Add test for RPIT defined with different hidden types with different substs

tests/ui/impl-trait/multiple-defining-usages-in-body.rs

@@ -0,0 +1,12 @@
+trait Trait {}
+impl Trait for () {}
+
+fn foo<T: Trait, U: Trait>() -> impl Trait {
+    //~^ WARN function cannot return without recursing [unconditional_recursion]
+    let a: T = foo::<T, U>();
+    //~^ ERROR concrete type differs from previous defining opaque type use
+    loop {}
+    let _: T = foo::<U, T>();
+}
+
+fn main() {}

tests/ui/impl-trait/multiple-defining-usages-in-body.stderr

@@ -0,0 +1,26 @@
+warning: function cannot return without recursing
+  --> $DIR/multiple-defining-usages-in-body.rs:4:1
+   |
+LL | fn foo<T: Trait, U: Trait>() -> impl Trait {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot return without recursing
+LL |
+LL |     let a: T = foo::<T, U>();
+   |                ------------- recursive call site
+   |
+   = help: a `loop` may express intention better if this is on purpose
+   = note: `#[warn(unconditional_recursion)]` on by default
+
+error: concrete type differs from previous defining opaque type use
+  --> $DIR/multiple-defining-usages-in-body.rs:6:16
+   |
+LL |     let a: T = foo::<T, U>();
+   |                ^^^^^^^^^^^^^ expected `U`, got `T`
+   |
+note: previous use here
+  --> $DIR/multiple-defining-usages-in-body.rs:9:16
+   |
+LL |     let _: T = foo::<U, T>();
+   |                ^^^^^^^^^^^^^
+
+error: aborting due to previous error; 1 warning emitted
+