Fix walk of multi-level if statement

Author: MikhailArkhipov

src/Analysis/Ast/Impl/Extensions/PythonWalkerExtensions.cs

@@ -20,41 +20,42 @@ namespace Microsoft.Python.Analysis {
     public static class PythonWalkerExtensions {
         public static bool WalkIfWithSystemConditions(this IfStatement node, PythonWalker walker, PythonLanguageVersion languageVersion, bool isWindows) {
             // System version, platform and os.path specializations
-            var someRecognized = false;
+            var executeElse = false;
             foreach (var test in node.Tests) {
+
                 var result = test.TryHandleSysVersionInfo(languageVersion);
-                if (result != ConditionTestResult.Unrecognized) {
-                    if (result == ConditionTestResult.WalkBody) {
-                        test.Walk(walker);
-                    } else {
-                        node.ElseStatement?.Walk(walker);
+                if (result == ConditionTestResult.Unrecognized) {
+                    result = test.TryHandleSysPlatform(isWindows);
+                    if (result == ConditionTestResult.Unrecognized) {
+                        result = test.TryHandleOsPath(isWindows);
                     }
-                    someRecognized = true;
-                    continue;
                 }
 
-                result = test.TryHandleSysPlatform(isWindows);
-                if (result != ConditionTestResult.Unrecognized) {
-                    if (result == ConditionTestResult.WalkBody) {
+                // If condition is satisfied, walk the corresponding block and 
+                // return false indicating that statement should not be walked again.
+                // If condition is false or was not recognized, continue but remember
+                // if we need to execute final else clause.
+                switch (result) {
+                    case ConditionTestResult.WalkBody:
                         test.Walk(walker);
-                    } else {
-                        node.ElseStatement?.Walk(walker);
-                    }
-                    someRecognized = true;
-                    continue;
+                        return false; // We only need to execute one of the clauses.
+                    case ConditionTestResult.DontWalkBody:
+                        // If condition is false, continue but remember
+                        // if we may need to execute the final else clause.
+                        executeElse = true;
+                        break;
+                    case ConditionTestResult.Unrecognized:
+                        continue; // See if other conditions may work.
                 }
+            }
 
-                result = test.TryHandleOsPath(isWindows);
-                if (result != ConditionTestResult.Unrecognized) {
-                    if (result == ConditionTestResult.WalkBody) {
-                        test.Walk(walker);
-                    } else {
-                        node.ElseStatement?.Walk(walker);
-                    }
-                    return false; // Execute only one condition.
-                }
+            if (executeElse) {
+                node.ElseStatement?.Walk(walker);
+                return false;
             }
-            return !someRecognized;
+
+            // We didn't walk anything, so me caller do their own thing.
+            return true;
         }
     }
 }

src/Analysis/Ast/Test/ConditionalsTests.cs

@@ -152,6 +152,22 @@ def func(a, b): ...
                 .Which.Should().HaveParameters(is3x ? new[] { "a" } : new[] { "a", "b" });
         }
 
+        [DataRow(false)]
+        [DataRow(true)]
+        [DataTestMethod, Priority(0)]
+        public async Task FunctionByVersionElif(bool is3x) {
+            const string code = @"
+if sys.version_info >= (3, 0):
+   def func(a): ...
+elif sys.version_info < (3, 0):
+   def func(a, b): ...
+";
+            var analysis = await GetAnalysisAsync(code, is3x ? PythonVersions.LatestAvailable3X : PythonVersions.LatestAvailable2X);
+            analysis.Should().HaveFunction("func")
+                .Which.Should().HaveSingleOverload()
+                .Which.Should().HaveParameters(is3x ? new[] { "a" } : new[] { "a", "b" });
+        }
+
         private IOSPlatform SubstitutePlatform(out IServiceManager sm) {
             sm = new ServiceManager();
             var platform = Substitute.For<IOSPlatform>();