Add a regression test for #47131

Author: varkor

src/test/ui/dead-code-impl.rs

@@ -0,0 +1,17 @@
+// run-pass
+
+#![deny(dead_code)]
+
+pub struct GenericFoo<T>(T);
+
+type Foo = GenericFoo<u32>;
+
+impl Foo {
+    fn bar(self) -> u8 {
+        0
+    }
+}
+
+fn main() {
+    println!("{}", GenericFoo(0).bar());
+}