Add correct diagnostic for missing parentheses

Author: kimdv

Sources/SwiftParser/Expressions.swift

@@ -1746,7 +1746,9 @@ extension Parser {
     var effectSpecifiers: RawTypeEffectSpecifiersSyntax?
     var returnClause: RawReturnClauseSyntax? = nil
     if !self.at(.keyword(.in)) {
-      if self.at(.leftParen) {
+      // If current token is not a leftParen but a colon it might be an failure
+      // as we need a left and right paren.
+      if self.at(.leftParen) || self.peek(isAt: .colon) {
         // Parse the closure arguments.
         let params = self.parseParameterClause(RawClosureParameterClauseSyntax.self) { parser in
           parser.parseClosureParameter()
@@ -2427,7 +2429,7 @@ extension Parser.Lookahead {
     }
 
     // Parse pattern-tuple func-signature-result? 'in'.
-    if lookahead.consume(if: .leftParen) != nil {  // Consume the ')'.
+    if lookahead.consume(if: .leftParen) != nil {  // Consume the '('.
 
       // While we don't have '->' or ')', eat balanced tokens.
       var skipProgress = LoopProgressCondition()
@@ -2453,8 +2455,18 @@ extension Parser.Lookahead {
       // Parse identifier (',' identifier)*
       lookahead.consumeAnyToken()
 
+      func consumeTypesIfAvailable() -> Bool {
+        // If identifiers have types, we need to have parens around.
+        // If they are missing but colon an type is there, it might be an error.
+        if lookahead.consume(if: .colon) != nil {
+          return lookahead.canParseType() && lookahead.consume(if: .comma) != nil
+        } else {
+          return lookahead.consume(if: .comma) != nil
+        }
+      }
+
       var parametersProgress = LoopProgressCondition()
-      while lookahead.consume(if: .comma) != nil && lookahead.hasProgressed(&parametersProgress) {
+      while consumeTypesIfAvailable() && lookahead.hasProgressed(&parametersProgress) {
         if lookahead.at(.identifier) || lookahead.at(.wildcard) {
           lookahead.consumeAnyToken()
           continue
@@ -2463,6 +2475,10 @@ extension Parser.Lookahead {
         return false
       }
 
+      if lookahead.at(.rightParen) {
+        lookahead.consumeAnyToken()
+      }
+
       lookahead.consumeEffectsSpecifiers()
 
       // Parse the func-signature-result, if present.

Tests/SwiftParserTest/ExpressionTests.swift

@@ -2689,4 +2689,49 @@ final class StatementExpressionTests: XCTestCase {
 
     assertParse("_ = { (@Wrapper x) in }")
   }
+
+  func testClosureWithMissingParentheses() {
+    assertParse(
+      """
+      _ = { 1️⃣a: Int, b: Int 2️⃣in
+      }
+      """,
+      diagnostics: [
+        DiagnosticSpec(
+          locationMarker: "1️⃣",
+          message: "expected '(' to start parameter clause",
+          fixIts: ["insert '('"]
+        ),
+        DiagnosticSpec(
+          locationMarker: "2️⃣",
+          message: "expected ')' to end parameter clause",
+          fixIts: ["insert ')'"]
+        ),
+      ],
+      fixedSource: """
+        _ = { (a: Int, b: Int) in
+        }
+        """
+    )
+  }
+
+  func testClosureWithMissingLeftParenthese() {
+    assertParse(
+      """
+      _ = { 1️⃣a: Int, b: Int) in
+      }
+      """,
+      diagnostics: [
+        DiagnosticSpec(
+          locationMarker: "1️⃣",
+          message: "expected '(' to start parameter clause",
+          fixIts: ["insert '('"]
+        )
+      ],
+      fixedSource: """
+        _ = { (a: Int, b: Int) in
+        }
+        """
+    )
+  }
 }