PY-21175 Don't lost explicit Any added by a definition in the result Union

We need to distinguish between initial synthetic value in the
incremental union used to combine types and the actual "Any" type
inferred from one of the definitions.

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

@@ -504,22 +504,23 @@ public class PyReferenceExpressionImpl extends PyElementImpl implements PyRefere
     final PyElement element = augAssignment != null ? augAssignment : anchor;
     try {
       final List<Instruction> defs = PyDefUseUtil.getLatestDefs(scopeOwner, name, element, true, false);
-      PyType type = null;
-      for (Instruction def : defs) {
-        final ReadWriteInstruction instruction = PyUtil.as(def, ReadWriteInstruction.class);
-        if (instruction != null) {
-          final Ref<PyType> instructionType = instruction.getType(context, anchor);
-          if (instructionType != null) {
-            if (type != null) {
-              type = PyUnionType.union(type, instructionType.get());
-            }
-            else {
-              type = instructionType.get();
-            }
+      // null means empty set of possible types, Ref(null) means Any
+      final @Nullable Ref<PyType> combinedType = StreamEx.of(defs)
+        .select(ReadWriteInstruction.class)
+        .map(instr -> instr.getType(context, anchor))
+        // don't use foldLeft(BiFunction) here, as it doesn't support null results
+        .foldLeft(null, (accType, defType) -> {
+          if (defType == null) {
+            return accType;
           }
-        }
-      }
-      return type;
+          else if (accType == null) {
+            return defType;
+          }
+          else {
+            return Ref.create(PyUnionType.union(accType.get(), defType.get()));
+          }
+        });
+      return Ref.deref(combinedType);
     }
     catch (PyDefUseUtil.InstructionNotFoundException ignored) {
     }

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

@@ -2001,6 +2001,17 @@ public class PyTypeTest extends PyTestCase {
            "expr = xs\n");
   }
 
+  // PY-21175
+  public void testAnyAddedByConditionalDefinition() {
+    doTest("Union[str, Any]",
+           "def f(x, y):\n" +
+           "    if x:\n" +
+           "        var = y\n" +
+           "    else:\n" +
+           "        var = 'foo'\n" +
+           "    expr = var");
+  }
+
   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());