Added simple test case

Author: Kivooeo

tests/ui/traits/issue-139999.rs

@@ -0,0 +1,6 @@
+fn do_something() -> Option<ForgotToImport> {
+    //~^ cannot find type `ForgotToImport` in this scope [E0412]
+    None
+}
+
+fn main() {}

tests/ui/traits/issue-139999.stderr

@@ -0,0 +1,9 @@
+error[E0412]: cannot find type `ForgotToImport` in this scope
+  --> $DIR/issue-139999.rs:1:29
+   |
+LL | fn do_something() -> Option<ForgotToImport> {
+   |                             ^^^^^^^^^^^^^^ not found in this scope
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0412`.