enum Enums
{
    a,
    b
}

void main()
{
    enum Fields = 
    [
        Enums.a : "a",
    ];
    
    mixin("auto x = " ~ Fields[Enums.b] ~ ";");
}

Errors:
test.d(16): Error: argument to mixin must be a string, not ("auto x = " ~ [cast(Enums)0:"a"][cast(Enums)1] ~ ";")
test.d(16): Error: argument to mixin must be a string, not ("auto x = " ~ [cast(Enums)0:"a"][cast(Enums)1] ~ ";")

I'd rather the errors be something like:

test.d(16): Error: CTFE: Key 'Enums.b' is not in 'Fields'
test.d(16): Error: argument to mixin must be a string, not ("auto x = " ~ (__error) ~ ";")

We could also get rid of duplicate error messages.