Add a test for const arguments

src/test/ui/const-generics/const-arg-in-fn.rs

@@ -0,0 +1,13 @@
+// run-pass
+
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
+
+fn const_u32_identity<const X: u32>() -> u32 {
+    X
+}
+
+ fn main() {
+    let val = const_u32_identity::<18>();
+    assert_eq!(val, 18);
+}

src/test/ui/const-generics/const-arg-in-fn.stderr

@@ -0,0 +1,6 @@
+warning: the feature `const_generics` is incomplete and may cause the compiler to crash
+  --> $DIR/const-arg-in-fn.rs:3:12
+   |
+LL | #![feature(const_generics)]
+   |            ^^^^^^^^^^^^^^
+