clang-format: [JS] handle union types in arrow functions.

[mprobst]

Summary

clang-format would previously fail to detect that an arrow functions parameter block is not an expression, and thus insert whitespace around the | and & type operators in it.

[djasper]

LG

lib/Format/TokenAnnotator.cpp

case tok::colon:
       if (Style.Language == FormatStyle::LK_JavaScript) {
         if (Contexts.back().ColonIsForRangeExpr || // colon in for loop
             (Contexts.size() == 1 &&               // switch/case labels
              !Line.First->isOneOf(tok::kw_enum, tok::kw_case)) ||
             Contexts.back().ContextKind == tok::l_paren ||  // function params
             Contexts.back().ContextKind == tok::l_square || // array type
             (Contexts.size() == 1 &&
              Line.MustBeDeclaration)) { // method/property declaration
+          Contexts.back().IsExpression = false;
           Tok->Type = TT_JsTypeColon;
           break;
         }
       }
       if (Contexts.back().ColonIsDictLiteral ||
           Style.Language == FormatStyle::LK_Proto ||
           Style.Language == FormatStyle::LK_TextProto) {
         Tok->Type = TT_DictLiteral;

unittests/Format/FormatTestJS.cpp

TEST_F(FormatTestJS, ArrowFunctions) {
 
   // FIXME: This is bad, we should be wrapping before "() => {".
   verifyFormat("someFunction(() => {\n"
                "  doSomething();  // break\n"
                "})\n"
                "    .doSomethingElse(\n"
                "        // break\n"
                "    );");
+  verifyFormat("const f = (x: string|null): string|null => {\n"
+               "  return x;\n"
+               "}\n");
 }
 
 TEST_F(FormatTestJS, ReturnStatements) {
   verifyFormat("function() {\n"
                "  return [hello, world];\n"
                "}");
 }