Add a regression test for issue-82792

Author: JohnTitor

src/test/ui/const-generics/defaults/repr-c-issue-82792.rs

@@ -0,0 +1,14 @@
+// Regression test for #82792.
+
+// run-pass
+
+#![feature(const_generics_defaults)]
+#![allow(incomplete_features)]
+
+#[repr(C)]
+pub struct Loaf<T: Sized, const N: usize = 1usize> {
+    head: [T; N],
+    slice: [T],
+}
+
+fn main() {}