Add a test showing that we don't infer across multiple uses of the same opaque type but with a different order of generic arguments

Author: oli-obk

src/test/ui/type-alias-impl-trait/multiple-def-uses-in-one-fn-infer.rs

@@ -0,0 +1,13 @@
+// https://github.com/rust-lang/rust/issues/73481
+// This test used to cause unsoundness, since one of the two possible
+// resolutions was chosen at random instead of erroring due to conflicts.
+
+#![feature(type_alias_impl_trait)]
+
+type Y<A, B> = impl std::fmt::Debug;
+
+fn g<A, B>() -> (Y<A, B>, Y<B, A>) {
+    (42_i64, 60) //~^ ERROR concrete type differs from previous defining opaque type use
+}
+
+fn main() {}

src/test/ui/type-alias-impl-trait/multiple-def-uses-in-one-fn-infer.stderr

@@ -0,0 +1,14 @@
+error: concrete type differs from previous defining opaque type use
+  --> $DIR/multiple-def-uses-in-one-fn-infer.rs:9:1
+   |
+LL | fn g<A, B>() -> (Y<A, B>, Y<B, A>) {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `i64`, got `i32`
+   |
+note: previous use here
+  --> $DIR/multiple-def-uses-in-one-fn-infer.rs:9:1
+   |
+LL | fn g<A, B>() -> (Y<A, B>, Y<B, A>) {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+