[Bug c++/67150] New: [c++-concepts] Expression constraint fails with dependent types

[Casey at Carter dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67150

            Bug ID: 67150
           Summary: [c++-concepts] Expression constraint fails with
                    dependent types
           Product: gcc
           Version: c++-concepts
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: Casey at Carter dot net
  Target Milestone: ---

r226725 fails to compile this program:

#include <type_traits>

template <class From, class To>
concept bool ExplicitlyConvertible =
  requires (From&& val) {
    requires std::is_same<From,bool>::value;
    requires std::is_same<To,bool>::value;
    requires std::is_same<decltype(val),bool&&>::value;
    static_cast<bool>((bool&&)val); // Line 9
    static_cast<To>((From&&)val);   // Line 10
  };

template <class T>
concept bool Boolean =
  requires (const T& t) {
    { t } -> ExplicitlyConvertible<bool>;
  };

constexpr bool f(Boolean) { return true; } // Line 19
static_assert(f(true));                    // Line 20

with error:

~/concept-gcc-r226725/bin/g++ -std=gnu++1z foo.cpp -c
foo.cpp:20:21: error: cannot call function ‘constexpr bool f(auto:1) [with
auto:1 = bool]’
 static_assert(f(true));                    // Line 20
                     ^
foo.cpp:19:16: note:   constraints not satisfied
 constexpr bool f(Boolean) { return true; } // Line 19
                ^
foo.cpp:19:16: note:   concept ‘Boolean<bool>’ was not satisfied

It does compile correctly if line 10 is commented out, demonstrating that the
expression "static_cast<To>((From&&)val)" is causing the constraint
dissatisfaction despite the fact that From is bool, To is bool, and
"static_cast<bool>((bool&&)val)" is acceptable.