Match linter conditional walk to analysis

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

@@ -16,8 +16,10 @@
 using System.Collections.Generic;
 using System.Linq;
 using Microsoft.Python.Analysis.Analyzer;
+using Microsoft.Python.Analysis.Analyzer.Handlers;
 using Microsoft.Python.Analysis.Types;
 using Microsoft.Python.Core;
+using Microsoft.Python.Core.OS;
 using Microsoft.Python.Core.Text;
 using Microsoft.Python.Parsing.Ast;
 using Microsoft.Python.Parsing.Extensions;
@@ -55,6 +57,7 @@ public override bool Walk(ListComprehension node) {
             return false;
         }
 
+
         public override bool Walk(SetComprehension node) {
             node.Walk(new ComprehensionWalker(_walker, _localNames, _localNameExpressions));
             return false;
@@ -75,6 +78,11 @@ public override bool Walk(NamedExpression node) {
             return false;
         }
 
+        public override bool Walk(IfStatement node) 
+            => node.WalkIfWithSystemConditions(_walker, 
+                _walker.Analysis.Document.Interpreter.LanguageVersion, 
+                _walker.Services.GetService<IOSPlatform>());
+
         public override bool Walk(NameExpression node) {
             if (_localNames?.Contains(node.Name) == true) {
                 return false;

src/Analysis/Ast/Test/LintUndefinedVarsTests.cs

@@ -832,6 +832,22 @@ public async Task AugmentedAssignToUndefined() {
             d[0].SourceSpan.Should().Be(2, 1, 2, 2);
         }
 
+        [TestMethod, Priority(0)]
+        public async Task PlatformSpecific() {
+            const string code = @"
+import sys
+
+if sys.platform == 'linux':
+    aVariable='Hello Linux'
+    print(aVariable)
+
+else:
+    aVariable = 'Hello Other''
+    print(aVariable)
+";
+            var d = await LintAsync(code);
+            d.Should().BeEmpty();
+        }
 
         private async Task<IReadOnlyList<DiagnosticsEntry>> LintAsync(string code, InterpreterConfiguration configuration = null) {
             var analysis = await GetAnalysisAsync(code, configuration ?? PythonVersions.LatestAvailable3X);