https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94819
Bug ID: 94819
Summary: [10 Regression] Inherited and constrained constructors
are "ambiguous" even if they aren't Pt. 2
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: gcc-bugs at marehr dot dialup.fu-berlin.de
Target Milestone: ---
Hi gcc-team,
a recent report of mine https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94549
fixed my reduced example, but my unreduced test case is still failing.
This is the second attempt at reducing my test case and this came out of it:
```c++
#include <utility>
template <typename T, typename U>
concept same_as = std::is_same_v<T, U>;
template <typename T, typename U>
concept convertible_to = std::is_convertible_v<T, U>;
template <typename... component_types>
struct alphabet_tuple_base
{
template <typename type>
static constexpr bool is_component = (same_as<type, component_types> ||
...);
template <typename type>
static constexpr bool is_indirect_component = !is_component<type> &&
(convertible_to<type, component_types> || ...);
// commenting out constexpr works?!
template <typename component_type>
requires is_component<component_type>
constexpr alphabet_tuple_base(component_type) {}
template <typename indirect_component_type>
requires is_indirect_component<indirect_component_type>
alphabet_tuple_base(indirect_component_type);
};
template <typename sequence_alphabet_t, typename structure_alphabet_t>
struct structured_rna : alphabet_tuple_base<sequence_alphabet_t,
structure_alphabet_t> {
using base_type = alphabet_tuple_base<sequence_alphabet_t,
structure_alphabet_t>;
using base_type::base_type;
};
struct dna4 {};
struct rna4 {
rna4(dna4);
};
struct wuss51 {};
// commenting out any of these works?!
structured_rna<rna4, wuss51> t1{wuss51{}};
structured_rna<rna4, wuss51> t2{dna4{}};
structured_rna<rna4, wuss51> t3{wuss51{}};
```
https://godbolt.org/z/WVvSxb
Note that `is_component` and `is_indirect_component` are mutual exclusive, so
there isn't any ambiguity even if they don't subsume each other.
This worked in gcc-9 with `-fconcepts` and apparently in msvc and clang, too.
```
> g++-git -std=c++2a
<source>:41:41: error: call of overloaded 'structured_rna(<brace-enclosed
initializer list>)' is ambiguous
41 | structured_rna<rna4, wuss51> t3{wuss51{}}; // commenting out any of
these works?!
| ^
<source>:20:15: note: candidate: 'constexpr
alphabet_tuple_base<component_types>::alphabet_tuple_base(component_type) [with
component_type = wuss51; component_types = {rna4, wuss51}]'
20 | constexpr alphabet_tuple_base(component_type) {} // commenting out
constexpr works?!
| ^~~~~~~~~~~~~~~~~~~
<source>:30:20: note: inherited here
30 | using base_type::base_type;
| ^~~~~~~~~
<source>:24:5: note: candidate:
'alphabet_tuple_base<component_types>::alphabet_tuple_base(indirect_component_type)
[with indirect_component_type = indirect_component_type; component_types =
{rna4, wuss51}]'
24 | alphabet_tuple_base(indirect_component_type);
| ^~~~~~~~~~~~~~~~~~~
<source>:30:20: note: inherited here
30 | using base_type::base_type;
| ^~~~~~~~~
<source>:28:8: note: candidate: 'constexpr structured_rna<rna4,
wuss51>::structured_rna(const structured_rna<rna4, wuss51>&)'
28 | struct structured_rna : alphabet_tuple_base<sequence_alphabet_t,
structure_alphabet_t> {
| ^~~~~~~~~~~~~~
<source>:28:8: note: candidate: 'constexpr structured_rna<rna4,
wuss51>::structured_rna(structured_rna<rna4, wuss51>&&)'
Compiler returned: 1
```
