Add regression test for rust-lang#102124

Author: Hosshii

src/test/ui/const-generics/issue-102124.rs

@@ -0,0 +1,20 @@
+// run-pass
+// compile-flags: -Zmir-opt-level=3
+
+// regression test for #102124
+
+const L: usize = 4;
+
+pub trait Print<const N: usize> {
+    fn print(&self) -> usize {
+        N
+    }
+}
+
+pub struct Printer;
+impl Print<L> for Printer {}
+
+fn main() {
+    let p = Printer;
+    assert_eq!(p.print(), 4);
+}