Fix diagnostic for wrong where in if statement

Sources/SwiftParser/Statements.swift

@@ -188,8 +188,8 @@ extension Parser {
       let condition = self.parseConditionElement(lastBindingKind: elements.last?.condition.as(RawOptionalBindingConditionSyntax.self)?.bindingKeyword)
       var unexpectedBeforeKeepGoing: RawUnexpectedNodesSyntax? = nil
       keepGoing = self.consume(if: .comma)
-      if keepGoing == nil, let andOperator = self.consumeIfContextualPunctuator("&&") {
-        unexpectedBeforeKeepGoing = RawUnexpectedNodesSyntax(combining: unexpectedBeforeKeepGoing, andOperator, arena: self.arena)
+      if keepGoing == nil, let token = self.consumeIfContextualPunctuator("&&") ?? self.consume(if: .keyword(.where)) {
+        unexpectedBeforeKeepGoing = RawUnexpectedNodesSyntax(combining: unexpectedBeforeKeepGoing, token, arena: self.arena)
         keepGoing = missingToken(.comma)
       }
       elements.append(

Sources/SwiftParserDiagnostics/ParseDiagnosticsGenerator.swift

@@ -582,7 +582,7 @@ public class ParseDiagnosticsGenerator: SyntaxAnyVisitor {
     if let trailingComma = node.trailingComma {
       exchangeTokens(
         unexpected: node.unexpectedBetweenConditionAndTrailingComma,
-        unexpectedTokenCondition: { $0.text == "&&" },
+        unexpectedTokenCondition: { $0.text == "&&" || $0.tokenKind == .keyword(.where) },
         correctTokens: [node.trailingComma],
         message: { _ in .joinConditionsUsingComma },
         moveFixIt: { ReplaceTokensFixIt(replaceTokens: $0, replacements: [trailingComma]) }

Tests/SwiftParserTest/translated/RecoveryTests.swift

@@ -2394,25 +2394,25 @@ final class RecoveryTests: XCTestCase {
   func testRecovery151() {
     // <rdar://problem/20489838> QoI: Nonsensical error and fixit if "let" is missing between 'if let ... where' clauses
     assertParse(
-      #"""
+      """
       if let y = x 1️⃣where y == 0, let z = x {
         _ = y
         _ = z
-      }2️⃣
-      """#,
+      }
+      """,
       diagnostics: [
-        // TODO: Old parser expected error on line 4: expected ',' joining parts of a multi-clause condition, Fix-It replacements: 15 - 21 = ','
-        DiagnosticSpec(locationMarker: "1️⃣", message: "expected '{' in 'if' statement", fixIts: ["insert '{'"]),
-        DiagnosticSpec(locationMarker: "1️⃣", message: "unexpected code 'where y == 0,' before variable"),
-        DiagnosticSpec(locationMarker: "2️⃣", message: "expected '}' to end 'if' statement", fixIts: ["insert '}'"]),
+        DiagnosticSpec(
+          message: "expected ',' joining parts of a multi-clause condition",
+          fixIts: ["replace 'where' with ','"]
+        )
       ],
-      fixedSource: #"""
-        if let y = x {where y == 0, let z = x {
+      fixedSource: """
+        if let y = x, y == 0, let z = x {
           _ = y
           _ = z
         }
-        }
-        """#
+        """
+
     )
   }