PY-21474 Fixed: Type checking confused by None default argument

PyTypeAssertionEvaluator: Mark reference which is used as condition in PyConditionalExpression as non-None

python/src/com/jetbrains/python/codeInsight/controlflow/PyTypeAssertionEvaluator.java

@@ -83,7 +83,7 @@ public class PyTypeAssertionEvaluator extends PyRecursiveElementVisitor {
 
   @Override
   public void visitPyReferenceExpression(final PyReferenceExpression node) {
-    if (node.getParent() instanceof PyIfPart) {
+    if (isUnderIf(node)) {
       pushAssertion(node, !myPositive, context -> PyNoneType.INSTANCE);
       return;
     }
@@ -195,6 +195,12 @@ public class PyTypeAssertionEvaluator extends PyRecursiveElementVisitor {
     myStack.push(new Assertion(target, typeCallback));
   }
 
+  private static boolean isUnderIf(@NotNull PyReferenceExpression node) {
+    final PsiElement parent = node.getParent();
+    return parent instanceof PyIfPart ||
+           parent instanceof PyConditionalExpression && node == ((PyConditionalExpression)parent).getCondition();
+  }
+
   static class Assertion {
     private final PyReferenceExpression element;
     private InstructionTypeCallback myFunction;

python/testData/codeInsight/controlflow/conditionalexpression.txt

@@ -8,9 +8,9 @@
 7(8) WRITE ACCESS: c
 8(9) element: PyAssignmentStatement
 9(10) element: PyConditionalExpression
-10(11,12) READ ACCESS: c
-11(13) READ ACCESS: x
-12(13) READ ACCESS: y
-13(14) WRITE ACCESS: z
-14() element: null
-
+10(11,13) READ ACCESS: c
+11(12) ASSERTTYPE ACCESS: c
+12(14) READ ACCESS: x
+13(14) READ ACCESS: y
+14(15) WRITE ACCESS: z
+15() element: null

python/testSrc/com/jetbrains/python/PyTypeTest.java

@@ -1504,6 +1504,16 @@ public class PyTypeTest extends PyTestCase {
            "expr = foo(a=1, b=\"2\")");
   }
 
+  // PY-21474
+  public void testReassigningOptionalListWithDefaultValue() {
+    doTest("Union[List[str], list]",
+           "def x(things):\n" +
+           "    \"\"\"\n" +
+           "    :type things: None | list[str]\n" +
+           "    \"\"\"\n" +
+           "    expr = things if things else []");
+  }
+
   private static List<TypeEvalContext> getTypeEvalContexts(@NotNull PyExpression element) {
     return ImmutableList.of(TypeEvalContext.codeAnalysis(element.getProject(), element.getContainingFile()).withTracing(),
                             TypeEvalContext.userInitiated(element.getProject(), element.getContainingFile()).withTracing());