[llvm-bugs] [Bug 48173] New: Increment incorrectly allowed for bool in requires-expressions

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=48173

            Bug ID: 48173
           Summary: Increment incorrectly allowed for bool in
                    requires-expressions
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++2a
          Assignee: unassignedclangbugs at nondot.org
          Reporter: Casey at Carter.net
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

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.

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