Don't normalize projections referencing type error

compiler/rustc_trait_selection/src/solve/normalize.rs

@@ -70,7 +70,10 @@ impl<'tcx> NormalizationFolder<'_, 'tcx> {
         // keep the projection unnormalized. This is the case for projections
         // with a `T: Trait` where-clause and opaque types outside of the defining
         // scope.
-        let result = if infcx.predicate_may_hold(&obligation) {
+        // HACK: if alias_ty references errors, then there's a possibility that the
+        // normalized type may remain unconstrained. In that case, let's just bail.
+        // See `dont-try-normalizing-proj-with-errors.rs` in the test suite.
+        let result = if infcx.predicate_may_hold(&obligation) && !alias.references_error() {
             self.fulfill_cx.register_predicate_obligation(infcx, obligation);
             let errors = self.fulfill_cx.select_all_or_error(infcx);
             if !errors.is_empty() {

tests/ui/traits/new-solver/dont-try-normalizing-proj-with-errors.rs

@@ -0,0 +1,19 @@
+// compile-flags: -Ztrait-solver=next
+
+trait Mirror {
+    type Assoc: ?Sized;
+}
+
+struct Wrapper<T: ?Sized>(T);
+impl<T: ?Sized> Mirror for Wrapper<T> {
+    type Assoc = T;
+}
+
+fn mirror<W: Mirror>(_: W) -> Box<W::Assoc> { todo!() }
+
+fn type_error() -> TypeError { todo!() }
+//~^ ERROR cannot find type `TypeError` in this scope
+
+fn main() {
+    let x = mirror(type_error());
+}

tests/ui/traits/new-solver/dont-try-normalizing-proj-with-errors.stderr

@@ -0,0 +1,9 @@
+error[E0412]: cannot find type `TypeError` in this scope
+  --> $DIR/dont-try-normalizing-proj-with-errors.rs:14:20
+   |
+LL | fn type_error() -> TypeError { todo!() }
+   |                    ^^^^^^^^^ not found in this scope
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0412`.