Add test for #71042

Author: JohnTitor

src/test/ui/pattern/issue-71042-opaquely-typed-constant-used-in-pattern.rs

@@ -0,0 +1,9 @@
+#![feature(impl_trait_in_bindings)]
+#![allow(incomplete_features)]
+
+fn main() {
+    const C: impl Copy = 0;
+    match C {
+        C | _ => {} //~ ERROR: opaque types cannot be used in patterns
+    }
+}

src/test/ui/pattern/issue-71042-opaquely-typed-constant-used-in-pattern.stderr

@@ -0,0 +1,8 @@
+error: opaque types cannot be used in patterns
+  --> $DIR/issue-71042-opaquely-typed-constant-used-in-pattern.rs:7:9
+   |
+LL |         C | _ => {}
+   |         ^
+
+error: aborting due to previous error
+