Fix ArgumentException in SeparatedSyntaxList.ReplaceSeparator to use proper message and paramName

src/Compilers/CSharp/Test/Syntax/Syntax/SeparatedSyntaxListTests.cs

@@ -320,5 +320,45 @@ public void ReplaceSeparator(bool collectionExpression)
             Assert.Equal(2, newList.SeparatorCount);
             Assert.Equal(1, newList.GetSeparator(1).GetLeadingTrivia().Count);
         }
+
+        [Fact]
+        public void ReplaceSeparator_ThrowsArgumentExceptionWithProperMessage_WhenSeparatorNotFound()
+        {
+            var list = SyntaxFactory.SeparatedList<SyntaxNode>(
+                new[] {
+                    SyntaxFactory.IdentifierName("A"),
+                    SyntaxFactory.IdentifierName("B"),
+                    SyntaxFactory.IdentifierName("C"),
+                });
+
+            var badToken = SyntaxFactory.Token(SyntaxKind.SemicolonToken);
+            var newSeparator = SyntaxFactory.Token(SyntaxKind.CommaToken);
+
+            var exception = Assert.Throws<ArgumentException>(() =>
+                list.ReplaceSeparator(badToken, newSeparator));
+
+            Assert.Equal("separatorToken", exception.ParamName);
+            Assert.Contains("item specified is not the element of a list", exception.Message);
+        }
+
+        [Fact]
+        public void ReplaceSeparator_ThrowsArgumentExceptionWithProperMessage_WhenNewSeparatorHasWrongKind()
+        {
+            var list = SyntaxFactory.SeparatedList<SyntaxNode>(
+                new[] {
+                    SyntaxFactory.IdentifierName("A"),
+                    SyntaxFactory.IdentifierName("B"),
+                    SyntaxFactory.IdentifierName("C"),
+                });
+
+            var firstSep = list.GetSeparator(0);
+            var wrongSep = SyntaxFactory.Token(SyntaxKind.SemicolonToken);
+
+            var exception = Assert.Throws<ArgumentException>(() =>
+                list.ReplaceSeparator(firstSep, wrongSep));
+
+            Assert.Equal("newSeparator", exception.ParamName);
+            Assert.Contains("must have the same RawKind and Language", exception.Message);
+        }
     }
 }

src/Compilers/Core/Portable/Syntax/SeparatedSyntaxList.cs

@@ -573,13 +573,13 @@ public SeparatedSyntaxList<TNode> ReplaceSeparator(SyntaxToken separatorToken, S
             var index = nodesWithSeps.IndexOf(separatorToken);
             if (index < 0)
             {
-                throw new ArgumentException("separatorToken");
+                throw new ArgumentException(CodeAnalysisResources.MissingListItem, nameof(separatorToken));
             }
 
             if (newSeparator.RawKind != nodesWithSeps[index].RawKind ||
                 newSeparator.Language != nodesWithSeps[index].Language)
             {
-                throw new ArgumentException("newSeparator");
+                throw new ArgumentException("New separator must have the same RawKind and Language as the separator being replaced.", nameof(newSeparator));
             }
 
             return new SeparatedSyntaxList<TNode>(nodesWithSeps.Replace(separatorToken, newSeparator));