Test removing unstable default parameter

Author: Avi-D-coder

src/test/ui/stability-attribute/auxiliary/unstable_generic_param.rs

@@ -109,3 +109,15 @@ impl<T> Box2<T, System> {
         Self { ptr: &mut t, alloc: System {} }
     }
 }
+
+#[stable(feature = "stable_test_feature", since = "1.0.0")]
+pub struct Box3<T> {
+    ptr: *mut T,
+}
+
+impl<T> Box3<T> {
+    #[stable(feature = "stable_test_feature", since = "1.0.0")]
+    pub fn new(mut t: T) -> Self {
+        Self { ptr: &mut t }
+    }
+}

src/test/ui/stability-attribute/generics-default-stability.rs

@@ -115,4 +115,6 @@ fn main() {
 
     let _: Box2<isize, System> = Box2::new(1); // ok
     let _: Box2<isize> = Box2::new(1); // ok
+
+    let _: Box3<isize> = Box3::new(1); // ok
 }