improve diagnostic for unnamed closure parameters

- instead of treating as unrecognized, try to parse the remaining tokens as a type even if the preceding colon is missing

Author: AppAppWorks

Sources/SwiftParser/Parameters.swift

@@ -160,11 +160,20 @@ extension Parser {
     let misplacedSpecifiers = parseMisplacedSpecifiers()
 
     let names = self.parseParameterNames()
-    let colon = self.consume(if: .colon)
-    let type: RawTypeSyntax?
-    if colon != nil {
-      type = self.parseType(misplacedSpecifiers: misplacedSpecifiers)
-    } else {
+    var colon = self.consume(if: .colon)
+    // try to parse the type regardless of the presence of the preceding colon
+    // because the parameter may be unnamed
+    // e.g. { [X] in } or { (:[X]) in }
+    var type: RawTypeSyntax? = self.parseType(misplacedSpecifiers: misplacedSpecifiers)
+    if type?.raw.kind != .missingType {
+      if colon == nil {
+        // mark the preceding colon as missing if the parameter is unnamed
+        // e.g. { [X] in }
+        colon = missingToken(.colon)
+      }
+    } else if colon == nil {
+      // don't mark the type as missing if the preceding colon is absent
+      // e.g. { _ in }
       type = nil
     }
 

Tests/SwiftParserTest/translated/RecoveryTests.swift

@@ -3355,4 +3355,19 @@ final class RecoveryTests: ParserTestCase {
       ]
     )
   }
+  
+  func testRecovery185() {
+    // <rdar://1181099123>
+    assertParse(
+      """
+      test { (1️⃣[X]) in }
+      """,
+      diagnostics: [
+        DiagnosticSpec(message: "expected identifier and ':' in parameter", fixIts: ["insert identifier and ':'"]),
+      ],
+      fixedSource: """
+        test { (<#identifier#>: [X]) in }
+        """
+    )
+  }
 }