Add regression test for #29821

Closes #29821

src/test/ui/issues/issue-29821.rs

@@ -0,0 +1,19 @@
+// build-pass
+
+pub trait Foo {
+    type FooAssoc;
+}
+
+pub struct Bar<F: Foo> {
+    id: F::FooAssoc
+}
+
+pub struct Baz;
+
+impl Foo for Baz {
+    type FooAssoc = usize;
+}
+
+static mut MY_FOO: Bar<Baz> = Bar { id: 0 };
+
+fn main() {}