src/test/ui/hrtb/hrtb-exists-forall-fn.rs

   1 // Test an `exists<'a> { forall<'b> { 'a = 'b } }` pattern -- which should not compile!
   2 //
   3 // In particular, we test this pattern in trait solving, where it is not connected
   4 // to any part of the source code.
   5
   6 fn foo<'a>() -> fn(&'a u32) {
   7     panic!()
   8 }
   9
  10 fn main() {
  11     // Here, proving that `fn(&'a u32) <: for<'b> fn(&'b u32)`:
  12     //
  13     // - instantiates `'b` with a placeholder `!b`,
  14     // - requires that `&!b u32 <: &'a u32` and hence that `!b: 'a`,
  15     // - but we can never know this.
  16
  17     let _: for<'b> fn(&'b u32) = foo(); //~ ERROR mismatched types
  18 }