Add missing testcase

Author: philberty

gcc/testsuite/rust.test/xfail_compile/generics8.rs

@@ -0,0 +1,15 @@
+struct Foo<A, B>(A, B);
+
+impl<T> Foo<i32, T> {
+    fn test(a: T) -> T { // { dg-error "duplicate definitions with name test" }
+        a
+    }
+}
+
+impl Foo<i32, f32> {
+    fn test() -> f32 { // { dg-error "duplicate definitions with name test" }
+        123f32
+    }
+}
+
+fn main() {}