check if res is trait def

Author: TaKO8Ki

Diff for: compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs

@@ -789,15 +789,17 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
 
                 Some(args.constraints.iter().filter_map(|constraint| {
                     let ident = constraint.ident;
-                    let trait_def = path.res.opt_def_id();
-                    let assoc_item = trait_def.and_then(|trait_def_id| {
-                        tcx.associated_items(trait_def_id).find_by_name_and_kind(
-                            tcx,
-                            ident,
-                            ty::AssocKind::Type,
-                            trait_def_id,
-                        )
-                    });
+
+                    let Res::Def(DefKind::Trait, trait_def) = path.res else {
+                        return None;
+                    };
+
+                    let assoc_item = tcx.associated_items(trait_def).find_by_name_and_kind(
+                        tcx,
+                        ident,
+                        ty::AssocKind::Type,
+                        trait_def,
+                    );
 
                     Some((ident.name, assoc_item?))
                 }))

Diff for: tests/ui/associated-types/ice-issue-137508.rs renamed to tests/ui/associated-types/avoid-getting-associated-items-of-undefined-trait.rs

@@ -1,9 +1,11 @@
+// Fix for https://github.com/rust-lang/rust/issues/137508>.
+
 trait Tr {
     type Item;
 }
 
 fn main() {
-    let _: dyn Tr + ?Foo();
+    let _: dyn Tr + ?Foo<Assoc = ()>;
     //~^ ERROR: `?Trait` is not permitted in trait object types
     //~| ERROR: cannot find trait `Foo` in this scope
     //~| ERROR: the value of the associated type `Item` in `Tr` must be specifi

Diff for: tests/ui/associated-types/ice-issue-137508.stderr renamed to tests/ui/associated-types/avoid-getting-associated-items-of-undefined-trait.stderr

@@ -1,25 +1,25 @@
 error[E0658]: `?Trait` is not permitted in trait object types
-  --> $DIR/issue-137508.rs:6:21
+  --> $DIR/avoid-getting-associated-items-of-undefined-trait.rs:8:21
    |
-LL |     let _: dyn Tr + ?Foo();
-   |                     ^^^^^^
+LL |     let _: dyn Tr + ?Foo<Assoc = ()>;
+   |                     ^^^^^^^^^^^^^^^^
    |
    = help: add `#![feature(more_maybe_bounds)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error[E0405]: cannot find trait `Foo` in this scope
-  --> $DIR/issue-137508.rs:6:22
+  --> $DIR/avoid-getting-associated-items-of-undefined-trait.rs:8:22
    |
-LL |     let _: dyn Tr + ?Foo();
+LL |     let _: dyn Tr + ?Foo<Assoc = ()>;
    |                      ^^^ not found in this scope
 
 error[E0191]: the value of the associated type `Item` in `Tr` must be specified
-  --> $DIR/issue-137508.rs:6:16
+  --> $DIR/avoid-getting-associated-items-of-undefined-trait.rs:8:16
    |
 LL |     type Item;
    |     --------- `Item` defined here
 ...
-LL |     let _: dyn Tr + ?Foo();
+LL |     let _: dyn Tr + ?Foo<Assoc = ()>;
    |                ^^ help: specify the associated type: `Tr<Item = Type>`
 
 error: aborting due to 3 previous errors