Make isAutoDecodableString independent of issue 21570

[MoonlightSentinel]

The current implementation relies on issue 21570 to reject enums with
static arrays as their base type.

Use another is-expression instead of isStaticArray to detect types
that are (convertible to) static arrays.

See https://issues.dlang.org/show_bug.cgi?id=21570

---

Blocking dlang/dmd#12142

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✗	21570	normal	__traits(isStaticArray, ...) accepts enums with static array as base type

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#7760"

[PetarKirov]

I think this should work:

    (is(T : const char[]) || is(T : const wchar[])) && !is(T : U[n], size_t n);

[PetarKirov]

After some testing, I think this is the cleanest solution:

template isAutodecodableString(T)
{
    import std.range.primitives : autodecodeStrings;

    enum isAutodecodableString = autodecodeStrings &&
        is(immutable T == immutable char[]) ||
        is(immutable T == immutable wchar[]);
}

unittest
{
    enum E1 : string { s = "asd" }
    enum E2 : const(char[3]) { s = "asd" }
    enum E3 : wstring { s = "asd" }

    static struct S(T)
    {
        T get() { return T.init; }
        alias get this;
    }
    
    import std.meta : AliasSeq;
    static foreach (i, T; AliasSeq!(
        string[true], wstring[true], dstring[false],
        char[2][false], const char[2][false], const(char)[2][false], wchar[2][false], dchar[2][false],
        E1[false], E2[false], E3[false],
    ))
    {{
        static if (is(T == U[expected], U, size_t expected))
        {{
            pragma(msg, U, i > 3 && i < 8 ? "\t= " : "\t\t= ", isAutodecodableString!U, "\t| isAutodecodableString!(S!U) = ", isAutodecodableString!(S!U));
            static assert(isAutodecodableString!U == expected);
            static assert(!isAutodecodableString!(S!U));
        }}
        else
            static assert(0);
    }}
}

Output:

string		= true	| isAutodecodableString!(S!U) = false
wstring		= true	| isAutodecodableString!(S!U) = false
dstring		= false	| isAutodecodableString!(S!U) = false
char[2]		= false	| isAutodecodableString!(S!U) = false
const(char[2])	= false	| isAutodecodableString!(S!U) = false
const(char[2])	= false	| isAutodecodableString!(S!U) = false
wchar[2]	= false	| isAutodecodableString!(S!U) = false
dchar[2]	= false	| isAutodecodableString!(S!U) = false
E1		= false	| isAutodecodableString!(S!U) = false
E2		= false	| isAutodecodableString!(S!U) = false
E3		= false	| isAutodecodableString!(S!U) = false

https://run.dlang.io/is/7kZkPR

[MoonlightSentinel]

That implementation changes the behaviour, e.g. isAutodecodableString!E1 currently yields true.

[PetarKirov]

That implementation changes the behaviour, e.g. isAutodecodableString!E1 currently yields true.

I didn't read the unittest right below the current implementation 🤦 and I thought that we want to exclude enum and alias this types, sorry for the confusion.