Seal the JSX attributes object if there is no spread

Summary:
Currently, if you try to use exact objects for react props, you get
errors about flowing inexact objects to exact objects. This diff does NOT fix
that. To fix that, we probably need to fiddle a bunch more with the complicated
react machinery.

However, the `jsx` pragma code is much more simple. So by sealing this object,
we allow the jsx function to expect an exact object type. See the test for an
example.

Reviewed By: samwgoldman

Differential Revision: D4015376

fbshipit-source-id: 9810691c1fe4601f1c0d19e10c5c75b764310e2d

Author: gabelevi

newtests/jsx_pragma/test.js

@@ -272,5 +272,34 @@ export default suite(({addFile, addFiles, addCode}) => [
                      ^^^^^^^^^^^^^^ JSX element \`Bar\`. Could not resolve name
         `,
       ),
-  ])
+  ]),
+  test('Exact prop type without spread should work', [
+    addCode(`
+      // @jsx Foo
+      function Foo(elem: number, props: {| x: string |}) {}
+      const Bar = 123;
+
+      <Bar x="hi" />;
+    `).noNewErrors(),
+  ]),
+  test('Exact prop type with spread still doesnt work', [
+    addCode(`
+      // @jsx Foo
+      function Foo(elem: number, props: {| x: string |}) {}
+      const Bar = 123;
+
+      const props = {x: "hi"};
+      <Bar {...props} />;
+    `).newErrors(
+        `
+          test.js:9
+            9:       <Bar {...props} />;
+                     ^^^^^^^^^^^^^^^^^^ JSX desugared to \`Foo(...)\`
+            9:       <Bar {...props} />;
+                              ^^^^^ spread of object literal. Inexact type is incompatible with exact type
+            5:       function Foo(elem: number, props: {| x: string |}) {}
+                                                       ^^^^^^^^^^^^^^^ exact type: object type
+        `,
+      ),
+  ]),
 ]);

src/typing/statement.ml

@@ -3496,7 +3496,7 @@ and jsx_title cx openingElement children = Ast.JSX.(
     let reason_props = replace_reason_const
       (if is_react then RReactElementProps name else RJSXElementProps name)
       reason in
-    let o = Flow.mk_object_with_map_proto cx reason_props
+    let o = Flow.mk_object_with_map_proto cx reason_props ~sealed:(!spread=None)
       !map (MixedT (reason_props, Mixed_everything))
     in
     match !spread with