PY-76243 Don't build implicit union types for conditional definitions and names imported from stub packages

Also fixes PY-59014, PY-39761.

PyResolveImportUtil returns both .pyi stubs and the corresponding .py files for stub packages
to support partial stub packages. See the line:

```
groupedResults.topResultIs(Priority.STUB_PACKAGE) -> firstResultWithFallback(groupedResults, Priority.STUB_PACKAGE)
```

in PyResolveImportUtil.filterTopPriorityResults.

It means that, for instance, resolving the QuerySet name in type hints led to QuerySet
definitions from both places. Then, PyTypingTypeProvider.getType() for the reference expression
"QuerySet" returned a union type containing PyClassTypes for both of them, we couldn't parameterize
it in PyTypingTypeProvider.getParameterizedType and returned Any.

It's wrong that while evaluating type hints, we interpret multiple declarations as
a union type. Those should only be explicitly expressed with typing.Union or "|" operator.
This behavior was originally added in PY-18427 as an ad-hoc way to support version checks
for type hints, but now it seems detrimental because it's unclear how to parameterize
such implicit unions of generic types then.

Other type checkers also don't treat conditional definitions like that. For instance, for
conditional type aliases, Mypy complains about the name being defined twice and then uses
only the first definition, and Pyright doesn't consider names under conditions other than
version checks as valid type aliases at all. Both type checkers also support partial stub
packages properly.

GitOrigin-RevId: 1ecc7ab5d09625d10850ddc0e1f7761332ccddd5

python/python-psi-impl/src/com/jetbrains/python/codeInsight/typing/PyTypingTypeProvider.java

@@ -765,20 +765,13 @@ public final class PyTypingTypeProvider extends PyTypeProviderWithCustomContext<
 
   @Nullable
   private static Ref<PyType> getType(@NotNull PyExpression expression, @NotNull Context context) {
-    final List<PyType> members = new ArrayList<>();
-    boolean foundAny = false;
     for (Pair<PyQualifiedNameOwner, PsiElement> pair : tryResolvingWithAliases(expression, context.getTypeContext())) {
       final Ref<PyType> typeRef = getTypeForResolvedElement(expression, pair.getFirst(), pair.getSecond(), context);
       if (typeRef != null) {
-        final PyType type = typeRef.get();
-        if (type == null) {
-          foundAny = true;
-        }
-        members.add(type);
+        return typeRef;
       }
     }
-    final PyType union = PyUnionType.union(members);
-    return union != null || foundAny ? Ref.create(union) : null;
+    return null;
   }
 
   @Nullable

python/testData/types/GenericClassDeclaredInStubPackage/site-packages/pkg-stubs/mod.pyi

@@ -0,0 +1,2 @@
+class MyClass[T]:
+    pass

python/testData/types/GenericClassDeclaredInStubPackage/site-packages/pkg/mod.py

@@ -0,0 +1,2 @@
+class MyClass:
+    pass

python/testSrc/com/jetbrains/python/PyTypingTest.java

@@ -593,17 +593,55 @@ public class PyTypingTest extends PyTestCase {
              """);
   }
 
-  // PY-18427
-  public void testConditionalType() {
-    doTest("int | str",
+  // PY-18427 PY-76243
+  public void testConditionalTypeAlias() {
+    doTest("int",
            """
              if something:
                  Type = int
              else:
                  Type = str
 
-             def f(expr: Type):
-                 pass
+             expr: Type
+             """);
+  }
+
+  public void testConditionalGenericTypeAlias() {
+    doTest("list[str]",
+           """
+             if something:
+                 Type = list
+             else:
+                 Type = set
+             
+             expr: Type[str]
+             """);
+  }
+
+  public void testTypeAliasOfUnionOfGenericTypes() {
+    doTest("list[str] | set[str]",
+           """
+             from typing import TypeVar
+             
+             T = TypeVar("T")
+             
+             Type = list[T] | set[T]
+             
+             expr: Type[str]
+             """);
+  }
+
+  public void testTypeAliasOfUnionOfGenericTypesWithDifferentArity() {
+    doTest("dict[str, int] | set[int]",
+           """
+             from typing import TypeVar
+             
+             T1 = TypeVar("T1")
+             T2 = TypeVar("T2")
+             
+             Type = dict[T1, T2] | set[T2]
+             
+             expr: Type[str, int]
              """);
   }
 
@@ -1950,28 +1988,24 @@ public class PyTypingTest extends PyTestCase {
              """
                from __future__ import annotations
                if something:
-                   Type = int
+                   x: int
                else:
-                   Type = str
-
-               def f(expr: Type):
-                   pass
+                   x: str
+               expr = x
                """);
     });
   }
 
   // PY-44974
-  public void testWithoutFromFutureImport() {
+  public void testBitwiseOrUnionWithoutFromFutureImport() {
     runWithLanguageLevel(LanguageLevel.PYTHON39, () -> {
       doTest("Union[int, str]",
              """
                if something:
-                   Type = int
+                   x: int
                else:
-                   Type = str
-
-               def f(expr: Type):
-                   pass
+                   x: str
+               expr = x
                """);
     });
   }
@@ -5787,6 +5821,17 @@ public class PyTypingTest extends PyTestCase {
       """);
   }
 
+  // PY-76243
+  public void testGenericClassDeclaredInStubPackage() {
+    runWithAdditionalClassEntryInSdkRoots("types/" + getTestName(false) + "/site-packages", () -> {
+      doTest("MyClass[int]",
+             """
+               from pkg.mod import MyClass
+               expr: MyClass[int]
+               """);
+    });
+  }
+
   private void doTestNoInjectedText(@NotNull String text) {
     myFixture.configureByText(PythonFileType.INSTANCE, text);
     final InjectedLanguageManager languageManager = InjectedLanguageManager.getInstance(myFixture.getProject());