[Bug c++/102262] New: No reason given for constexpr function that uses non-constexpr destructor

[Bug c++/102262] New: No reason given for constexpr function that uses non-constexpr destructor

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102262

            Bug ID: 102262
           Summary: No reason given for constexpr function that uses
                    non-constexpr destructor
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

template<typename T>
struct foo
{
  constexpr foo() { }
  ~foo() { }
};

template<typename T>
constexpr bool check()
{
  T t;
  return true;
}

static_assert( check<foo<int>>() );


The error doesn't tell you what's wrong:

ce.C:15:31: error: non-constant condition for static assertion
   15 | static_assert( check<foo<int>>() );
      |                ~~~~~~~~~~~~~~~^~
ce.C:15:31: error: 'constexpr bool check() [with T = foo<int>]' called in a
constant expression
ce.C:9:16: note: 'constexpr bool check() [with T = foo<int>]' is not usable as
a 'constexpr' function because:
    9 | constexpr bool check()
      |                ^~~~~


The problem is that the destructor is not constexpr.

[Bug c++/102262] No reason given for constexpr function that uses non-constexpr destructor

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102262

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2021-09-09
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.  Interesting only MSVC provides the reason:

<source>(15): error C2131: expression did not evaluate to a constant
<source>(11): note: failure was caused by call of undefined function or one not
declared 'constexpr'
<source>(11): note: see usage of 'foo<int>::~foo'


clang does not either:
<source>:15:16: error: static_assert expression is not an integral constant
expression
static_assert( check<foo<int>>() );
               ^~~~~~~~~~~~~~~~~
<source>:11:5: note: non-literal type 'foo<int>' cannot be used in a constant
expression
  T t;
    ^
<source>:15:16: note: in call to 'check()'
static_assert( check<foo<int>>() );
               ^

Nor EDG (ICC):
<source>(15): error: expression must have a constant value
  static_assert( check<foo<int>>() );
                 ^
<source>(15): note: cannot call non-constexpr function "check<T>() [with
T=foo<int>]" (declared at line 9)
  static_assert( check<foo<int>>() );
                 ^

[Bug c++/102262] No reason given for constexpr function that uses non-constexpr destructor

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102262

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Simpler testcase, the class doesn't need to be a template, but check does:
struct S
{
  constexpr S () { }
  ~S () { }
};

template <typename T>
constexpr bool check ()
{
  T t;
  return true;
}

constexpr bool a = check<S> ();

constexpr bool check2 ()
{
  S s;
  return true;
}

constexpr bool b = check2 ();

because check2 is diagnosed properly:
pr102262.C: In function ‘constexpr bool check2()’:
pr102262.C:18:5: error: variable ‘s’ of non-literal type ‘S’ in ‘constexpr’
function
   18 |   S s;
      |     ^
pr102262.C:1:8: note: ‘S’ is not literal because:
    1 | struct S
      |        ^
pr102262.C:1:8: note:   ‘S’ does not have ‘constexpr’ destructor

[Bug c++/102262] No reason given for constexpr function that uses non-constexpr destructor

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102262

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at gcc dot gnu.org

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Seems ensure_literal_type_for_constexpr_object doesn't emit any errors in
instatiations of constexpr functions:
      if (CLASS_TYPE_P (stype) && !COMPLETE_TYPE_P (complete_type (stype)))
        /* Don't complain here, we'll complain about incompleteness
           when we try to initialize the variable.  */;
      else if (!literal_type_p (type))
        {
          if (DECL_DECLARED_CONSTEXPR_P (decl))
...
          else
            {
              if (!is_instantiation_of_constexpr (current_function_decl))
                {
                  auto_diagnostic_group d;
                  error_at (DECL_SOURCE_LOCATION (decl),
                            "variable %qD of non-literal type %qT in "
                            "%<constexpr%> function", decl, type);
                  explain_non_literal_class (type);
                  decl = error_mark_node;
                }
              cp_function_chain->invalid_constexpr = true;
            }

When parsing, this isn't reached because of && !processing_template_decl, and
when it is instantiated, is_instantiation_of_constexpr (current_function_decl)
is true and so it doesn't emit the error, but just sets invalid_constexpr.

[Bug c++/102262] No reason given for constexpr function that uses non-constexpr destructor

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102262

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Another variation:

template<typename _Tp>
struct vector
{
      struct _Temp_value
      {
          constexpr
          _Temp_value()
          {
          }

#ifndef ICE
        constexpr
        ~_Temp_value()
        { }
#endif

        union _Storage
        {
          constexpr _Storage() : _M_byte() { }
          ~_Storage() { }
          unsigned char _M_byte;
        };

        _Storage _M_storage;
      };

    constexpr void
    insert()
    {
      _Temp_value v;
    }
};

constexpr bool
test()
{
  vector<int> v;
  v.insert();

  return true;
}

static_assert( test() );


With -std=c++20 this prints:

ce.C:43:20: error: non-constant condition for static assertion
   43 | static_assert( test() );
      |                ~~~~^~
ce.C:43:20:   in 'constexpr' expansion of 'test()'
ce.C:38:11: error: 'constexpr void vector<_Tp>::insert() [with _Tp = int]'
called in a constant expression
   38 |   v.insert();
      |   ~~~~~~~~^~
ce.C:28:5: note: 'constexpr void vector<_Tp>::insert() [with _Tp = int]' is not
usable as a 'constexpr' function because:
   28 |     insert()
      |     ^~~~~~


But with -std=c++20 -DICE it ICEs:

ce.C:43:20: error: non-constant condition for static assertion
   43 | static_assert( test() );
      |                ~~~~^~
ce.C:43:20:   in ‘constexpr’ expansion of ‘test()’
ce.C:38:11: error: ‘constexpr void vector<_Tp>::insert() [with _Tp = int]’
called in a constant expression
   38 |   v.insert();
      |   ~~~~~~~~^~
ce.C:28:5: note: ‘constexpr void vector<_Tp>::insert() [with _Tp = int]’ is not
usable as a ‘constexpr’ function because:
   28 |     insert()
      |     ^~~~~~
ce.C:31:5: error: call to non-‘constexpr’ function
‘vector<int>::_Temp_value::~_Temp_value()’
   31 |     }
      |     ^
ce.C:4:14: note: ‘vector<int>::_Temp_value::~_Temp_value()’ is not usable as a
‘constexpr’ function because:
    4 |       struct _Temp_value
      |              ^~~~~~~~~~~
ce.C:4:14: internal compiler error: in synthesized_method_walk, at
cp/method.c:2531
0x6b9809 synthesized_method_walk
        /home/jwakely/src/gcc/gcc/gcc/cp/method.c:2531
0xa3f243 explain_implicit_non_constexpr(tree_node*)
        /home/jwakely/src/gcc/gcc/gcc/cp/method.c:2887
0x977a17 explain_invalid_constexpr_fn(tree_node*)
        /home/jwakely/src/gcc/gcc/gcc/cp/constexpr.c:976
0x9767f7 potential_constant_expression_1
        /home/jwakely/src/gcc/gcc/gcc/cp/constexpr.c:8220
0x9758a1 potential_constant_expression_1
        /home/jwakely/src/gcc/gcc/gcc/cp/constexpr.c:8445
0x976d38 potential_constant_expression_1(tree_node*, bool, bool, bool, int)
        /home/jwakely/src/gcc/gcc/gcc/cp/constexpr.c:9126
0x976d38 require_potential_rvalue_constant_expression(tree_node*)
        /home/jwakely/src/gcc/gcc/gcc/cp/constexpr.c:9160
0x977638 explain_invalid_constexpr_fn(tree_node*)
        /home/jwakely/src/gcc/gcc/gcc/cp/constexpr.c:984
0x96d8aa cxx_eval_call_expression
        /home/jwakely/src/gcc/gcc/gcc/cp/constexpr.c:2629
0x96f406 cxx_eval_constant_expression
        /home/jwakely/src/gcc/gcc/gcc/cp/constexpr.c:6344
0x96f362 cxx_eval_constant_expression
        /home/jwakely/src/gcc/gcc/gcc/cp/constexpr.c:6674
0x96f1c8 cxx_eval_constant_expression
        /home/jwakely/src/gcc/gcc/gcc/cp/constexpr.c:6542
0x97054a cxx_eval_statement_list
        /home/jwakely/src/gcc/gcc/gcc/cp/constexpr.c:5881
0x97054a cxx_eval_constant_expression
        /home/jwakely/src/gcc/gcc/gcc/cp/constexpr.c:7013
0x96f3e8 cxx_eval_constant_expression
        /home/jwakely/src/gcc/gcc/gcc/cp/constexpr.c:7048
0x96cae0 cxx_eval_call_expression
        /home/jwakely/src/gcc/gcc/gcc/cp/constexpr.c:2789
0x96f406 cxx_eval_constant_expression
        /home/jwakely/src/gcc/gcc/gcc/cp/constexpr.c:6344
0x973296 cxx_eval_outermost_constant_expr
        /home/jwakely/src/gcc/gcc/gcc/cp/constexpr.c:7409
0xb5bb39 finish_static_assert(tree_node*, tree_node*, unsigned int, bool, bool)
        /home/jwakely/src/gcc/gcc/gcc/cp/semantics.c:10457
0xaa1b5d cp_parser_static_assert
        /home/jwakely/src/gcc/gcc/gcc/cp/parser.c:16166
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