Fix parsing invalid function parameters

#2254

Author: kimdv

Sources/SwiftParser/Parameters.swift

@@ -96,11 +96,31 @@ extension Parser {
     let modifiers = parseParameterModifiers(isClosure: false)
     let misplacedSpecifiers = parseMisplacedSpecifiers()
 
-    var names = self.parseParameterNames()
-    let (unexpectedBeforeColon, colon) = self.expect(.colon)
+    var names: ParameterNames
 
+    let unexpectedBeforeColon: RawUnexpectedNodesSyntax?
+    let colon: RawTokenSyntax
     let type: RawTypeSyntax
 
+    // try to parse the type regardless of the presence of the preceding colon
+    // to tackle any unnamed parameter or missing colon
+    // e.g. [X], (:[X]) or (x [X])
+    let canParseType = withLookahead { $0.canParseType() && $0.at(.comma, .rightParen) }
+
+    if canParseType {
+      names = ParameterNames(
+        unexpectedBeforeFirstName: nil,
+        firstName: nil,
+        unexpectedBeforeSecondName: nil,
+        secondName: nil
+      )
+      unexpectedBeforeColon = nil
+      colon = missingToken(.colon)
+    } else {
+      names = self.parseParameterNames()
+      (unexpectedBeforeColon, colon) = self.expect(.colon)
+    }
+
     if colon.presence == .missing,
       let secondName = names.secondName,
       secondName.tokenKind == .identifier,

Tests/SwiftParserTest/ExpressionTests.swift

@@ -3369,4 +3369,30 @@ final class StatementExpressionTests: ParserTestCase {
       substructureAfterMarker: "1️⃣"
     )
   }
+
+  func testFunctionParameterWithMalformedParameters() {
+    assertParse(
+      """
+      init(1️⃣String.Index, x:(Int) -> Int) rethrows
+      """,
+      diagnostics: [
+        DiagnosticSpec(message: "expected identifier and ':' in parameter", fixIts: ["insert identifier and ':'"])
+      ],
+      fixedSource: """
+        init(<#identifier#>: String.Index, x:(Int) -> Int) rethrows
+        """
+    )
+
+    assertParse(
+      """
+      init(1️⃣String.Index) rethrows
+      """,
+      diagnostics: [
+        DiagnosticSpec(message: "expected identifier and ':' in parameter", fixIts: ["insert identifier and ':'"])
+      ],
+      fixedSource: """
+        init(<#identifier#>: String.Index) rethrows
+        """
+    )
+  }
 }