[Issue 16030] New: bad error message when trying to use union in CTFE

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=16030

          Issue ID: 16030
           Summary: bad error message when trying to use union in CTFE
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Keywords: diagnostic
          Severity: minor
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: ag0aep6g at gmail.com

Encountered by Stefan who posted to D.learn:
http://forum.dlang.org/post/gsnvzfrglquebmwqcmib@forum.dlang.org

Reduced code:
----
union U
{
    int i;
    byte[4] b;
}

byte[4] f(int val)
{
    U u;
    u.i = val;
    return u.b;
}

static byte[4] forceCtfe = f(1); /* line 14 */
----

Fails compilation with:
----
test.d(14): Error: uninitialized variable 'i' cannot be returned from CTFE
----

I guess it's expected that unions don't work in CTFE, but the error message is
bad:

* There is no variable `i` on line 14. The function that actually fails CTFE
may be hidden anywhere in the call graph of `f`.
* The function tries to return `u.b`, not `u.i`.
* It's not obvious that "uninitialized" means "from a union".

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