Avoid ICE on ReFree region from malformed code

Author: estebank

src/librustc_typeck/outlives/utils.rs

@@ -1,6 +1,7 @@
 use rustc::ty::outlives::Component;
 use rustc::ty::subst::{GenericArg, GenericArgKind};
 use rustc::ty::{self, Region, RegionKind, Ty, TyCtxt};
+use syntax_pos::DUMMY_SP;
 use smallvec::smallvec;
 use std::collections::BTreeSet;
 
@@ -161,9 +162,21 @@ fn is_free_region(tcx: TyCtxt<'_>, region: Region<'_>) -> bool {
         // ignore it.  We can't put it on the struct header anyway.
         RegionKind::ReLateBound(..) => false,
 
+        // This can appear with malformed code (#64855):
+        //
+        //     struct Bar<T>(<Self as Foo>::Type) where Self: ;
+        //
+        // We accept it only to avoid an ICE.
+        RegionKind::ReEmpty => {
+            tcx.sess.delay_span_bug(
+                DUMMY_SP,
+                &format!("unexpected region in outlives inference: {:?}", region),
+            );
+            false
+        }
+
         // These regions don't appear in types from type declarations:
-        RegionKind::ReEmpty
-        | RegionKind::ReErased
+        RegionKind::ReErased
         | RegionKind::ReClosureBound(..)
         | RegionKind::ReScope(..)
         | RegionKind::ReVar(..)

src/test/ui/associated-types/issue-64855.rs

@@ -0,0 +1,8 @@
+pub trait Foo {
+    type Type;
+}
+
+pub struct Bar<T>(<Self as Foo>::Type) where Self: ;
+//~^ ERROR the trait bound `Bar<T>: Foo` is not satisfied
+
+fn main() {}

src/test/ui/associated-types/issue-64855.stderr

@@ -0,0 +1,9 @@
+error[E0277]: the trait bound `Bar<T>: Foo` is not satisfied
+  --> $DIR/issue-64855.rs:5:19
+   |
+LL | pub struct Bar<T>(<Self as Foo>::Type) where Self: ;
+   |                   ^^^^^^^^^^^^^^^^^^^ the trait `Foo` is not implemented for `Bar<T>`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.