Don't suppress LHS diagnostics on augmented assign

Author: Mikhail Arkhipov

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

@@ -49,9 +49,7 @@ public override bool Walk(SuiteStatement node) {
                         HandleNonLocal(nls);
                         break;
                     case AugmentedAssignStatement augs:
-                        _suppressDiagnostics = true;
                         augs.Left?.Walk(new ExpressionWalker(this));
-                        _suppressDiagnostics = false;
                         augs.Right?.Walk(new ExpressionWalker(this));
                         break;
                     case AssignmentStatement asst:

src/Analysis/Ast/Test/AssignmentTests.cs

@@ -219,6 +219,15 @@ def __init__(self):
                 .Which.Should().HaveMembers("abc", "y", "__class__");
         }
 
+        [TestMethod, Priority(0)]
+        public async Task AugmentedAssignToUndefined() {
+            const string code = @"
+x += 1
+";
+            var analysis = await GetAnalysisAsync(code);
+            analysis.Should().NotHaveVariable("x");
+        }
+
         [TestMethod, Priority(0)]
         public async Task BaseInstanceVariable() {
             const string code = @"

src/Analysis/Ast/Test/LintUndefinedVarsTests.cs

@@ -821,6 +821,18 @@ class Subclass(MyClass):
             d.Should().BeEmpty();
         }
 
+        [TestMethod, Priority(0)]
+        public async Task AugmentedAssignToUndefined() {
+            const string code = @"
+x += 1
+";
+            var d = await LintAsync(code);
+            d.Should().HaveCount(1);
+            d[0].ErrorCode.Should().Be(ErrorCodes.UndefinedVariable);
+            d[0].SourceSpan.Should().Be(2, 1, 2, 2);
+        }
+
+
         private async Task<IReadOnlyList<DiagnosticsEntry>> LintAsync(string code, InterpreterConfiguration configuration = null) {
             var analysis = await GetAnalysisAsync(code, configuration ?? PythonVersions.LatestAvailable3X);
             var a = Services.GetService<IPythonAnalyzer>();