Add diagnostic for wrong where requirements separation

Sources/SwiftParser/Declarations.swift

@@ -759,9 +759,21 @@ extension Parser {
         }
 
         keepGoing = self.consume(if: .comma)
+        let unexpectedBetweenBodyAndTrailingComma: RawUnexpectedNodesSyntax?
+
+        // If there's a comma, keep parsing the list.
+        // If there's a "&&", diagnose replace with a comma and keep parsing
+        if let token = self.consumeIfContextualPunctuator("&&") {
+          keepGoing = self.missingToken(.comma)
+          unexpectedBetweenBodyAndTrailingComma = RawUnexpectedNodesSyntax([token], arena: self.arena)
+        } else {
+          unexpectedBetweenBodyAndTrailingComma = nil
+        }
+
         elements.append(
           RawGenericRequirementSyntax(
             body: requirement,
+            unexpectedBetweenBodyAndTrailingComma,
             trailingComma: keepGoing,
             arena: self.arena
           )

Sources/SwiftParserDiagnostics/ParseDiagnosticsGenerator.swift

@@ -542,6 +542,38 @@ public class ParseDiagnosticsGenerator: SyntaxAnyVisitor {
     return handleEffectSpecifiers(node)
   }
 
+  public override func visit(_ node: GenericRequirementSyntax) -> SyntaxVisitorContinueKind {
+    if shouldSkip(node) {
+      return .skipChildren
+    }
+
+    if let unexpected = node.unexpectedBetweenBodyAndTrailingComma,
+      let token = unexpected.tokens(satisfying: { $0.tokenKind == .binaryOperator("&&") }).first,
+      let trailingComma = node.trailingComma,
+      trailingComma.presence == .missing,
+      let previous = node.unexpectedBetweenBodyAndTrailingComma?.previousToken(viewMode: .sourceAccurate)
+    {
+
+      addDiagnostic(
+        unexpected,
+        .expectedCommaInWhereClause,
+        fixIts: [
+          FixIt(
+            message: ReplaceTokensFixIt(replaceTokens: [token], replacement: .commaToken()),
+            changes: [
+              .makeMissing(token),
+              .makePresent(trailingComma),
+              FixIt.MultiNodeChange(.replaceTrailingTrivia(token: previous, newTrivia: [])),
+            ]
+          )
+        ],
+        handledNodes: [unexpected.id, trailingComma.id]
+      )
+    }
+
+    return .visitChildren
+  }
+
   public override func visit(_ node: DeinitializerDeclSyntax) -> SyntaxVisitorContinueKind {
     if shouldSkip(node) {
       return .skipChildren

Sources/SwiftParserDiagnostics/ParserDiagnosticMessages.swift

@@ -131,6 +131,9 @@ extension DiagnosticMessage where Self == StaticParserError {
   public static var expectedAssignmentInsteadOfComparisonOperator: Self {
     .init("expected '=' instead of '==' to assign default value for parameter")
   }
+  public static var expectedCommaInWhereClause: Self {
+    .init("expected ',' to separate the requirements of this 'where' clause")
+  }
   public static var expectedLeftBraceOrIfAfterElse: Self {
     .init("expected '{' or 'if' after 'else'")
   }

Tests/SwiftParserTest/translated/RecoveryTests.swift

@@ -2174,15 +2174,17 @@ final class RecoveryTests: XCTestCase {
   }
 
   func testRecovery177() {
+    // rdar://38225184
     assertParse(
       """
-      // rdar://38225184
       extension Collection where Element == Int 1️⃣&& Index == Int {}
       """,
       diagnostics: [
-        DiagnosticSpec(message: "unexpected code '&& Index == Int' in extension")
-        // TODO: Old parser expected error on line 1: expected ',' to separate the requirements of this 'where' clause, Fix-It replacements: 43 - 45 = ','
-      ]
+        DiagnosticSpec(message: "expected ',' to separate the requirements of this 'where' clause", fixIts: ["replace '&&' with ','"])
+      ],
+      fixedSource: """
+        extension Collection where Element == Int, Index == Int {}
+        """
     )
   }