Wrap ternaries in Flow typecasts in parens (#3940)

* Wrap TypeCastExpression in parens

* Tests

src/common/fast-path.js

@@ -560,6 +560,7 @@ FastPath.prototype.needsParens = function(options) {
         case "AwaitExpression":
         case "JSXSpreadAttribute":
         case "TSTypeAssertionExpression":
+        case "TypeCastExpression":
         case "TSAsExpression":
         case "TSNonNullExpression":
           return true;

tests/flow_type_cast/__snapshots__/jsfmt.spec.js.snap

@@ -0,0 +1,25 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`expression.js 1`] = `
+let x: string = (foo: string);
+
+// https://github.com/prettier/prettier/issues/3936
+const foo = ((1?2:3): number);
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+let x: string = (foo: string);
+
+// https://github.com/prettier/prettier/issues/3936
+const foo = ((1 ? 2 : 3): number);
+
+`;
+
+exports[`statement.js 1`] = `
+foo: string;
+bar: number;
+(foo.bar: SomeType);
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+foo: string;
+bar: number;
+(foo.bar: SomeType);
+
+`;

tests/flow_type_cast/expression.js

@@ -0,0 +1,4 @@
+let x: string = (foo: string);
+
+// https://github.com/prettier/prettier/issues/3936
+const foo = ((1?2:3): number);

tests/flow_type_cast/jsfmt.spec.js

@@ -0,0 +1 @@
+run_spec(__dirname, ["flow", "babylon"]);

tests/flow_type_cast/statement.js

@@ -0,0 +1,3 @@
+foo: string;
+bar: number;
+(foo.bar: SomeType);