[Issue 19926] New: Wrong instantiation of opDispatch when used in WithStatement

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19926

          Issue ID: 19926
           Summary: Wrong instantiation of opDispatch when used in
                    WithStatement
           Product: D
           Version: D2
          Hardware: x86
                OS: Windows
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: simen.kjaras at gmail.com

DMD is of two minds when invoking opDispatch inside WithStatements, and
requires that the template be possible to instantiate with the arguments
passed, followed by instatiating it with an empty parameter tuple:

unittest {
    with (S1.init) {
        a(1); // Compiles
    }
    with (S2.init) {
        a(1); // is not callable using argument types (int)
    }
}

struct S1 {
    auto opDispatch(string name, Args...)(Args args) {
        static assert(Args.length == 0);
        return (int i){};
    }
}

struct S2 {
    auto opDispatch(string name)() {
        return (int i){};
    }
}

As the example with S2 shows, the compiler complains when opDispatch takes no
parameters (this is sensible). However, as S1 shows, Args is actually empty
when opDispatch is instantiated, and instead opDispatch needs to return a
lambda that actually handles the run-time arguments (this is not sensible, on
more than one level).

The correct behavior would of course be for this to compile and run:

unittest {
    with (S3.init) {
        a(1);
    }
}

struct S3 {
    auto opDispatch(string name, Args...)(Args args) {
        static assert(Args.length == 1);
        static assert(is(Args[0] == int));
        assert(args[0] == 1);
    }
}

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