Merge pull request #1302 from TiagoMaiaL/fix-else_if-diagnostic

Author: ahoppen

Sources/SwiftParser/Directives.swift

@@ -106,15 +106,21 @@ extension Parser {
       while let poundIfHandle = firstIteration ? self.canRecoverTo(.poundIfKeyword) : self.canRecoverTo(anyIn: IfConfigContinuationClauseStartKeyword.self)?.handle,
         loopProgress.evaluate(self.currentToken)
       {
-        let (unexpectedBeforePoundIf, poundIf) = self.eat(poundIfHandle)
+        var (unexpectedBeforePoundIf, poundIf) = self.eat(poundIfHandle)
         firstIteration = false
         // Parse the condition.
         let condition: RawExprSyntax?
         switch poundIf.tokenKind {
         case .poundIfKeyword, .poundElseifKeyword:
           condition = RawExprSyntax(self.parseSequenceExpression(.basic, forDirective: true))
         case .poundElseKeyword:
-          condition = nil
+          if let ifToken = self.consume(if: .init(.if, allowAtStartOfLine: false)) {
+            unexpectedBeforePoundIf = RawUnexpectedNodesSyntax(combining: unexpectedBeforePoundIf, poundIf, ifToken, arena: self.arena)
+            poundIf = self.missingToken(.poundElseifKeyword)
+            condition = RawExprSyntax(self.parseSequenceExpression(.basic, forDirective: true))
+          } else {
+            condition = nil
+          }
         default:
           preconditionFailure("The loop condition should guarantee that we are at one of these tokens")
         }

Sources/SwiftParserDiagnostics/ParseDiagnosticsGenerator.swift

@@ -668,6 +668,37 @@ public class ParseDiagnosticsGenerator: SyntaxAnyVisitor {
     return .visitChildren
   }
 
+  public override func visit(_ node: IfConfigDeclSyntax) -> SyntaxVisitorContinueKind {
+    for clause in node.clauses where clause.hasError {
+      if let unexpectedBeforePoundKeyword = clause.unexpectedBeforePoundKeyword,
+        clause.poundKeyword.tokenKind == .poundElseifKeyword,
+        clause.poundKeyword.presence == .missing
+      {
+        let unexpectedTokens =
+          unexpectedBeforePoundKeyword
+          .suffix(2)
+          .compactMap { $0.as(TokenSyntax.self) }
+        if unexpectedTokens.map(\.tokenKind) == [.poundElseKeyword, .keyword(.if)] {
+          addDiagnostic(
+            unexpectedBeforePoundKeyword,
+            StaticParserError.unexpectedPoundElseSpaceIf,
+            fixIts: [
+              FixIt(
+                message: ReplaceTokensFixIt(replaceTokens: unexpectedTokens, replacement: clause.poundKeyword),
+                changes: [
+                  .makeMissing(unexpectedBeforePoundKeyword, transferTrivia: false),
+                  .makePresent(clause.poundKeyword, leadingTrivia: unexpectedBeforePoundKeyword.leadingTrivia),
+                ]
+              )
+            ],
+            handledNodes: [unexpectedBeforePoundKeyword.id, clause.poundKeyword.id]
+          )
+        }
+      }
+    }
+    return .visitChildren
+  }
+
   public override func visit(_ node: InitializerClauseSyntax) -> SyntaxVisitorContinueKind {
     if shouldSkip(node) {
       return .skipChildren

Sources/SwiftParserDiagnostics/ParserDiagnosticMessages.swift

@@ -197,6 +197,9 @@ extension DiagnosticMessage where Self == StaticParserError {
   public static var tryOnInitialValueExpression: Self {
     .init("'try' must be placed on the initial value expression")
   }
+  public static var unexpectedPoundElseSpaceIf: Self {
+    .init("unexpected 'if' keyword following '#else' conditional compilation directive; did you mean '#elseif'?")
+  }
   public static var unexpectedSemicolon: Self {
     .init("unexpected ';' separator")
   }

Tests/SwiftParserTest/translated/IfconfigExprTests.swift

@@ -428,6 +428,35 @@ final class IfconfigExprTests: XCTestCase {
     )
   }
 
+  func testIfconfigExpr31() {
+    AssertParse(
+      """
+      #if arch(x86_64)
+        debugPrint("x86_64")
+      1️⃣#else if arch(arm64)
+        debugPrint("arm64")
+      #else
+        debugPrint("Some other architecture.")
+      #endif
+      """,
+      diagnostics: [
+        DiagnosticSpec(
+          message: "unexpected 'if' keyword following '#else' conditional compilation directive; did you mean '#elseif'?",
+          fixIts: ["replace '#else if' with '#elseif'"]
+        )
+      ],
+      fixedSource: """
+        #if arch(x86_64)
+          debugPrint("x86_64")
+        #elseif arch(arm64)
+          debugPrint("arm64")
+        #else
+          debugPrint("Some other architecture.")
+        #endif
+        """
+    )
+  }
+
   func testUnknownPlatform1() {
     AssertParse(
       """