Explicit struct constructors aren't smart when the structs contain anonymous unions

[dlangBugzillaToGithub]

basile-z reported this on 2018-10-14T22:16:16Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=19306

Description

struct Foo
{
    union
    {
        void delegate() a;
        void function() b;
    }
    ulong c;
}

void b(){}
struct A {void a(){}}

void main()
{
    A a;
    Foo f0 = Foo(&a.a);      // ok
    Foo f1 = Foo(&a.a, 0UL); // not allowed but should
    Foo f2 = Foo(&b, 0UL);   // not allowed but should
    Foo f3 = Foo(&b);        // not allowed but should
}