Test that TAIT and RPIT are in sync

compiler/rustc_typeck/src/check/fn_ctxt/_impl.rs

@@ -772,6 +772,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // Note: this check is pessimistic, as the inference type could be matched with something other
         // than the opaque type, but then we need a new `TypeRelation` just for this specific case and
         // can't re-use `sup` below.
+        // See src/test/ui/impl-trait/hidden-type-is-opaque.rs and
+        // src/test/ui/impl-trait/hidden-type-is-opaque-2.rs for examples that hit this path.
         if formal_ret.has_infer_types() {
             for ty in ret_ty.walk() {
                 if let ty::subst::GenericArgKind::Type(ty) = ty.unpack() {

src/test/ui/impl-trait/hidden-type-is-opaque-2.rs

@@ -2,6 +2,8 @@
 // into function arguments via the function's generic parameters
 // FIXME(oli-obk): make `expected_inputs_for_expected_output` support this
 
+#![feature(type_alias_impl_trait)]
+
 fn reify_as() -> Thunk<impl FnOnce(Continuation) -> Continuation> {
     Thunk::new(|mut cont| { //~ ERROR type annotations needed
         cont.reify_as();
@@ -9,6 +11,15 @@ fn reify_as() -> Thunk<impl FnOnce(Continuation) -> Continuation> {
     })
 }
 
+type Tait = impl FnOnce(Continuation) -> Continuation;
+
+fn reify_as_tait() -> Thunk<Tait> {
+    Thunk::new(|mut cont| { //~ ERROR type annotations needed
+        cont.reify_as();
+        cont
+    })
+}
+
 #[must_use]
 struct Thunk<F>(F);
 

src/test/ui/impl-trait/hidden-type-is-opaque-2.stderr

@@ -1,11 +1,19 @@
 error[E0282]: type annotations needed
-  --> $DIR/hidden-type-is-opaque-2.rs:6:17
+  --> $DIR/hidden-type-is-opaque-2.rs:8:17
    |
 LL |     Thunk::new(|mut cont| {
    |                 ^^^^^^^^ consider giving this closure parameter a type
    |
    = note: type must be known at this point
 
-error: aborting due to previous error
+error[E0282]: type annotations needed
+  --> $DIR/hidden-type-is-opaque-2.rs:17:17
+   |
+LL |     Thunk::new(|mut cont| {
+   |                 ^^^^^^^^ consider giving this closure parameter a type
+   |
+   = note: type must be known at this point
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0282`.

src/test/ui/impl-trait/hidden-type-is-opaque.rs

@@ -1,4 +1,5 @@
 // check-pass
+#![feature(type_alias_impl_trait)]
 
 fn reify_as() -> Thunk<impl ContFn> {
     Thunk::new(|mut cont| {
@@ -7,6 +8,15 @@ fn reify_as() -> Thunk<impl ContFn> {
     })
 }
 
+type Tait = impl ContFn;
+
+fn reify_as_tait() -> Thunk<Tait> {
+    Thunk::new(|mut cont| {
+        cont.reify_as();
+        cont
+    })
+}
+
 #[must_use]
 struct Thunk<F>(F);