[Bug c++/115296] New: CTAD fails

[Bug c++/115296] New: CTAD fails

[tiagomacarios at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115296

            Bug ID: 115296
           Summary: CTAD fails
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tiagomacarios at gmail dot com
  Target Milestone: ---

The following code compiles fine with clang and msvc, but fails with gcc.

https://godbolt.org/z/e4dP7rzar
```
#include <span>

template <typename T>
using array_view = std::span<T>;

void fun(std::span<int> s) {
    [[maybe_unused]] auto _1 = std::span{s};
    [[maybe_unused]] auto _2 = array_view{s};
}
```

Error:
```
In file included from <source>:1:
/opt/compiler-explorer/gcc-trunk-20240530/include/c++/15.0.0/span: In function
'void fun(std::span<int>)':
/opt/compiler-explorer/gcc-trunk-20240530/include/c++/15.0.0/span:450:5: error:
size '18446744073709551615' of array exceeds maximum object size
'9223372036854775807'
  450 |     span(_Type(&)[_ArrayExtent]) -> span<_Type, _ArrayExtent>;
      |     ^~~~
/opt/compiler-explorer/gcc-trunk-20240530/include/c++/15.0.0/span:450:10:
error: size '18446744073709551615' of array exceeds maximum object size
'9223372036854775807'
  450 |     span(_Type(&)[_ArrayExtent]) -> span<_Type, _ArrayExtent>;
      |          ^~~~~~~~~~~~~~~~~~~~~~
<source>:8:44: error: class template argument deduction failed:
    8 |     [[maybe_unused]] auto _2 = array_view{s};
      |                                            ^
<source>:8:44: error: call of overloaded 'span(std::span<int>&)' is ambiguous
/opt/compiler-explorer/gcc-trunk-20240530/include/c++/15.0.0/span:464:5: note:
candidate: 'std::span(_Range&&)-> span<typename
remove_reference<ranges::range_reference_t<_Range&> >::type> requires 
__is_deducible (array_view, std::span<typename
std::remove_reference<decltype(*(declval<decltype(std::ranges::__access::__begin((declval<_Container&>)()))&>)())>::type,
18446744073709551615>) [with _Range = span<int>&; typename
remove_reference<ranges::range_reference_t<_Range&> >::type = int;
ranges::range_reference_t<_Range&> = int&]'
  464 |     span(_Range &&)
      |     ^~~~
/opt/compiler-explorer/gcc-trunk-20240530/include/c++/15.0.0/span:54:11: note:
candidate: 'span(std::span<_Type, 18446744073709551615>)-> std::span<_Type,
18446744073709551615> [with T = int]'
   54 |     class span;
      |           ^~~~
/opt/compiler-explorer/gcc-trunk-20240530/include/c++/15.0.0/span:225:7: note:
candidate: 'span(const std::span<_Type, 18446744073709551615>&)->
std::span<_Type, 18446744073709551615> [with T = int]'
  225 |       span(const span&) noexcept = default;
      |       ^~~~
Compiler returned: 1
```

[Bug c++/115296] CTAD fails

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115296

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
                 CC|                            |mpolacek at gcc dot gnu.org
   Last reconfirmed|                            |2024-05-31
     Ever confirmed|0                           |1

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Doesn't look like a regression.

#0  error_at (loc=74842051, gmsgid=0x38de910 "size %qE of array exceeds maximum
object size %qE")
    at /home/mpolacek/src/gcc/gcc/diagnostic.cc:2137
#1  0x000000000129b72e in invalid_array_size_error (loc=74842051,
error=cst_size_too_big, 
    size=<integer_cst 0x7fffea203680>, name=<tree 0x0>)
    at /home/mpolacek/src/gcc/gcc/c-family/c-common.cc:9212
#2  0x000000000129b8e2 in valid_array_size_p (loc=74842051, t=<integer_cst
0x7fffea203680>, name=<tree 0x0>, 
    complain=true) at /home/mpolacek/src/gcc/gcc/c-family/c-common.cc:9263
#3  0x0000000000ee5c74 in compute_array_index_type_loc (name_loc=74842051,
name=<tree 0x0>, 
    size=<integer_cst 0x7fffea203680>, complain=3) at
/home/mpolacek/src/gcc/gcc/cp/decl.cc:11653
#4  0x0000000000ee61cd in compute_array_index_type (name=<tree 0x0>,
size=<integer_cst 0x7fffea203680>, 
    complain=3) at /home/mpolacek/src/gcc/gcc/cp/decl.cc:11778
#5  0x000000000111fc4b in tsubst (t=<integer_type 0x7fffe941c2a0>,
args=<tree_vec 0x7fffe910c5f0>, 
    complain=3, in_decl=<function_decl 0x7fffe923ce00 __dguide_span>)
    at /home/mpolacek/src/gcc/gcc/cp/pt.cc:16317
#6  0x00000000011217bf in tsubst (t=<array_type 0x7fffe941c348>, args=<tree_vec
0x7fffe910c5f0>, complain=3, 
    in_decl=<function_decl 0x7fffe923ce00 __dguide_span>) at
/home/mpolacek/src/gcc/gcc/cp/pt.cc:16742
#7  0x000000000111f4a7 in tsubst (t=<reference_type 0x7fffe941c3f0>,
args=<tree_vec 0x7fffe910c5f0>, 
    complain=3, in_decl=<function_decl 0x7fffe923ce00 __dguide_span>)
    at /home/mpolacek/src/gcc/gcc/cp/pt.cc:16239
#8  0x000000000111d85f in tsubst_arg_types (arg_types=<tree_list
0x7fffe9799d48>, 
    args=<tree_vec 0x7fffe910c5f0>, end=<tree 0x0>, complain=3, 
    in_decl=<function_decl 0x7fffe923ce00 __dguide_span>) at
/home/mpolacek/src/gcc/gcc/cp/pt.cc:15749
#9  0x000000000111dc2d in tsubst_function_type (t=<function_type
0x7fffe941c888>, 
    args=<tree_vec 0x7fffe910c5f0>, complain=3, in_decl=<function_decl
0x7fffe923ce00 __dguide_span>)
    at /home/mpolacek/src/gcc/gcc/cp/pt.cc:15854
#10 0x00000000011216de in tsubst (t=<function_type 0x7fffe941c888>,
args=<tree_vec 0x7fffe910c5f0>, 
    complain=3, in_decl=<function_decl 0x7fffe923ce00 __dguide_span>)
    at /home/mpolacek/src/gcc/gcc/cp/pt.cc:16727
#11 0x0000000001112526 in tsubst_function_decl (t=<function_decl 0x7fffe923ce00
__dguide_span>, 
    args=<tree_vec 0x7fffe910c5f0>, complain=3, lambda_fntype=<tree 0x0>,
use_spec_table=false)
    at /home/mpolacek/src/gcc/gcc/cp/pt.cc:14560
#12 0x000000000111a255 in tsubst_decl (t=<function_decl 0x7fffe923ce00
__dguide_span>, 
    args=<tree_vec 0x7fffe910c5f0>, complain=3, use_spec_table=false)
    at /home/mpolacek/src/gcc/gcc/cp/pt.cc:15131
#13 0x0000000001165243 in alias_ctad_tweaks (tmpl=<template_decl 0x7fffe941ec00
array_view>, 
    uguides=<overload 0x7fffe9a571a0>) at
/home/mpolacek/src/gcc/gcc/cp/pt.cc:30380
#14 0x0000000001166b4d in deduction_guides_for (tmpl=<template_decl
0x7fffe941ec00 array_view>, 
    any_dguides_p=@0x7fffffffb7ef: true, complain=3) at
/home/mpolacek/src/gcc/gcc/cp/pt.cc:30714
#15 0x0000000001167a17 in do_class_deduction (ptype=<template_type_parm
0x7fffe91099d8 auto>, 
    tmpl=<template_decl 0x7fffe941ec00 array_view>, init=<constructor
0x7fffe910f000>, 
    outer_targs=<tree 0x0>, flags=1, complain=3) at
/home/mpolacek/src/gcc/gcc/cp/pt.cc:30878
#16 0x0000000001168d77 in do_auto_deduction (type=<template_type_parm
0x7fffe91099d8 auto>, 
    init=<constructor 0x7fffe910f000>, auto_node=<template_type_parm
0x7fffe91099d8 auto>, complain=3, 
    context=adc_variable_type, outer_targs=<tree 0x0>, flags=1, tmpl=<tree
0x0>)
    at /home/mpolacek/src/gcc/gcc/cp/pt.cc:31089