[Issue 21332] New: Circular reference to a static field of a struct for inferred types

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21332

          Issue ID: 21332
           Summary: Circular reference to a static field of a struct for
                    inferred types
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: lsferreira169 at gmail.com

On this code:

```d
immutable _a = S.c;

struct S {
    immutable a = _a;
    static immutable c = "bar";
}
```

_a only depends on S.c which is "bar", so this should work, because logically,
this is the same as:

```d
immutable _a = _c;
private immutable _c = "bar";

struct S {
    immutable a = _a;
    static immutable c = _c;
}
```

The problem here seems to be the type inference on `immutable a`, because this
code works fine:

```d
immutable _a = S.c;

struct S {
    immutable string a = _a;
    static immutable c = "bar";
}
```

In the worst case, this should give a more clear message like: "Can't infer
type of variable `a`"

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