[BUG] Unable to use enums in inspect expressions - `no matching function for call to 'is'`

[filipsajdak]

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The current implementation is not supporting enums in inspect expressions.

This cpp2 code

enum class lexeme : std::uint8_t {
    hash,
};

to_string: (e:lexeme) -> auto = {
    return inspect (e) -> std::string {
        is lexeme::hash = "hash";
        is _ = "INTERNAL_ERROR";
    };
}

compiles by cppfront to (skipping enum declaration):

[[nodiscard]] auto to_string(cpp2::in<lexeme> e) -> auto{
    return [&] () -> std::string { auto&& __expr = (e);
        if (cpp2::is<lexeme::hash>(__expr)) { if constexpr( requires{"hash";} ) if constexpr( std::is_convertible_v<CPP2_TYPEOF("hash"),std::string> ) return "hash"; else return std::string{}; else return std::string{}; }
        else return "INTERNAL_ERROR"; }
    ()
; }

and fails to compile with the cpp1 compiler with the error:

main.cpp2:26:13: error: no matching function for call to 'is'
[build]         if (cpp2::is<lexeme::hash>(__expr)) { if constexpr( requires{"hash";} ) if constexpr( std::is_convertible_v<CPP2_TYPEOF("hash"),std::string> ) return "hash"; else return std::string{}; else return std::string{}; }
[build]             ^~~~~~~~~~~~~~~~~~~~~~
[build] /Users/filipsajdak/dev/execspec/external/cppfront/include/cpp2util.h:517:6: note: candidate template ignored: invalid explicitly-specified argument for template parameter 'C'

The issue is caused that enums cannot match to any provided is function overloads. To handle that we need special overload for enums.