mixin template silently `override` without even a warning message!

[mw]

Hi,

I just found another problem:

```
import std;

template Foo() {
     void foo() {writeln("F");}
     void bar() {foo();}  // fixed to Foo.foo()
}

class A {
     mixin Foo;
     void foo() {writeln("A");}  // I accidentally added this new 
name-clashed method
}

void main() {
     A a = new A();
     a.bar();  // print F
     a.foo();  // print A
}
```

1) class A ends with two methods `foo()` with the same signature!

2) whether the foo() in bar() should be fixed to Foo.foo() 
(non-virtual) is debatable (I can see the reason on both sides).

3) BUT, the compiler should at least issue a warning message, so 
the programmer is aware of this name clash. Even when a sub-class 
override a super-class virtual method, the compiler enforces the 
user to add `override` keyword, but why not in this case?


Has this problem be discussed before? and what's the conclusion?