[Issue 21229] New: [REG 2.080.2] Constructor flow analysis doesn't understand unions

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21229

          Issue ID: 21229
           Summary: [REG 2.080.2] Constructor flow analysis doesn't
                    understand unions
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: regression
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: snarwin+bugzilla at gmail.com

Example program:

---
struct NeedsInit
{
    int n;
    @disable this();
}

union U
{
    NeedsInit a;
}

struct S
{
    U u;
    this(NeedsInit arg){ u.a = arg; }
}
---

Compiler output, from run.dlang.io:

Up to      2.060  : Failure with output:
-----
onlineapp.d(15): Error: can only initialize const member a inside constructor
onlineapp.d(15): Error: can only initialize const member a inside constructor
onlineapp.d(15): Error: constructor onlineapp.S.this field u must be
initialized in constructor
-----

2.061   to 2.080.1: Success and no output
Since      2.081.2: Failure with output: onlineapp.d(15): Error: field `u` must
be initialized in constructor

This appears to be a known bug, since there is a comment about unions in
`dmd.semantic3.Semantic3Visitor.visit(FuncDeclaration)`:

/* Current bugs in the flow analysis:
 * 1. union members should not produce error messages even if
 *    not assigned to
 * 2. structs should recognize delegating opAssign calls as well
 *    as delegating calls to other constructors
 */

--