Don't cancel stashed OpaqueHiddenTypeMismatch errors.

This gives one extra error message on one test, but is necessary to fix
bigger problems caused by the cancellation of stashed errors.

(Note: why not just avoid stashing altogether? Because that resulted in
additional output changes.)

compiler/rustc_middle/src/ty/mod.rs

@@ -852,7 +852,11 @@ impl<'tcx> OpaqueHiddenType<'tcx> {
             .dcx()
             .steal_diagnostic(tcx.def_span(opaque_def_id), StashKey::OpaqueHiddenTypeMismatch)
         {
-            diag.cancel();
+            // We used to cancel here for slightly better error messages, but
+            // cancelling stashed diagnostics is no longer allowed because it
+            // causes problems when tracking whether errors have actually
+            // occurred.
+            diag.emit();
         }
         (self.ty, other.ty).error_reported()?;
         // Found different concrete types for the opaque type.

tests/ui/type-alias-impl-trait/different_defining_uses_never_type-2.rs

@@ -11,6 +11,7 @@ fn foo<'a, 'b>() -> Tait<'a> {
     }
     let x: Tait<'a> = ();
     x
+    //~^ ERROR concrete type differs from previous defining opaque type use
 }
 
 fn main() {}

tests/ui/type-alias-impl-trait/different_defining_uses_never_type-2.stderr

@@ -1,3 +1,15 @@
+error: concrete type differs from previous defining opaque type use
+  --> $DIR/different_defining_uses_never_type-2.rs:13:5
+   |
+LL |     x
+   |     ^ expected `i32`, got `()`
+   |
+note: previous use here
+  --> $DIR/different_defining_uses_never_type-2.rs:8:31
+   |
+LL |             let y: Tait<'b> = 1i32;
+   |                               ^^^^
+
 error: concrete type differs from previous defining opaque type use
   --> $DIR/different_defining_uses_never_type-2.rs:8:31
    |
@@ -10,5 +22,5 @@ note: previous use here
 LL |         if { return } {
    |              ^^^^^^
 
-error: aborting due to 1 previous error
+error: aborting due to 2 previous errors