Test tait use in a fn type

src/test/ui/type-alias-impl-trait/type-alias-impl-trait-fn-type.rs

@@ -0,0 +1,13 @@
+#![feature(type_alias_impl_trait)]
+#![allow(dead_code)]
+
+// FIXME: this is ruled out for now but should work
+
+type Foo = fn() -> impl Send;
+//~^ ERROR: `impl Trait` not allowed outside of function and method return types
+
+fn make_foo() -> Foo {
+    || 15
+}
+
+fn main() {}

src/test/ui/type-alias-impl-trait/type-alias-impl-trait-fn-type.stderr

@@ -0,0 +1,9 @@
+error[E0562]: `impl Trait` not allowed outside of function and method return types
+  --> $DIR/type-alias-impl-trait-fn-type.rs:6:20
+   |
+LL | type Foo = fn() -> impl Send;
+   |                    ^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0562`.