add test for assoc type norm wf check

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

@@ -235,8 +235,9 @@ where
             // See <https://github.com/rust-lang/trait-system-refactor-initiative/issues/185>.
             ecx.try_evaluate_added_goals()?;
 
-            // Add GAT where clauses from the trait's definition.
-            // FIXME: We don't need these, since these are the type's own WF obligations.
+            // Add GAT where clauses from the trait's definition. This is necessary
+            // for soundness until we properly handle implied bounds on binders,
+            // see tests/ui/generic-associated-types/must-prove-where-clauses-on-norm.rs.
             ecx.add_goals(
                 GoalSource::AliasWellFormed,
                 cx.own_predicates_of(goal.predicate.def_id())

compiler/rustc_trait_selection/src/traits/project.rs

@@ -2055,6 +2055,9 @@ fn confirm_impl_candidate<'cx, 'tcx>(
 
 // Get obligations corresponding to the predicates from the where-clause of the
 // associated type itself.
+//
+// This is necessary for soundness until we properly handle implied bounds on binders.
+// see tests/ui/generic-associated-types/must-prove-where-clauses-on-norm.rs.
 // FIXME(mgca): While this supports constants, it is only used for types by default right now
 fn assoc_term_own_obligations<'cx, 'tcx>(
     selcx: &mut SelectionContext<'cx, 'tcx>,

tests/ui/generic-associated-types/must-prove-where-clauses-on-norm.current.stderr

@@ -0,0 +1,10 @@
+error: higher-ranked lifetime error
+  --> $DIR/must-prove-where-clauses-on-norm.rs:23:61
+   |
+LL |     let func: for<'a, 'b> fn((), &'b str) -> &'static str = foo::<()>;
+   |                                                             ^^^^^^^^^
+   |
+   = note: could not normalize `for<'b> fn(<() as Trait>::Assoc<'_, 'b>, &'b str) -> &str`
+
+error: aborting due to 1 previous error
+

tests/ui/generic-associated-types/must-prove-where-clauses-on-norm.next.stderr

@@ -0,0 +1,14 @@
+error[E0308]: mismatched types
+  --> $DIR/must-prove-where-clauses-on-norm.rs:23:61
+   |
+LL |     let func: for<'a, 'b> fn((), &'b str) -> &'static str = foo::<()>;
+   |               -------------------------------------------   ^^^^^^^^^ one type is more general than the other
+   |               |
+   |               expected due to this
+   |
+   = note: expected fn pointer `for<'b> fn((), &'b _) -> &'static _`
+                 found fn item `for<'b> fn(<() as Trait>::Assoc<'_, 'b>, &'b _) -> &_ {foo::<'_, ()>}`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/generic-associated-types/must-prove-where-clauses-on-norm.rs

@@ -0,0 +1,28 @@
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+
+// We have to prove implied bounds of higher-ranked types at some point.
+// Normalizing associated types can drop requirements. This means we need
+// to prove well-formedness when normalizing them, at least as long as
+// implied bounds are implicit.
+
+trait Trait {
+    type Assoc<'a, 'b: 'a>;
+}
+
+impl Trait for () {
+    type Assoc<'a, 'b: 'a> = ();
+}
+
+fn foo<'a, 'b, T: Trait>(_: <T as Trait>::Assoc<'a, 'b>, x: &'b str) -> &'a str {
+    x
+}
+
+fn main() {
+    let func: for<'a, 'b> fn((), &'b str) -> &'static str = foo::<()>;
+    //[current]~^ ERROR higher-ranked lifetime error
+    //[next]~^^ ERROR mismatched types
+    let x: &'static str = func((), &String::from("temporary"));
+    println!("{x}");
+}