https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98824
Bug ID: 98824
Summary: [C++-20] function template non-type-class-arg
deduction fails with a reason that looks bogus
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: dimitri.gorokhovik at free dot fr
Target Milestone: ---
The code:
template <auto> struct a {};
template <int i, a <i> v> constexpr auto f (a <v>) { return true; };
constexpr a <1> b;
auto const p = f (a <b> {});
when compiled with 'g++ -std=c++20 -c bug-14.cpp' (g++ (GCC) 11.0.0 20210123
(experimental)), produces:
bug-14.cpp:5:27: error: no matching function for call to ‘f(a<a<1>()>)’
5 | auto const p = f (a <b> {});
| ^
bug-14.cpp:2:42: note: candidate: ‘template<int i, a<i> v> constexpr auto
f(a<((const a<i>)v)>)’
2 | template <int i, a <i> v> constexpr auto f (a <v>) { return true; };
| ^
bug-14.cpp:2:42: note: template argument deduction/substitution failed:
bug-14.cpp:5:27: note: types ‘a<i>’ and ‘const a<1>’ have incompatible
cv-qualifiers
5 | auto const p = f (a <b> {});
clang-12 seems to accept it (hits a non-implemented feature in a subsequent
pass).
The diagnostics looks confusing/misleading:
-- compiler is aware of the constness of 'a <i>' -- see the signature of f() in
the note for line 2. And then it seemingly removes 'const' from 'const a <i>',
maybe expecting a certain strictness level in 'unify()'. However, unify() is
called with strict=UNIFY_ALLOW_NONE.
