Increment incorrectly allowed for bool in unevaluated contexts

[CaseyCarter]

Bugzilla Link	48173
Version	trunk
OS	All
CC	@Quuxplusone,@zygoloid

Extended Description

Compiling this well-formed program:

template <class T>
concept can_increment = requires(T t) {
    ++t;
};

template <class T>
void f() {
    static_assert(requires(T t) { ++t; }); // Incorrectly allowed
}

int main() {
    f<bool>();

    static_assert(!can_increment<bool>); // Incorrectly fails

    bool b = false;
    ++b; // Correctly rejected
}

with "clang++ -std=c++2a" (https://godbolt.org/z/39xM1E) diagnoses:

<source>:14:5: error: static_assert failed due to requirement '!can_increment<bool>'
    static_assert(!can_increment<bool>); // Incorrectly fails
    ^             ~~~~~~~~~~~~~~~~~~~~
<source>:17:5: error: ISO C++17 does not allow incrementing expression of type bool [-Wincrement-bool]
    ++b;
    ^ ~

The compiler knows that the language forbids increment of bools outside a requires-expression, but apparently not within a requires-expression.

[Quuxplusone]

Relatedly, the ill-formedness of "++bool" is not SFINAE-friendly.

https://godbolt.org/z/on6rhE6nr

template<class T> auto f(T t) -> decltype(++t);
auto f(...) -> void;
void g() {
    f(true);  // Clang wrongly makes this a hard error
}

[llvmbot]

@llvm/issue-subscribers-c-17

[yronglin]

Fixed in https://reviews.llvm.org/D152259