Improve linting of LHS expressions

src/Analysis/Ast/Impl/Linting/UndefinedVariables/ExpressionWalker.cs

@@ -17,7 +17,6 @@
 using System.Linq;
 using Microsoft.Python.Analysis.Analyzer;
 using Microsoft.Python.Analysis.Types;
-using Microsoft.Python.Core;
 using Microsoft.Python.Core.Text;
 using Microsoft.Python.Parsing.Ast;
 using Microsoft.Python.Parsing.Extensions;

src/Analysis/Ast/Impl/Linting/UndefinedVariables/UndefinedVariablesWalker.cs

@@ -53,9 +53,13 @@ public override bool Walk(SuiteStatement node) {
                         augs.Right?.Walk(new ExpressionWalker(this));
                         break;
                     case AssignmentStatement asst:
+                        var lhs = asst.Left.MaybeEnumerate().ToArray();
+                        if (lhs.Length > 0) {
+                            lhs[0]?.Walk(new ExpressionWalker(this));
+                        }
                         _suppressDiagnostics = true;
-                        foreach (var lhs in asst.Left ?? Enumerable.Empty<Expression>()) {
-                            lhs?.Walk(new ExpressionWalker(this));
+                        foreach (var l in lhs.Skip(1)) {
+                            l?.Walk(new ExpressionWalker(this));
                         }
                         _suppressDiagnostics = false;
                         asst.Right?.Walk(new ExpressionWalker(this));
@@ -73,7 +77,7 @@ public void ReportUndefinedVariable(NameExpression node) {
             ReportUndefinedVariable(node.Name, eval.GetLocation(node).Span);
         }
 
-        public void ReportUndefinedVariable(string name, SourceSpan span) {
+        private void ReportUndefinedVariable(string name, SourceSpan span) {
             if (!_suppressDiagnostics) {
                 _diagnostics.Add(new DiagnosticsEntry(
                     Resources.UndefinedVariable.FormatInvariant(name),

src/Analysis/Ast/Test/LintNoSelfArgumentTests.cs

@@ -19,7 +19,6 @@
 using Microsoft.Python.Analysis.Diagnostics;
 using Microsoft.Python.Analysis.Tests.FluentAssertions;
 using Microsoft.Python.Core;
-using Microsoft.Python.Parsing.Tests;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using TestUtilities;
 

src/Analysis/Ast/Test/LintReturnInInitTests.cs

@@ -18,7 +18,6 @@
 using FluentAssertions;
 using Microsoft.Python.Analysis.Tests.FluentAssertions;
 using Microsoft.Python.Parsing;
-using Microsoft.Python.Parsing.Tests;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using TestUtilities;
 

src/Analysis/Ast/Test/LintUndefinedVarsTests.cs

@@ -832,6 +832,30 @@ public async Task AugmentedAssignToUndefined() {
             d[0].SourceSpan.Should().Be(2, 1, 2, 2);
         }
 
+        [TestMethod, Priority(0)]
+        public async Task UndefinedInInit() {
+            const string code = @"
+class TestIt():
+    def __init__(self):
+        slf.y = 6
+";
+            var d = await LintAsync(code);
+            d.Should().HaveCount(1);
+            d[0].ErrorCode.Should().Be(ErrorCodes.UndefinedVariable);
+            d[0].SourceSpan.Should().Be(4, 9, 4, 12);
+        }
+
+        [TestMethod, Priority(0)]
+        public async Task UndefinedIndex() {
+            const string code = @"
+x = [1, 2]
+x[y] = 1
+";
+            var d = await LintAsync(code);
+            d.Should().HaveCount(1);
+            d[0].ErrorCode.Should().Be(ErrorCodes.UndefinedVariable);
+            d[0].SourceSpan.Should().Be(3, 3, 3, 4);
+        }
 
         private async Task<IReadOnlyList<DiagnosticsEntry>> LintAsync(string code, InterpreterConfiguration configuration = null) {
             var analysis = await GetAnalysisAsync(code, configuration ?? PythonVersions.LatestAvailable3X);