Resolve an ambiguity with the multiple-trailing-closure syntax in favor of parsing default: labels.

Author: rjmccall

lib/Parse/ParseExpr.cpp

@@ -3149,8 +3149,12 @@ ParserStatus Parser::parseExprList(tok leftTok, tok rightTok,
 }
 
 static bool isStartOfLabelledTrailingClosure(Parser &P) {
-  // Fast path: the next two tokens are a label and a colon.
-  if (!P.Tok.canBeArgumentLabel() || !P.peekToken().is(tok::colon))
+  // Fast path: the next two tokens must be a label and a colon.
+  // But 'default:' is ambiguous with switch cases and we disallow it
+  // (unless escaped) even outside of switches.
+  if (!P.Tok.canBeArgumentLabel() ||
+      P.Tok.is(tok::kw_default) ||
+      !P.peekToken().is(tok::colon))
     return false;
 
   // Do some tentative parsing to distinguish `label: { ... }` and

test/Parse/multiple_trailing_closures.swift

@@ -116,3 +116,10 @@ func test_multiple_trailing_syntax_without_labels() {
     _: {}
     _: {}
 }
+
+func produce(fn: () -> Int?, default d: () -> Int) -> Int { // expected-note {{declared here}}
+  return fn() ?? d()
+}
+// TODO: The diagnostics here are perhaps a little overboard.
+_ = produce { 0 } default: { 1 } // expected-error {{missing argument for parameter 'fn' in call}} expected-error {{consecutive statements}} expected-error {{'default' label can only appear inside a 'switch' statement}} expected-error {{top-level statement cannot begin with a closure expression}} expected-error {{closure expression is unused}} expected-note {{did you mean to use a 'do' statement?}}
+_ = produce { 2 } `default`: { 3 }