Add a regression test for issue-84408

Author: JohnTitor

src/test/ui/const-generics/issues/issue-84408.rs

@@ -0,0 +1,38 @@
+// Regression test for #84408.
+// check-pass
+
+#![feature(const_generics, const_evaluatable_checked)]
+#![allow(incomplete_features)]
+
+trait Melon<const X: usize> {
+    fn new(arr: [i32; X]) -> Self;
+    fn change<T: Melon<X>>(self) -> T;
+}
+
+struct Foo([i32; 5]);
+struct Bar<const A: usize, const B: usize>([i32; A + B])
+where
+    [(); A + B]: ;
+
+impl Melon<5> for Foo {
+    fn new(arr: [i32; 5]) -> Self {
+        Foo(arr)
+    }
+    fn change<T: Melon<5>>(self) -> T {
+        T::new(self.0)
+    }
+}
+
+impl<const A: usize, const B: usize> Melon<{ A + B }> for Bar<A, B>
+where
+    [(); A + B]: ,
+{
+    fn new(arr: [i32; A + B]) -> Self {
+        Bar(arr)
+    }
+    fn change<T: Melon<{ A + B }>>(self) -> T {
+        T::new(self.0)
+    }
+}
+
+fn main() {}