Don't leak unnameable types in -> _ recover

tests/ui/variance/leaking-unnameables.rs

@@ -0,0 +1,13 @@
+// Test variance computation doesn't explode when we leak unnameable
+// types due to `-> _` recovery.
+
+pub struct Type<'a>(&'a ());
+
+pub fn g() {}
+
+pub fn f<T>() -> _ {
+   //~^ ERROR the placeholder `_` is not allowed within types on item signatures
+   g
+}
+
+fn main() {}

tests/ui/variance/leaking-unnameables.stderr

@@ -0,0 +1,12 @@
+error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
+  --> $DIR/leaking-unnameables.rs:8:18
+   |
+LL | pub fn f<T>() -> _ {
+   |                  ^
+   |                  |
+   |                  not allowed in type signatures
+   |                  help: replace with the correct return type: `fn()`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0121`.