Add examples of what can and cannot be used in bindings to functions.

Author: cristianoc

jscomp/gentype_tests/typescript-react-example/src/Core.bs.js

@@ -2,13 +2,28 @@
 /* eslint-disable import/first */
 
 
+import {someFunWithNullThenOptionalArgs as someFunWithNullThenOptionalArgsNotChecked} from './CoreTS';
+
+import {someFunWithNullUndefinedArg as someFunWithNullUndefinedArgNotChecked} from './CoreTS';
+
 const $$toJS552311971: { [key: string]: any } = {"0": "A"};
 
 const $$toRE552311971: { [key: string]: any } = {"A": 0};
 
-// @ts-ignore: Implicit any on import
-import * as CoreBS__Es6Import from './Core.bs';
-const CoreBS: any = CoreBS__Es6Import;
+// In case of type error, check the type of 'someFunWithNullThenOptionalArgs' in 'Core.res' and './CoreTS'.
+export const someFunWithNullThenOptionalArgsTypeChecked: (_1:(null | string), _2:(undefined | string)) => string = someFunWithNullThenOptionalArgsNotChecked;
+
+// Export 'someFunWithNullThenOptionalArgs' early to allow circular import from the '.bs.js' file.
+export const someFunWithNullThenOptionalArgs: unknown = someFunWithNullThenOptionalArgsTypeChecked as (_1:(null | string), _2:(undefined | string)) => string;
+
+// In case of type error, check the type of 'someFunWithNullUndefinedArg' in 'Core.res' and './CoreTS'.
+export const someFunWithNullUndefinedArgTypeChecked: (_1:(null | undefined | string), _2:number) => string = someFunWithNullUndefinedArgNotChecked;
+
+// Export 'someFunWithNullUndefinedArg' early to allow circular import from the '.bs.js' file.
+export const someFunWithNullUndefinedArg: unknown = someFunWithNullUndefinedArgTypeChecked as (_1:(null | undefined | string), _2:number) => string;
+
+// tslint:disable-next-line:no-var-requires
+const CoreBS = require('./Core.bs');
 
 // tslint:disable-next-line:interface-over-type-literal
 export type variant = "A" | { tag: "B"; value: string };

jscomp/gentype_tests/typescript-react-example/src/Core.gen.tsx

@@ -73,4 +73,16 @@ let option1 = (x: option<variant>) => x
 type t1 = {x?: string}
 
 @genType
-type t2 = {x: Js.undefined<string>}
+type t2 = {x: Js.undefined<string>}
+
+@genType.import("./CoreTS")
+external someFunWithNullThenOptionalArgs: (
+  Null.t<string> /* Cannot be Nullable.t or option */,
+  option<string> /* Cannot be Null.t or Nullable.t */,
+) => string = "someFunWithNullThenOptionalArgs"
+
+@genType.import("./CoreTS")
+external someFunWithNullUndefinedArg: (
+  Nullable.t<string> /* Can also be Null.t or option as they are subtypes */,
+  int,
+) => string = "someFunWithNullUndefinedArg"

jscomp/gentype_tests/typescript-react-example/src/Core.res

@@ -0,0 +1,9 @@
+export declare function someFunWithNullThenOptionalArgs(
+  nullable: null | string,
+  app?: string
+): string;
+
+export declare function someFunWithNullUndefinedArg(
+  nullUndefined: null | undefined | string,
+  other: number
+): string;