Implement flag to allow typechecking of untyped modules

Fixes python#8545

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"],

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

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.

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