[Issue 21975] New: is expression ignores implicit conversion of struct via alias this when pattern matching

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21975

          Issue ID: 21975
           Summary: is expression ignores implicit conversion of struct
                    via alias this when pattern matching
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: snarwin+bugzilla at gmail.com

The following program fails to compile as of DMD 2.096.1:

---
struct Outer(T) {
    Inner!T inner;
    alias inner this;
}

struct Inner(T) {
    T t;
}

static assert(is(Outer!int : Inner!int)); // ok
static assert(is(Outer!int : Inner!T, T)); // error
---

The error message is:

--
bug.d(11): Error: static assert:  `is(Outer!int : Inner!T, __isexp_id2, T)` is
false
---

The expected behavior is for both static assert statements to pass.

Surprisingly, changing Inner and Outer from structs to classes causes the
second static assert statement to pass:

---
class Outer(T) {
    Inner!T inner;
    alias inner this;
}

class Inner(T) {
    T t;
}

static assert(is(Outer!int : Inner!int)); // ok
static assert(is(Outer!int : Inner!T, T)); // ok
---

--