GAT's and broken LLVM

[DutchGhost]

I know GAT's are far from complete, but I've played around with them for a bit, and found some fun ICE's and LLVM errors.

The following code produces wrong LLVM output:

#![feature(generic_associated_types)]

trait Fun {
    type F<'a>: Fn() -> u32;
    
    fn callme<'a>(f: Self::F<'a>) -> u32 {
        f()
    }
}

impl <T> Fun for T {
    type F<'a> = Self;
}

fn main() {
    <fn() -> usize>::callme(|| {1});
}

It produces

Function return type does not match operand type of return inst!
  ret i64 %4, !dbg !155
 i32in function _ZN10playground3Fun6callme17h68e94299655f2c3eE
LLVM ERROR: Broken function found, compilation aborted!
error: could not compile `playground`.

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