Implement flag to allow typechecking of untyped modules (#17712)

Add a flag and config ini options `"follow_untyped_imports"`. Setting it
to `True` instructs mypy to typecheck also modules that do not have
stubs or a `py.typed` marker.

Fixes #8545

Author: DeinAlptraum

Diff for: docs/source/command_line.rst

@@ -166,6 +166,10 @@ imports.
 
     For more details, see :ref:`ignore-missing-imports`.
 
+.. option:: --follow-untyped-imports
+
+    This flag makes mypy analyze imports without stubs or a py.typed marker.
+
 .. option:: --follow-imports {normal,silent,skip,error}
 
     This flag adjusts how mypy follows imported modules that were not

Diff for: docs/source/config_file.rst

@@ -315,6 +315,18 @@ section of the command line docs.
     match the name of the *imported* module, not the module containing the
     import statement.
 
+.. confval:: follow_untyped_imports
+
+    :type: boolean
+    :default: False
+
+    Typechecks imports from modules that do not have stubs or a py.typed marker.
+
+    If this option is used in a per-module section, the module name should
+    match the name of the *imported* module, not the module containing the
+    import statement. Note that scanning all unannotated modules might
+    significantly increase the runtime of your mypy calls.
+
 .. confval:: follow_imports
 
     :type: string

Diff for: docs/source/running_mypy.rst

@@ -321,6 +321,12 @@ not catch errors in its use.
     recommend avoiding ``--ignore-missing-imports`` if possible: it's equivalent
     to adding a ``# type: ignore`` to all unresolved imports in your codebase.
 
+4.  To make mypy typecheck imports from modules without stubs or a py.typed
+    marker, you can set the :option:`--follow-untyped-imports <mypy --follow-untyped-imports>`
+    command line flag or set the :confval:`follow_untyped_imports` config file option to True,
+    either in the global section of your mypy config file, or individually on a
+    per-module basis.
+
 
 Library stubs not installed
 ---------------------------

Diff for: mypy/main.py

@@ -580,6 +580,11 @@ def add_invertible_flag(
         action="store_true",
         help="Silently ignore imports of missing modules",
     )
+    imports_group.add_argument(
+        "--follow-untyped-imports",
+        action="store_true",
+        help="Typecheck modules without stubs or py.typed marker",
+    )
     imports_group.add_argument(
         "--follow-imports",
         choices=["normal", "silent", "skip", "error"],

Diff for: mypy/modulefinder.py

@@ -334,10 +334,11 @@ def _typeshed_has_version(self, module: str) -> bool:
         return version >= min_version and (max_version is None or version <= max_version)
 
     def _find_module_non_stub_helper(
-        self, components: list[str], pkg_dir: str
+        self, id: str, pkg_dir: str
     ) -> OnePackageDir | ModuleNotFoundReason:
         plausible_match = False
         dir_path = pkg_dir
+        components = id.split(".")
         for index, component in enumerate(components):
             dir_path = os_path_join(dir_path, component)
             if self.fscache.isfile(os_path_join(dir_path, "py.typed")):
@@ -350,6 +351,10 @@ def _find_module_non_stub_helper(
             if not self.fscache.isdir(dir_path):
                 break
         if plausible_match:
+            if self.options:
+                module_specific_options = self.options.clone_for_module(id)
+                if module_specific_options.follow_untyped_imports:
+                    return os.path.join(pkg_dir, *components[:-1]), False
             return ModuleNotFoundReason.FOUND_WITHOUT_TYPE_HINTS
         else:
             return ModuleNotFoundReason.NOT_FOUND
@@ -463,7 +468,7 @@ def _find_module(self, id: str, use_typeshed: bool) -> ModuleSearchResult:
                             third_party_stubs_dirs.append((path, True))
                     else:
                         third_party_stubs_dirs.append((path, True))
-            non_stub_match = self._find_module_non_stub_helper(components, pkg_dir)
+            non_stub_match = self._find_module_non_stub_helper(id, pkg_dir)
             if isinstance(non_stub_match, ModuleNotFoundReason):
                 if non_stub_match is ModuleNotFoundReason.FOUND_WITHOUT_TYPE_HINTS:
                     found_possible_third_party_missing_type_hints = True

Diff for: mypy/options.py

@@ -42,6 +42,7 @@ class BuildType:
     "extra_checks",
     "follow_imports_for_stubs",
     "follow_imports",
+    "follow_untyped_imports",
     "ignore_errors",
     "ignore_missing_imports",
     "implicit_optional",
@@ -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.follow_untyped_imports = 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 for: 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 --follow-untyped-imports
+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