https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95066
Bug ID: 95066
Summary: [C++ 20] Incorrect valid compilation with a
conditional explicit
Product: gcc
Version: 9.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: ojman101 at protonmail dot com
Target Milestone: ---
The code below is invalid C++, the line "Foo<int> b = a;" should fail to
compile as implicitly casting is made illegal by the conditional explicit using
the "IsSafelyCastable" predicate.
----------------------------------------------------------------
#include <type_traits>
template <typename, typename>
class IsSafelyCastable : public std::false_type {};
template <>
class IsSafelyCastable<int, float> : public std::true_type {};
template <typename T>
struct Foo {
template <typename U>
explicit(!IsSafelyCastable<T, U>::value) operator Foo<U>();
};
template <typename T>
template <typename U>
Foo<T>::operator Foo<U>() {
return {};
}
int main() {
Foo<float> a;
Foo<int> b = a;
}
----------------------------------------------------------------
Clang 10 correctly evaluates the explicit condition to be true and blocks the
implicit cast. However, GCC 9.3.0 successfully compiles without any errors. I
believe this to be a GCC bug as subtle changes can make GCC produce the correct
error. For example, moving the definition of the function to be inline with the
declaration.
