Consume Balanced Braces in Trailing Closure Lookahead

Consuming single tokens isn't enough in this position because,
as this test case demonstrates, any true trailing closures inside
of the braced block will end our lookahead early.

rdar://101330805

Author: CodaFi

Sources/SwiftParser/Expressions.swift

@@ -2294,7 +2294,6 @@ extension Parser.Lookahead {
     //   {...}()
     // by looking ahead for the ()'s, but this has been replaced by do{}, so this
     // probably isn't worthwhile.
-    //
     guard case .basic = flavor else {
       return true
     }
@@ -2317,8 +2316,8 @@ extension Parser.Lookahead {
     var backtrack = self.lookahead()
     backtrack.eat(.leftBrace)
     var loopProgress = LoopProgressCondition()
-    while !backtrack.at(any: [.eof, .rightBrace]) && loopProgress.evaluate(backtrack.currentToken) {
-      backtrack.consumeAnyToken()
+    while !backtrack.at(any: [.eof, .rightBrace, .poundEndifKeyword, .poundElseKeyword, .poundElseifKeyword ]) && loopProgress.evaluate(backtrack.currentToken) {
+      backtrack.skipSingle()
     }
 
     guard backtrack.consume(if: .rightBrace) != nil else {

Tests/SwiftParserTest/Statements.swift

@@ -49,6 +49,11 @@ final class StatementTests: XCTestCase {
         DiagnosticSpec(message: "unexpected code '* ! = x' in 'if' statement"),
       ]
     )
+
+    AssertParse(
+      """
+      if includeSavedHints { a = a.flatMap{ $0 } ?? nil }
+      """)
   }
 
   func testNestedIfs() {