Add diagnostic for comparison in default value

Author: kimdv

Sources/SwiftParser/Declarations.swift

@@ -1229,7 +1229,7 @@ extension Parser {
     }
 
     let defaultArgument: RawInitializerClauseSyntax?
-    if self.at(.equal) {
+    if self.at(.equal) || self.atContextualPunctuator("==") {
       defaultArgument = self.parseDefaultArgument()
     } else {
       defaultArgument = nil

Sources/SwiftParser/Expressions.swift

@@ -1671,7 +1671,18 @@ extension Parser {
 extension Parser {
   @_spi(RawSyntax)
   public mutating func parseDefaultArgument() -> RawInitializerClauseSyntax {
-    let (unexpectedBeforeEq, eq) = self.expect(.equal)
+    let unexpectedBeforeEq: RawUnexpectedNodesSyntax?
+    let eq: RawTokenSyntax
+    if let comparison = self.consumeIfContextualPunctuator("==") {
+      unexpectedBeforeEq = RawUnexpectedNodesSyntax(
+        elements: [RawSyntax(comparison)],
+        arena: self.arena
+      )
+      eq = missingToken(.equal)
+    } else {
+      (unexpectedBeforeEq, eq) = self.expect(.equal)
+    }
+
     let expr = self.parseExpression()
     return RawInitializerClauseSyntax(
       unexpectedBeforeEq,

Sources/SwiftParserDiagnostics/ParseDiagnosticsGenerator.swift

@@ -771,6 +771,23 @@ public class ParseDiagnosticsGenerator: SyntaxAnyVisitor {
     if shouldSkip(node) {
       return .skipChildren
     }
+
+    if let unexpected = node.unexpectedBeforeEqual,
+      unexpected.first?.as(TokenSyntax.self)?.tokenKind == .binaryOperator("==")
+    {
+      addDiagnostic(
+        unexpected,
+        .expectedAssignmentInsteadOfComparisonOperator,
+        fixIts: [
+          FixIt(
+            message: ReplaceTokensFixIt(replaceTokens: [.binaryOperator("==")], replacement: node.equal),
+            changes: [.makeMissing(unexpected), .makePresent(node.equal, leadingTrivia: [])]
+          )
+        ],
+        handledNodes: [unexpected.id, node.equal.id]
+      )
+    }
+
     if node.equal.presence == .missing {
       exchangeTokens(
         unexpected: node.unexpectedBeforeEqual,

Sources/SwiftParserDiagnostics/ParserDiagnosticMessages.swift

@@ -128,6 +128,9 @@ extension DiagnosticMessage where Self == StaticParserError {
   public static var expectedExpressionAfterTry: Self {
     .init("expected expression after 'try'")
   }
+  public static var expectedAssignmentInsteadOfComparisonOperator: Self {
+    .init("expected '=' instead of '==' to assign default value for parameter")
+  }
   public static var expectedSequenceExpressionInForEachLoop: Self {
     .init("expected Sequence expression for for-each loop")
   }

Tests/SwiftParserTest/translated/RecoveryTests.swift

@@ -2146,9 +2146,13 @@ final class RecoveryTests: XCTestCase {
       func f_53396(a: Int 1️⃣== 0) {}
       """,
       diagnostics: [
-        // TODO: Old parser expected error on line 3: expected '=' instead of '==' to assign default value for parameter, Fix-It replacements: 21 - 23 = '='
-        DiagnosticSpec(message: "unexpected code '== 0' in parameter clause")
-      ]
+        DiagnosticSpec(message: "expected '=' instead of '==' to assign default value for parameter", fixIts: ["replace '==' with '='"])
+      ],
+      fixedSource: """
+        // rdar://problem/22478168
+        // https://github.com/apple/swift/issues/53396
+        func f_53396(a: Int = 0) {}
+        """
     )
   }