Fix ICE in suggest_param_env_shadowing with incompatible args

compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs

@@ -301,6 +301,9 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 let rebased_args = alias.args.rebase_onto(tcx, trait_def_id, impl_substs);
 
                 let impl_item_def_id = leaf_def.item.def_id;
+                if !tcx.check_args_compatible(impl_item_def_id, rebased_args) {
+                    return false;
+                }
                 let impl_assoc_ty = tcx.type_of(impl_item_def_id).instantiate(tcx, rebased_args);
 
                 self.infcx.can_eq(param_env, impl_assoc_ty, concrete)

tests/ui/associated-types/suggest-param-env-shadowing-incompatible-args.rs

@@ -0,0 +1,18 @@
+//@ compile-flags: -Znext-solver=globally
+
+// Regression test for https://github.com/rust-lang/rust/issues/152684.
+
+#![feature(associated_type_defaults)]
+
+trait Foo {
+    type Assoc<T = u8> = T;
+    //~^ ERROR defaults for generic parameters are not allowed here
+    fn foo() -> Self::Assoc;
+}
+impl Foo for () {
+    fn foo() -> Self::Assoc {
+        [] //~ ERROR mismatched types
+    }
+}
+
+fn main() {}

tests/ui/associated-types/suggest-param-env-shadowing-incompatible-args.stderr

@@ -0,0 +1,20 @@
+error: defaults for generic parameters are not allowed here
+  --> $DIR/suggest-param-env-shadowing-incompatible-args.rs:8:16
+   |
+LL |     type Assoc<T = u8> = T;
+   |                ^^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/suggest-param-env-shadowing-incompatible-args.rs:14:9
+   |
+LL |     fn foo() -> Self::Assoc {
+   |                 ----------- expected `<() as Foo>::Assoc` because of return type
+LL |         []
+   |         ^^ expected `u8`, found `[_; 0]`
+   |
+   = note: expected type `u8`
+             found array `[_; 0]`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.