Bug 12884 – implicit conversion wrongly prefers casting immutable away over alias this when target type is base class

--- class B {int x;} class C : B { this(int x) pure {this.x = x;} @property C mutable() const {return new C(42);} alias mutable this; } void main() { immutable c = new C(1); B m1 = c; /* should call alias this */ assert(m1.x == 42); assert(m1 !is c); } --- Pull request is on the way.

With class types, the possibility of implicit class reference conversion from derived D to base class B is always determined in type system. In other words, conversion from immutable D to mutable B is a defined but disabled by default, and 'alias this' cannot override it. (Note that 'alias this' is invoked when the conversion is _not_ defined) So, 'alias this' never be considered in the case, and this is invalid issue.