.

The INIT_EXPR that stores into _ZGRL1a_[0] is created by
split_nonconstant_init_1 called indirectly from set_up_extended_ref_temp, so at
that point the initializer clearly didn't go through manifestly constant
evaluation.  Should that be done somewhere if set_up_extended_ref_temp is
called for a constexpr variable?

[Bug c++/102921] error: modification of '<temporary>' is not a constant expression

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102921

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
--- gcc/cp/call.c.jj    2021-10-21 16:06:42.231940456 +0200
+++ gcc/cp/call.c       2021-10-25 10:20:00.970635564 +0200
@@ -12735,7 +12735,9 @@ set_up_extended_ref_temp (tree decl, tre

   /* If the initializer is constant, put it in DECL_INITIAL so we get
      static initialization and use in constant expressions.  */
-  init = maybe_constant_init (expr);
+  init = maybe_constant_init (expr, NULL_TREE,
+                             VAR_P (decl)
+                             && DECL_DECLARED_CONSTEXPR_P (decl));
   /* As in store_init_value.  */
   init = cp_fully_fold (init);
   if (TREE_CONSTANT (init))
makes this accepted, but not the PR101588 testcase.

[Bug c++/102921] error: modification of '<temporary>' is not a constant expression

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102921

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED
           Keywords|                            |rejects-valid
                 CC|                            |ppalka at gcc dot gnu.org
   Target Milestone|---                         |12.3

--- Comment #4 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Looks like this is fixed for GCC 12.3+ by the fix for PR107079