Type inference failure when cloning

[sfackler]

fn foo() -> String {
    let x = std::default::Default::default();
    drop(x.clone());
    x
}

fn main() {}

test.rs:3:10: 3:19 error: the type of this value must be known in this context
test.rs:3     drop(x.clone());
                   ^~~~~~~~~
error: aborting due to previous error

cc @nikomatsakis

[michaelsproul]

Probably related? #19851 and #19853.

[nikomatsakis]

This is expected behavior. When doing a method call, we must know the type at that point in type checking. Here, the constraint on the type of x only arises at the point of the return. It'd be nice to fix this but it's not on the current plans (and requires a lot of work).