[flow-remove-types] handle as cast with generics (#9208)

Summary:
Added support for correctly removing `as` casts when generics are involved.

Input:
```js
'm' as $NonMaybeType<string>;
```

Output before (current):
```js
'm'    $NonMaybeType        ;
```

Output after (expected):
```js
'm'                         ;
```

Pull Request resolved: #9208

Reviewed By: gkz

Differential Revision: D62982339

fbshipit-source-id: 286d8627b84bd800f260946ec0eb4a93e25f97ac

packages/flow-remove-types/index.js

@@ -168,7 +168,11 @@ var removeFlowVisitor = {
   AsExpression: function (context, node, ast) {
     var typeIdx = findTokenIndexAtStartOfNode(ast.tokens, node.typeAnnotation);
     removeNode(context, ast.tokens[typeIdx - 1]); // `as` token
-    removeNode(context, node.typeAnnotation);
+    if (node.typeAnnotation.type === 'GenericTypeAnnotation') {
+      removeNode(context, ast.tokens[typeIdx]);
+    } else {
+      removeNode(context, node.typeAnnotation);
+    }
   },
 
   AsConstExpression: function (context, node, ast) {

packages/flow-remove-types/test/expected-pretty.js

@@ -229,6 +229,13 @@ const kk = function(a,) {}
 1;
 [1];
 
+// `as` cast with generics
+'m';
+['a', 'b', 'c'];
+['x', 'y', 'z'];
+const ga = {a: 'b'};
+const gb = {a: 'x', b: 1};
+
 // `as const`
 's';
 ['s'];

packages/flow-remove-types/test/expected-uninitialized-fields.js

@@ -293,6 +293,13 @@ a        ,) {}
 1                   ;
 [1]       ;
 
+// `as` cast with generics
+'m'                         ;
+['a', 'b', 'c']                                            ;
+['x', 'y', 'z']                                     ;
+const ga = {a: 'b'}                                              ;
+const gb = {a: 'x', b: 1}                                  ;
+
 // `as const`
 's'         ;
 ['s']         ;

packages/flow-remove-types/test/expected.js

@@ -293,6 +293,13 @@ a        ,) {}
 1                   ;
 [1]       ;
 
+// `as` cast with generics
+'m'                         ;
+['a', 'b', 'c']                                            ;
+['x', 'y', 'z']                                     ;
+const ga = {a: 'b'}                                              ;
+const gb = {a: 'x', b: 1}                                  ;
+
 // `as const`
 's'         ;
 ['s']         ;

packages/flow-remove-types/test/source.js

@@ -293,6 +293,13 @@ a: string,) {}
 1 as number as mixed;
 [1] as [1];
 
+// `as` cast with generics
+'m' as $NonMaybeType<string>;
+['a', 'b', 'c'] as $Keys<{a: string, b: string, c: number}>;
+['x', 'y', 'z'] as $Values<{a: 'x', b: 'y', c: 'z'}>;
+const ga = {a: 'b'} as $Rest<{a: string, c: number}, {c: number}>;
+const gb = {a: 'x', b: 1} as $Shape<{a: string, b: number}>;
+
 // `as const`
 's' as const;
 ['s'] as const;