Fix NullReferenceException in VB SyntaxFactory.ParseArgumentList when parentheses are missing

src/Compilers/VisualBasic/Portable/Parser/ParseExpression.vb

@@ -1310,14 +1310,15 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Syntax.InternalSyntax
         ' Lines: 16304 - 16304
         ' ParenthesizedArgumentList .Parser::ParseParenthesizedArguments( [ _Inout_ bool& ErrorInConstruct ] )
         Friend Function ParseParenthesizedArguments(Optional RedimOrNewParent As Boolean = False, Optional attributeListParent As Boolean = False) As ArgumentListSyntax
-            Debug.Assert(CurrentToken.Kind = SyntaxKind.OpenParenToken, "should be at tkLParen.")
-
             Dim arguments As CodeAnalysis.Syntax.InternalSyntax.SeparatedSyntaxList(Of ArgumentSyntax) = Nothing
             Dim openParen As PunctuationSyntax = Nothing
             Dim closeParen As PunctuationSyntax = Nothing
 
-            Debug.Assert(CurrentToken.Kind = SyntaxKind.OpenParenToken)
-            TryGetTokenAndEatNewLine(SyntaxKind.OpenParenToken, openParen)
+            ' Try to get the opening parenthesis, create a missing one if not found
+            If Not TryGetTokenAndEatNewLine(SyntaxKind.OpenParenToken, openParen) Then
+                openParen = InternalSyntaxFactory.MissingPunctuation(SyntaxKind.OpenParenToken)
+                openParen = ReportSyntaxError(openParen, ERRID.ERR_ExpectedLparen)
+            End If
 
             Dim unexpected As GreenNode = Nothing
             arguments = ParseArguments(unexpected, RedimOrNewParent, attributeListParent)

src/Compilers/VisualBasic/Test/Syntax/Syntax/SyntaxFactoryTests.vb

@@ -134,5 +134,30 @@ Integer
             Dim typeName = SyntaxFactory.ParseTypeName("", options:=parseOptions)
             Assert.Same(parseOptions, typeName.SyntaxTree.Options)
         End Sub
+
+        <Fact>
+        Public Shared Sub TestParseArgumentListWithoutParentheses()
+            ' This should not throw NullReferenceException or assertion failure
+            ' when input lacks parentheses
+            Dim result = SyntaxFactory.ParseArgumentList("42")
+
+            ' Should still produce a valid ArgumentListSyntax with errors
+            Assert.NotNull(result)
+            Assert.True(result.ContainsDiagnostics)
+
+            ' Should have an error for missing opening parenthesis
+            Dim errors = result.GetDiagnostics().ToArray()
+            Assert.True(errors.Length > 0, "Expected diagnostic errors for missing parentheses")
+
+            ' Verify that the parse produces a valid tree with missing tokens
+            Assert.NotNull(result.OpenParenToken)
+            Assert.True(result.OpenParenToken.IsMissing)
+
+            ' Test with parentheses - should work correctly
+            Dim resultWithParens = SyntaxFactory.ParseArgumentList("(42)")
+            Assert.NotNull(resultWithParens)
+            Assert.Equal("(42)", resultWithParens.ToString())
+            Assert.False(resultWithParens.ContainsDiagnostics)
+        End Sub
     End Class
 End Namespace