Fix wrong diagnostic for generics

Author: kimdv

Sources/SwiftParser/Declarations.swift

@@ -465,7 +465,7 @@ extension Parser {
     }
 
     precondition(self.currentToken.starts(with: "<"))
-    let langle = self.consumeAnyToken(remapping: .leftAngle)
+    let langle = self.consumeStartingCharacterOfCurrentToken(as: .leftAngle)
     var elements = [RawGenericParameterSyntax]()
     do {
       var keepGoing: RawTokenSyntax? = nil
@@ -477,7 +477,7 @@ extension Parser {
         var each = self.consume(if: .keyword(.each))
 
         let (unexpectedBetweenEachAndName, name) = self.expectIdentifier(allowSelfOrCapitalSelfAsIdentifier: true)
-        if attributes == nil && each == nil && unexpectedBetweenEachAndName == nil && name.isMissing && elements.isEmpty {
+        if attributes == nil && each == nil && unexpectedBetweenEachAndName == nil && name.isMissing && elements.isEmpty && !self.currentToken.starts(with: ">") {
           break
         }
 

Sources/SwiftParser/Parser.swift

@@ -557,6 +557,37 @@ extension Parser {
     )
     return tok
   }
+
+  /// Consumes a single token and advances the current token by 1.
+  mutating func consumeStartingCharacterOfCurrentToken(
+    as tokenKind: RawTokenKind
+  ) -> RawTokenSyntax {
+    let current = self.currentToken
+
+    let endIndex = current.textRange.lowerBound.advanced(by: 1)
+    var tokenDiagnostic = current.diagnostic
+    if let error = tokenDiagnostic, error.byteOffset > current.leadingTriviaByteLength + 1 {
+      // The lexer error isn't in the prefix. Drop it.
+      tokenDiagnostic = nil
+    }
+
+    let tok = RawTokenSyntax(
+      kind: tokenKind,
+      wholeText: SyntaxText(rebasing: current.wholeText[..<endIndex]),
+      textRange: current.textRange.lowerBound..<endIndex,
+      presence: .present,
+      tokenDiagnostic: tokenDiagnostic,
+      arena: self.arena
+    )
+
+    self.adjustNestingLevel(for: tokenKind)
+
+    self.currentToken = self.lexemes.resetForSplit(
+      splitToken: self.currentToken,
+      consumedPrefix: self.currentToken.leadingTriviaByteLength + 1
+    )
+    return tok
+  }
 }
 
 extension SyntaxText {

Tests/SwiftParserTest/translated/RecoveryTests.swift

@@ -1031,7 +1031,7 @@ final class RecoveryTests: XCTestCase {
     assertParse(
       """
       // Note: Don't move braces to a different line here.
-      struct ErrorGenericParameterList4< 1️⃣
+      struct ErrorGenericParameterList4<1️⃣
       {
       }
       """,
@@ -2245,4 +2245,23 @@ final class RecoveryTests: XCTestCase {
     )
   }
 
+  // https://github.com/apple/swift-syntax/issues/1483
+  func testRecovery183() {
+    // Can be parsed and produces no diagnostics.
+    assertParse(
+      "func f<1️⃣ >() {}",
+      diagnostics: [
+        DiagnosticSpec(message: "expected generic parameter in generic parameter clause")
+      ]
+    )
+
+    // Can be parsed. Printing the node or asking for the diagnostics leads to a crash.
+    assertParse(
+      "func f<1️⃣>() {}",
+      diagnostics: [
+        DiagnosticSpec(message: "expected generic parameter in generic parameter clause")
+      ]
+    )
+  }
+
 }