[SwiftParser] Fix lookahead accepting invalid closure signatures

Sources/SwiftParser/Expressions.swift

@@ -2718,8 +2718,10 @@ extension Parser.Lookahead {
     }
 
     // Parse pattern-tuple func-signature-result? 'in'.
+    var isBracketed = false
     if lookahead.at(.leftParen) {  // Consume the '('.
       // While we don't have '->' or ')', eat balanced tokens.
+      isBracketed = true
       var skipProgress = LoopProgressCondition()
       while !lookahead.at(.endOfFile, .rightBrace, .keyword(.in)) && !lookahead.at(.arrow)
         && lookahead.hasProgressed(&skipProgress)
@@ -2754,7 +2756,10 @@ extension Parser.Lookahead {
     }
 
     // Consume the ')', if it's there.
-    lookahead.consume(if: .rightParen)
+    var hasRightParen = false
+    if isBracketed {
+      hasRightParen = lookahead.consume(if: .rightParen) != nil
+    }
 
     lookahead.consumeEffectsSpecifiers()
 
@@ -2775,9 +2780,14 @@ extension Parser.Lookahead {
       return true
     }
 
+    // If we started with '(', we MUST have found a ')' to trust the arrow.
+    if isBracketed && !hasRightParen {
+      return false
+    }
     // Even if 'in' is missing, the presence of a top-level '->' makes this look like a
     // closure signature. There's no other valid syntax that could legally
     // contain '->' at this position.
+
     return sawTopLevelArrowInLookahead
   }
 }

Tests/SwiftParserTest/Issue3163.swift

@@ -0,0 +1,23 @@
+import SwiftParser
+import SwiftParserDiagnostics
+import SwiftSyntax
+import XCTest
+
+public class Issue3163Tests: XCTestCase {
+  func testFixForIssue3163() {
+    let source = "struct Foo {func a(s: S [{{g) -> Int {}}}}"
+
+    // Parse the code
+    let tree = Parser.parse(source: source)
+
+    // Get the error messages
+    let diagnostics = ParseDiagnosticsGenerator.diagnostics(for: tree)
+
+    // CHECK: Ensure the specific "missing 'in'" error is NOT present
+    let hasBadError = diagnostics.contains { $0.message.contains("expected 'in' in closure signature") }
+
+    // This will pass if the error is gone (FIXED).
+    // This will fail if the error is still there (BROKEN).
+    XCTAssertFalse(hasBadError, "The parser incorrectly identified a closure signature and asked for 'in'")
+  }
+}