Add a regression test for issue-52843

src/test/ui/type-alias-impl-trait/issue-52843.rs

@@ -0,0 +1,15 @@
+#![feature(type_alias_impl_trait)]
+
+type Foo<T> = impl Default;
+//~^ ERROR: the trait bound `T: Default` is not satisfied
+
+#[allow(unused)]
+fn foo<T: Default>(t: T) -> Foo<T> {
+    t
+}
+
+struct NotDefault;
+
+fn main() {
+    let _ = Foo::<NotDefault>::default();
+}

src/test/ui/type-alias-impl-trait/issue-52843.stderr

@@ -0,0 +1,14 @@
+error[E0277]: the trait bound `T: Default` is not satisfied
+  --> $DIR/issue-52843.rs:3:15
+   |
+LL | type Foo<T> = impl Default;
+   |               ^^^^^^^^^^^^ the trait `Default` is not implemented for `T`
+   |
+help: consider restricting type parameter `T`
+   |
+LL | type Foo<T: Default> = impl Default;
+   |           ^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.