From 64913137ae47cfb7c5a3fcf2e840f32c7e7db09e Mon Sep 17 00:00:00 2001
From: Jannick Kremer <jannick.kremer@mailbox.org>
Date: Tue, 27 Aug 2024 13:46:55 +0200
Subject: [PATCH 1/2] Implement flag to allow typechecking of untyped modules

Fixes #8545
---
 mypy/main.py               |  5 +++++
 mypy/modulefinder.py       | 15 ++++++++++++++-
 mypy/options.py            |  3 +++
 test-data/unit/pep561.test |  7 +++++++
 4 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/mypy/main.py b/mypy/main.py
index f177bb1c2062..79ac530dcd94 100644
--- a/mypy/main.py
+++ b/mypy/main.py
@@ -572,6 +572,11 @@ def add_invertible_flag(
         action="store_true",
         help="Silently ignore imports of missing modules",
     )
+    imports_group.add_argument(
+        "--enable-installed-packages",
+        action="store_true",
+        help="Typecheck modules without stubs or py.typed marker",
+    )
     imports_group.add_argument(
         "--follow-imports",
         choices=["normal", "silent", "skip", "error"],
diff --git a/mypy/modulefinder.py b/mypy/modulefinder.py
index 452cfef20f4c..4d37f26f00bc 100644
--- a/mypy/modulefinder.py
+++ b/mypy/modulefinder.py
@@ -13,7 +13,7 @@
 import subprocess
 import sys
 from enum import Enum, unique
-from typing import Dict, Final, List, NamedTuple, Optional, Tuple, Union
+from typing import cast, Dict, Final, List, NamedTuple, Optional, Tuple, Union
 from typing_extensions import TypeAlias as _TypeAlias
 
 from mypy import pyinfo
@@ -334,6 +334,19 @@ def _find_module_non_stub_helper(
             if approved_stub_package_exists(".".join(components[:i])):
                 return ModuleNotFoundReason.APPROVED_STUBS_NOT_INSTALLED
         if plausible_match:
+            if self.options:
+                enable_installed_packages = self.options.enable_installed_packages
+                try:
+                    enable_installed_packages = cast(
+                        bool,
+                        self.options.per_module_options[components[0]][
+                            "enable_installed_packages"
+                        ],
+                    )
+                except KeyError:
+                    pass
+                if enable_installed_packages:
+                    return os.path.join(pkg_dir, *components[:-1]), False
             return ModuleNotFoundReason.FOUND_WITHOUT_TYPE_HINTS
         else:
             return ModuleNotFoundReason.NOT_FOUND
diff --git a/mypy/options.py b/mypy/options.py
index 5e64d5e40035..0a1676ccc7ae 100644
--- a/mypy/options.py
+++ b/mypy/options.py
@@ -39,6 +39,7 @@ class BuildType:
     "disallow_untyped_defs",
     "enable_error_code",
     "enabled_error_codes",
+    "enable_installed_packages",
     "extra_checks",
     "follow_imports_for_stubs",
     "follow_imports",
@@ -113,6 +114,8 @@ def __init__(self) -> None:
         self.ignore_missing_imports = False
         # Is ignore_missing_imports set in a per-module section
         self.ignore_missing_imports_per_module = False
+        # Typecheck modules without stubs or py.typed marker
+        self.enable_installed_packages = False
         self.follow_imports = "normal"  # normal|silent|skip|error
         # Whether to respect the follow_imports setting even for stub files.
         # Intended to be used for disabling specific stubs.
diff --git a/test-data/unit/pep561.test b/test-data/unit/pep561.test
index 9969c2894c36..acde01d0e891 100644
--- a/test-data/unit/pep561.test
+++ b/test-data/unit/pep561.test
@@ -187,6 +187,13 @@ b.bf(1)
 testNamespacePkgWStubsWithNamespacePackagesFlag.py:7: error: Argument 1 to "bf" has incompatible type "int"; expected "bool"
 testNamespacePkgWStubsWithNamespacePackagesFlag.py:8: error: Argument 1 to "bf" has incompatible type "int"; expected "bool"
 
+[case testMissingPytypedFlag]
+# pkgs: typedpkg_ns_b
+# flags: --namespace-packages --enable-installed-packages
+import typedpkg_ns.b.bbb as b
+b.bf("foo", "bar")
+[out]
+testMissingPytypedFlag.py:4: error: Too many arguments for "bf"
 
 [case testTypedPkgNamespaceRegFromImportTwiceMissing]
 # pkgs: typedpkg_ns_a

From 05466a3269caf02d23102228d6f70aed217537de Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Tue, 27 Aug 2024 12:14:33 +0000
Subject: [PATCH 2/2] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 mypy/modulefinder.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mypy/modulefinder.py b/mypy/modulefinder.py
index 4d37f26f00bc..f3ed955dcc5a 100644
--- a/mypy/modulefinder.py
+++ b/mypy/modulefinder.py
@@ -13,7 +13,7 @@
 import subprocess
 import sys
 from enum import Enum, unique
-from typing import cast, Dict, Final, List, NamedTuple, Optional, Tuple, Union
+from typing import Dict, Final, List, NamedTuple, Optional, Tuple, Union, cast
 from typing_extensions import TypeAlias as _TypeAlias
 
 from mypy import pyinfo