[Issue 15638] New: no common type for const classes

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=15638

          Issue ID: 15638
           Summary: no common type for const classes
           Product: D
           Version: D2
          Hardware: x86_64
               URL: https://github.com/D-Programming-Language/dmd/pull/125
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: schuetzm at gmx.net

import std.stdio;

    class A     { int val; }
    class B : A { this() { val = 3; } } 
    class C : A { this() { val = 4; } } 

    void main()
    {   
        const B b;
        const C c;

        writeln(true ? b : c);
    }   

Error: incompatible types for ((b) : (c)): 'const(B)' and 'const(C)'

AFAICS there is no reason why this should be disallowed, considering that the
following works:

    const A a1 = b;
    const A a2 = c;

The common type should be `const(A)`.

This seems to be an unintended side-effect of this PR:
https://github.com/D-Programming-Language/dmd/pull/125

Discovered by SimonN:
http://forum.dlang.org/post/vgwdoqmdzxyegnautwgv@forum.dlang.org

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