Restrict construction of `enum class` values manifestly outside intended usage

Consider

enum class TestEnum : uint32_t { One, Two, Three };

int main() {
   TestEnum t { 17 };       // L1
}

Prior to C++17, line L1 was invalid and diagnosed at compile-time. The proposal P0138R0, P0138R1 for C++17 also made that ill-formed. However, the adopted revision P0138R2 -- at the insistence of CWG -- allowed it. That allowance has proven to easy source of mistakes in practice, in more realistic situations such as

enum class AccessType : uint32_t { Read, Write, Execute, };

where no other values of AccessType is expected or tolerated.

The suggestion here is to have a rule that bans the construction of enum class values that are manifestly outside the set expressly indicated in the definition of the enum class. For example,

enum class Foo : int { 
    Apple = 0,
    Banana = 1,
    Cookie = 2,
};

// Allowed
Foo foo { 0 };
Foo foo1 { 1 };
Foo foo2 { 2 };

// Not allowed
Foo foo3 { 3 };

// Allowed
Foo foo { static_cast<Foo>(3) };