Add test for issue-63279

src/test/ui/type-alias-impl-trait/issue-63279.rs

@@ -0,0 +1,9 @@
+#![feature(type_alias_impl_trait)]
+
+type Closure = impl FnOnce(); //~ ERROR: type mismatch resolving
+
+fn c() -> Closure {
+    || -> Closure { || () }
+}
+
+fn main() {}

src/test/ui/type-alias-impl-trait/issue-63279.stderr

@@ -0,0 +1,13 @@
+error[E0271]: type mismatch resolving `<[closure@$DIR/issue-63279.rs:6:5: 6:28] as std::ops::FnOnce<()>>::Output == ()`
+  --> $DIR/issue-63279.rs:3:1
+   |
+LL | type Closure = impl FnOnce();
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected opaque type, found ()
+   |
+   = note: expected type `Closure`
+              found type `()`
+   = 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 E0271`.