Fix ICE with const generic param in struct

src/test/ui/const-generics/struct-with-invalid-const-param.rs

@@ -0,0 +1,6 @@
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
+
+struct S<const C: u8>(C); //~ ERROR expected type, found const parameter
+
+fn main() {}

src/test/ui/const-generics/struct-with-invalid-const-param.stderr

@@ -0,0 +1,15 @@
+warning: the feature `const_generics` is incomplete and may cause the compiler to crash
+  --> $DIR/struct-with-invalid-const-param.rs:1:12
+   |
+LL | #![feature(const_generics)]
+   |            ^^^^^^^^^^^^^^
+
+error[E0573]: expected type, found const parameter `C`
+  --> $DIR/struct-with-invalid-const-param.rs:4:23
+   |
+LL | struct S<const C: u8>(C);
+   |                       ^ help: a struct with a similar name exists: `S`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0573`.