don't do Sized/other fn signature checks on RPIT's real type

compiler/rustc_typeck/src/check/check.rs

@@ -103,12 +103,6 @@ pub(super) fn check_fn<'a, 'tcx>(
             DUMMY_SP,
             param_env,
         ));
-    // HACK(oli-obk): we rewrite the declared return type, too, so that we don't end up inferring all
-    // unconstrained RPIT to have `()` as their hidden type. This would happen because further down we
-    // compare the ret_coercion with declared_ret_ty, and anything uninferred would be inferred to the
-    // opaque type itself. That again would cause writeback to assume we have a recursive call site
-    // and do the sadly stabilized fallback to `()`.
-    let declared_ret_ty = ret_ty;
     fcx.ret_coercion = Some(RefCell::new(CoerceMany::new(ret_ty)));
     fcx.ret_type_span = Some(decl.output.span());
 
@@ -252,7 +246,12 @@ pub(super) fn check_fn<'a, 'tcx>(
             fcx.next_ty_var(TypeVariableOrigin { kind: TypeVariableOriginKind::DynReturnFn, span });
         debug!("actual_return_ty replaced with {:?}", actual_return_ty);
     }
-    fcx.demand_suptype(span, declared_ret_ty, actual_return_ty);
+
+    // HACK(oli-obk, compiler-errors): We should be comparing this against
+    // `declared_ret_ty`, but then anything uninferred would be inferred to
+    // the opaque type itself. That again would cause writeback to assume
+    // we have a recursive call site and do the sadly stabilized fallback to `()`.
+    fcx.demand_suptype(span, ret_ty, actual_return_ty);
 
     // Check that a function marked as `#[panic_handler]` has signature `fn(&PanicInfo) -> !`
     if let Some(panic_impl_did) = tcx.lang_items().panic_impl()

src/test/ui/impl-trait/rpit-not-sized.rs

@@ -0,0 +1,6 @@
+fn foo() -> impl ?Sized {
+    //~^ ERROR the size for values of type `impl ?Sized` cannot be known at compilation time
+    ()
+}
+
+fn main() {}

src/test/ui/impl-trait/rpit-not-sized.stderr

@@ -0,0 +1,12 @@
+error[E0277]: the size for values of type `impl ?Sized` cannot be known at compilation time
+  --> $DIR/rpit-not-sized.rs:1:13
+   |
+LL | fn foo() -> impl ?Sized {
+   |             ^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `impl ?Sized`
+   = note: the return type of a function must have a statically known size
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.