PY-24383 Fixed: type inference broken for Union[Any, List[int]]

Update PySubscriptionExpressionImpl to correctly process weak operands.
Also update PyTypeChecker to not use original qualifier type while calculating substitutions for receiver, use current class type instead.

python/src/com/jetbrains/python/psi/impl/PySubscriptionExpressionImpl.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2000-2016 JetBrains s.r.o.
+ * Copyright 2000-2017 JetBrains s.r.o.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -42,6 +42,7 @@ public class PySubscriptionExpressionImpl extends PyElementImpl implements PySub
     super(astNode);
   }
 
+  @Override
   @NotNull
   public PyExpression getOperand() {
     return childToPsiNotNull(PythonDialectsTokenSetProvider.INSTANCE.getExpressionTokens(), 0);
@@ -57,6 +58,7 @@ public class PySubscriptionExpressionImpl extends PyElementImpl implements PySub
     return operand;
   }
 
+  @Override
   @Nullable
   public PyExpression getIndexExpression() {
     return childToPsi(PythonDialectsTokenSetProvider.INSTANCE.getExpressionTokens(), 1);
@@ -98,6 +100,11 @@ public class PySubscriptionExpressionImpl extends PyElementImpl implements PySub
       }
       members.add(res);
     }
+
+    if (type instanceof PyUnionType && ((PyUnionType)type).isWeak()) {
+      return PyUnionType.createWeakType(PyUnionType.union(members));
+    }
+
     return PyUnionType.union(members);
   }
 

python/src/com/jetbrains/python/psi/types/PyTypeChecker.java

@@ -561,7 +561,7 @@ public class PyTypeChecker {
         for (PyTypeProvider provider : Extensions.getExtensions(PyTypeProvider.EP_NAME)) {
           final PyType genericType = provider.getGenericType(type.getPyClass(), context);
           if (genericType != null) {
-            match(genericType, qualifierType, context, substitutions);
+            match(genericType, type, context, substitutions);
           }
           for (Map.Entry<PyType, PyType> entry : provider.getGenericSubstitutions(type.getPyClass(), context).entrySet()) {
             final PyGenericType genericKey = as(entry.getKey(), PyGenericType.class);

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

@@ -1897,6 +1897,13 @@ public class PyTypeTest extends PyTestCase {
     );
   }
 
+  // PY-24383
+  public void testSubscriptionOnWeakType() {
+    doTest("Union[int, Any]",
+           "foo = bar() if 42 != 42 else [1, 2, 3, 4]\n" +
+           "expr = foo[0]");
+  }
+
   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());