void doExclude(string exclude) {
    string[string] aa;

    int[] keep;
    foreach(int i, h; ["a", "b"]) {
        auto ptr = h in aa;
        pragma(msg, typeof(ptr));  // string*
        pragma(msg, typeof(*ptr)); // string
        if(*h != exclude) keep ~= i;
    }
}

test.d(9): Error: incompatible types for ((*cast(immutable(char)*)h) != (exclude)): 'immutable(char)' and 'string'