Make unnormalizable item ambiguous in coherence

compiler/rustc_next_trait_solver/src/solve/normalizes_to/mod.rs

@@ -238,8 +238,24 @@ where
                 // delay a bug because we can have trivially false where clauses, so we
                 // treat it as rigid.
                 if cx.impl_self_is_guaranteed_unsized(impl_def_id) {
-                    ecx.structurally_instantiate_normalizes_to_term(goal, goal.predicate.alias);
-                    return ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes);
+                    match ecx.typing_mode() {
+                        ty::TypingMode::Coherence => {
+                            return ecx.evaluate_added_goals_and_make_canonical_response(
+                                Certainty::AMBIGUOUS,
+                            );
+                        }
+                        ty::TypingMode::Analysis { .. }
+                        | ty::TypingMode::Borrowck { .. }
+                        | ty::TypingMode::PostBorrowckAnalysis { .. }
+                        | ty::TypingMode::PostAnalysis => {
+                            ecx.structurally_instantiate_normalizes_to_term(
+                                goal,
+                                goal.predicate.alias,
+                            );
+                            return ecx
+                                .evaluate_added_goals_and_make_canonical_response(Certainty::Yes);
+                        }
+                    }
                 } else {
                     return error_response(ecx, cx.delay_bug("missing item"));
                 }

tests/ui/traits/trivial-unsized-projection-in-coherence.rs

@@ -0,0 +1,45 @@
+// Make sure we don't treat missing associated items as rigid
+// during coherence, even if we know they've got an impossible
+// `Sized`-bound. As we check whether the self type is definitely
+// not `Sized` outside of coherence, this check can be incomplete.
+//
+// In this test we only use `impl<T> Overlap<u32> for T` to normalize
+// the field of `MaybeUnsized<T, u32>` when checking whether it's
+// definitely not `Sized`. However, for `MaybeUnsized<u32, u32>` we
+// could also use `impl<U> Overlap<U> for u32` for normalization, which
+// would result in a `Sized` type. cc #139000
+
+struct MaybeUnsized<T: Overlap<U>, U>(<T as Overlap<U>>::MaybeUnsized);
+
+trait ReqSized {
+    type Missing1
+    where
+        Self: Sized;
+    type Missing2
+    where
+        Self: Sized;
+}
+impl<T> ReqSized for MaybeUnsized<T, u32> {}
+
+struct W<T: ?Sized>(T);
+trait Eq<T> {}
+impl<T> Eq<T> for W<T> {}
+
+trait RelateReqSized {}
+impl<T: ReqSized> RelateReqSized for T where W<T::Missing1>: Eq<T::Missing2> {}
+
+trait Overlap<U> {
+    type MaybeUnsized: ?Sized;
+}
+impl<T> Overlap<u32> for T {
+    type MaybeUnsized = str;
+}
+impl<U> Overlap<U> for u32
+//~^ ERROR conflicting implementations of trait `Overlap<u32>` for type `u32`
+where
+    MaybeUnsized<U, u32>: RelateReqSized,
+{
+    type MaybeUnsized = u32;
+}
+
+fn main() {}

tests/ui/traits/trivial-unsized-projection-in-coherence.stderr

@@ -0,0 +1,15 @@
+error[E0119]: conflicting implementations of trait `Overlap<u32>` for type `u32`
+  --> $DIR/trivial-unsized-projection-in-coherence.rs:37:1
+   |
+LL |   impl<T> Overlap<u32> for T {
+   |   -------------------------- first implementation here
+...
+LL | / impl<U> Overlap<U> for u32
+LL | |
+LL | | where
+LL | |     MaybeUnsized<U, u32>: RelateReqSized,
+   | |_________________________________________^ conflicting implementation for `u32`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0119`.