[Bug c++/103333] New: [accepts-invalid] function template argument deduction for incompatible 'transformed A' / 'deduced A' pair

[davveston at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103333

            Bug ID: 103333
           Summary: [accepts-invalid] function template argument deduction
                    for incompatible 'transformed A' / 'deduced A' pair
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: davveston at gmail dot com
  Target Milestone: ---

The following program is accepted by GCC but rejected by Clang/MSVC
(-std=c++20):

---
#include <type_traits>

template<typename T, bool b>
struct S{
    S() = default;

    template<bool sfinae = true,
             typename = std::enable_if_t<sfinae && !std::is_const<T>::value>>
    operator S<T const, b>() { return S<T const, b>{}; }
};

template<typename T, bool b1, bool b2>
void f(S<const std::type_identity_t<T>, b1>,
                                 // ^- T in non-deduced context for func-param
#1 
       S<T, b2>)
      // ^- T deduced from here
{}                         

int main() {
    S<int, true> s1{};
    S<int, false> s2{};
    f(s1, s2);
}
---

The program is ill-formed as function template argument deduction
([temp.deduct.call]) for the first argument resolves in a mismatched deduced A
vs. transformed A that is S<const int, true> and S<int, true>, respectively,
and none of the three special cases of [temp.deduct.call]/4 allows this
mismatch.

[temp.arg.explicit]/7 allows a function argument to be converted to the type of
the corresponding function parameter, but only if the parameter type contains
to template arguments that participate in template argument deduction [from
that function argument]. This does not apply here, as the non-type template
parameter 'b1' participates in template argument deduction for the call.

---

https://timsong-cpp.github.io/cppwp/n4861/temp.deduct.call#4
https://timsong-cpp.github.io/cppwp/n4861/temp.arg.explicit#7