require type defaults to be after const generic parameters

as if this is currently possible. HA!

src/test/ui/const-generics/defaults/wrong-order.rs

@@ -0,0 +1,8 @@
+#![feature(const_generics)] //~ WARN the feature `const_generics` is incomplete
+
+struct A<T = u32, const N: usize> {
+    //~^ ERROR type parameters with a default must be trailing
+    arg: T,
+}
+
+fn main() {}

src/test/ui/const-generics/defaults/wrong-order.stderr

@@ -0,0 +1,19 @@
+error: type parameters with a default must be trailing
+  --> $DIR/wrong-order.rs:3:10
+   |
+LL | struct A<T = u32, const N: usize> {
+   |          ^
+   |
+   = note: using type defaults and const parameters in the same parameter listing is currently not possible
+
+warning: the feature `const_generics` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/wrong-order.rs:1:12
+   |
+LL | #![feature(const_generics)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #44580 <https://github.com/rust-lang/rust/issues/44580> for more information
+
+error: aborting due to previous error; 1 warning emitted
+