Fix fill wrapping when text starts with whitespace (#1666)

src/printer.js

@@ -3585,6 +3585,11 @@ function printJSXElement(path, options, print) {
         multilineChildren.push(rawJsxWhitespace);
         return;
       } else if (i === 0) {
+        // Fill expects alternating content & whitespace parts
+        // always starting with content.
+        // So we add a dummy content element if we would otherwise start
+        // with whitespace.
+        multilineChildren.push("");
         multilineChildren.push(concat([rawJsxWhitespace, hardline]));
         return;
       } else if (i === children.length - 1) {

tests/jsx-text-wrap/__snapshots__/jsfmt.spec.js.snap

@@ -96,6 +96,12 @@ x =
       Second
     </div> Third
   </div>
+
+leading_whitespace =
+  <div> First Second Third Fourth Fifth Sixth Seventh Eighth Ninth Tenth Eleventh Twelfth Thirteenth Fourteenth</div>
+
+no_leading_whitespace =
+  <div>First Second Third Fourth Fifth Sixth Seventh Eighth Ninth Tenth Eleventh Twelfth Thirteenth Fourteenth</div>
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 // Wrapping text
 x = (
@@ -241,4 +247,19 @@ x = (
   </div>
 );
 
+leading_whitespace = (
+  <div>
+    {" "}
+    First Second Third Fourth Fifth Sixth Seventh Eighth Ninth Tenth Eleventh
+    Twelfth Thirteenth Fourteenth
+  </div>
+);
+
+no_leading_whitespace = (
+  <div>
+    First Second Third Fourth Fifth Sixth Seventh Eighth Ninth Tenth Eleventh
+    Twelfth Thirteenth Fourteenth
+  </div>
+);
+
 `;

tests/jsx-text-wrap/test.js

@@ -93,3 +93,9 @@ x =
       Second
     </div> Third
   </div>
+
+leading_whitespace =
+  <div> First Second Third Fourth Fifth Sixth Seventh Eighth Ninth Tenth Eleventh Twelfth Thirteenth Fourteenth</div>
+
+no_leading_whitespace =
+  <div>First Second Third Fourth Fifth Sixth Seventh Eighth Ninth Tenth Eleventh Twelfth Thirteenth Fourteenth</div>