Fix single optional arrow param printing (#1002)

src/printer.js

@@ -315,6 +315,7 @@ function genericPrintNoParens(path, options, print) {
         !n.params[0].typeAnnotation &&
         !n.params[0].leadingComments &&
         !n.params[0].trailingComments &&
+        !n.params[0].optional &&
         !n.predicate &&
         !n.returnType
       ) {

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

@@ -30,6 +30,7 @@ x => ({}().b);
 (a => b)::c;
 a::(b => c);
 a = b => c;
+a = (b?) => c;
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 (a => {}).length;
 typeof (() => {});
@@ -61,6 +62,7 @@ x => ({}().b);
 (a => b)::c;
 a::(b => c);
 a = b => c;
+a = (b?) => c;
 "
 `;
 

tests/arrows/arrow_function_expression.js

@@ -27,3 +27,4 @@ x => ({}().b);
 (a => b)::c;
 a::(b => c);
 a = b => c;
+a = (b?) => c;