Emits error if has bound regions

compiler/rustc_hir_analysis/src/check/entry.rs

@@ -124,7 +124,10 @@ fn main_fn_return_type_span(tcx: TyCtxt<'_>, def_id: DefId) -> Option<Span> {
     if let Some(term_did) = tcx.lang_items().termination() {
         let return_ty = main_fnsig.output();
         let return_ty_span = main_fn_return_type_span(tcx, main_def_id).unwrap_or(main_span);
-        let return_ty = return_ty.skip_binder();
+        let Some(return_ty) = return_ty.no_bound_vars() else {
+            tcx.sess.emit_err(errors::MainFunctionReturnTypeGeneric { span: return_ty_span });
+            return;
+        };
         let infcx = tcx.infer_ctxt().build();
         let cause = traits::ObligationCause::new(
             return_ty_span,

tests/ui/entry-point/return-ty-has-bound-vars.rs

@@ -0,0 +1,5 @@
+// issue-119209
+
+type Foo<'a> = impl PartialEq; //~ERROR `impl Trait` in type aliases is unstable
+
+fn main<'a>(_: &'a i32) -> Foo<'a> {} //~ERROR `main` function return type is not allowed to have generic parameters

tests/ui/entry-point/return-ty-has-bound-vars.stderr

@@ -0,0 +1,19 @@
+error[E0658]: `impl Trait` in type aliases is unstable
+  --> $DIR/return-ty-has-bound-vars.rs:3:16
+   |
+LL | type Foo<'a> = impl PartialEq;
+   |                ^^^^^^^^^^^^^^
+   |
+   = note: see issue #63063 <https://github.com/rust-lang/rust/issues/63063> for more information
+   = help: add `#![feature(type_alias_impl_trait)]` to the crate attributes to enable
+
+error[E0131]: `main` function return type is not allowed to have generic parameters
+  --> $DIR/return-ty-has-bound-vars.rs:5:28
+   |
+LL | fn main<'a>(_: &'a i32) -> Foo<'a> {}
+   |                            ^^^^^^^
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0131, E0658.
+For more information about an error, try `rustc --explain E0131`.