Diagnose if class is being used as an inherited type in a generic parameter clause

Sources/SwiftParser/Declarations.swift

@@ -349,14 +349,25 @@ extension Parser {
 
         // Parse the ':' followed by a type.
         let colon = self.consume(if: .colon)
+        let unexpectedBeforeInherited: RawUnexpectedNodesSyntax?
         let inherited: RawTypeSyntax?
         if colon != nil {
           if self.at(any: [.identifier, .protocolKeyword, .anyKeyword]) {
+            unexpectedBeforeInherited = nil
             inherited = self.parseType()
+          } else if let classKeyword = self.consume(if: .classKeyword) {
+            unexpectedBeforeInherited = RawUnexpectedNodesSyntax([classKeyword], arena: self.arena)
+            inherited = RawTypeSyntax(RawSimpleTypeIdentifierSyntax(
+              name: missingToken(.identifier, text: "AnyObject"),
+              genericArgumentClause: nil,
+              arena: self.arena
+            ))
           } else {
-            inherited = nil
+            unexpectedBeforeInherited = nil
+            inherited = RawTypeSyntax(RawMissingTypeSyntax(arena: self.arena))
           }
         } else {
+          unexpectedBeforeInherited = nil
           inherited = nil
         }
         keepGoing = self.consume(if: .comma)
@@ -366,6 +377,7 @@ extension Parser {
           name: name,
           ellipsis: ellipsis,
           colon: colon,
+          unexpectedBeforeInherited,
           inheritedType: inherited,
           trailingComma: keepGoing,
           arena: self.arena))

Sources/SwiftParser/Diagnostics/ParseDiagnosticsGenerator.swift

@@ -491,6 +491,22 @@ public class ParseDiagnosticsGenerator: SyntaxAnyVisitor {
     return .visitChildren
   }
 
+  public override func visit(_ node: GenericParameterSyntax) -> SyntaxVisitorContinueKind {
+    if shouldSkip(node) {
+      return .skipChildren
+    }
+    if let inheritedTypeName = node.inheritedType?.as(SimpleTypeIdentifierSyntax.self)?.name {
+      exchangeTokens(
+        unexpected: node.unexpectedBetweenColonAndInheritedType,
+        unexpectedTokenCondition: { $0.tokenKind == .classKeyword },
+        correctTokens: [inheritedTypeName],
+        message: { _ in StaticParserError.classConstraintCanOnlyBeUsedInProtocol },
+        moveFixIt: { ReplaceTokensFixIt(replaceTokens: $0, replacement: inheritedTypeName) }
+      )
+    }
+    return .visitChildren
+  }
+
   public override func visit(_ node: IdentifierExprSyntax) -> SyntaxVisitorContinueKind {
     if shouldSkip(node) {
       return .skipChildren

Sources/SwiftParser/Diagnostics/ParserDiagnosticMessages.swift

@@ -68,6 +68,7 @@ public extension ParserFixIt {
 public enum StaticParserError: String, DiagnosticMessage {
   case allStatmentsInSwitchMustBeCoveredByCase = "all statements inside a switch must be covered by a 'case' or 'default' label"
   case caseOutsideOfSwitchOrEnum = "'case' can only appear inside a 'switch' statement or 'enum' declaration"
+  case classConstraintCanOnlyBeUsedInProtocol = "'class' constraint can only appear on protocol declarations"
   case consecutiveDeclarationsOnSameLine = "consecutive declarations on a line must be separated by ';'"
   case consecutiveStatementsOnSameLine = "consecutive statements on a line must be separated by ';'"
   case cStyleForLoop = "C-style for statement has been removed in Swift 3"

Tests/SwiftParserTest/translated/InvalidTests.swift

@@ -489,19 +489,17 @@ final class InvalidTests: XCTestCase {
   func testInvalid29() {
     AssertParse(
       """
-      struct Weak<T: 1️⃣class2️⃣> {
+      struct Weak<T: 1️⃣class> {
         weak let value: T
       }
       """,
       diagnostics: [
-        // TODO: Old parser expected error on line 1: 'class' constraint can only appear on protocol declarations
-        // TODO: Old parser expected note on line 1: did you mean to write an 'AnyObject' constraint?, Fix-It replacements: 16 - 21 = 'AnyObject'
-        DiagnosticSpec(locationMarker: "1️⃣", message: "expected '>' to end generic parameter clause"),
-        DiagnosticSpec(locationMarker: "1️⃣", message: "expected '{' in struct"),
-        DiagnosticSpec(locationMarker: "2️⃣", message: "expected name and member block in class"),
-        DiagnosticSpec(locationMarker: "2️⃣", message: "expected '}' to end struct"),
-        DiagnosticSpec(locationMarker: "2️⃣", message: "extraneous code at top level"),
-      ]
+        DiagnosticSpec(message: "'class' constraint can only appear on protocol declarations", fixIts: ["replace 'class' by 'AnyObject'"]),
+      ], fixedSource: """
+      struct Weak<T: AnyObject> {
+        weak let value: T
+      }
+      """
     )
   }