Support exported names starting with an underscore in __all__ (#3746)

Fixes #3745

Author: daboross

mypy/semanal.py

@@ -1483,7 +1483,9 @@ def visit_import_all(self, i: ImportAll) -> None:
             self.add_submodules_to_parent_modules(i_id, True)
             for name, node in m.names.items():
                 node = self.normalize_type_alias(node, i)
-                if not name.startswith('_') and node.module_public:
+                # if '__all__' exists, all nodes not included have had module_public set to
+                # False, and we can skip checking '_' because it's been explicitly included.
+                if node.module_public and (not name.startswith('_') or '__all__' in m.names):
                     existing_symbol = self.globals.get(name)
                     if existing_symbol:
                         # Import can redefine a variable. They get special treatment.

test-data/unit/check-modules.test

@@ -427,6 +427,25 @@ __all__ = [u'a', u'b', u'c']
 
 [out]
 
+[case testUnderscoreExportedValuesInImportAll]
+import typing
+from m import *
+_ = a
+_ = _b
+_ = __c__
+_ = ___d
+_ = e
+_ = f # E: Name 'f' is not defined
+_ = _g # E: Name '_g' is not defined
+[file m.py]
+__all__ = ['a']
+__all__ += ('_b',)
+__all__.append('__c__')
+__all__.extend(('___d', 'e'))
+
+a = _b = __c__ = ___d = e = f = _g = 1
+[builtins fixtures/module_all.pyi]
+
 [case testEllipsisInitializerInStubFileWithType]
 import m
 m.x = '' # E: Incompatible types in assignment (expression has type "str", variable has type "int")