Pick one possible lifetime in case there are multiple choices

Author: oli-obk

compiler/rustc_borrowck/src/region_infer/opaque_types.rs

@@ -124,7 +124,22 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             ty::ReVar(vid) => {
                 // Find something that we can name
                 let upper_bound = self.approx_universal_upper_bound(vid);
-                self.definitions[upper_bound].external_name.unwrap_or(region)
+                let upper_bound = &self.definitions[upper_bound];
+                match upper_bound.external_name {
+                    Some(reg) => reg,
+                    None => {
+                        // Nothing exact found, so we pick the first one that we find.
+                        let scc = self.constraint_sccs.scc(vid);
+                        for vid in self.rev_scc_graph.as_ref().unwrap().upper_bounds(scc) {
+                            match self.definitions[vid].external_name {
+                                None => {}
+                                Some(&ty::ReStatic) => {}
+                                Some(region) => return region,
+                            }
+                        }
+                        region
+                    }
+                }
             }
             _ => region,
         })

src/test/ui/impl-trait/multiple-lifetimes/ordinary-bounds-unrelated.nll.stderr

@@ -4,7 +4,11 @@ error[E0700]: hidden type for `impl Trait` captures lifetime that does not appea
 LL | fn upper_bounds<'a, 'b, 'c, 'd, 'e>(a: Ordinary<'a>, b: Ordinary<'b>) -> impl Trait<'d, 'e>
    |                                                                          ^^^^^^^^^^^^^^^^^^
    |
-   = note: hidden type `Ordinary<'_>` captures lifetime '_#9r
+note: hidden type `Ordinary<'b>` captures the lifetime `'b` as defined on the function body at 16:21
+  --> $DIR/ordinary-bounds-unrelated.rs:16:21
+   |
+LL | fn upper_bounds<'a, 'b, 'c, 'd, 'e>(a: Ordinary<'a>, b: Ordinary<'b>) -> impl Trait<'d, 'e>
+   |                     ^^
 
 error: aborting due to previous error
 

src/test/ui/impl-trait/multiple-lifetimes/ordinary-bounds-unsuited.nll.stderr

@@ -4,7 +4,11 @@ error[E0700]: hidden type for `impl Trait` captures lifetime that does not appea
 LL | fn upper_bounds<'a, 'b>(a: Ordinary<'a>, b: Ordinary<'b>) -> impl Trait<'a, 'b>
    |                                                              ^^^^^^^^^^^^^^^^^^
    |
-   = note: hidden type `Ordinary<'_>` captures lifetime '_#6r
+note: hidden type `Ordinary<'b>` captures the lifetime `'b` as defined on the function body at 18:21
+  --> $DIR/ordinary-bounds-unsuited.rs:18:21
+   |
+LL | fn upper_bounds<'a, 'b>(a: Ordinary<'a>, b: Ordinary<'b>) -> impl Trait<'a, 'b>
+   |                     ^^
 
 error: aborting due to previous error