stubgen: infer None return type for functions without return statement (#4467)

Fixes #4181.

Author: hoefling

mypy/stubgen.py

@@ -59,7 +59,7 @@
     Expression, IntExpr, UnaryExpr, StrExpr, BytesExpr, NameExpr, FloatExpr, MemberExpr, TupleExpr,
     ListExpr, ComparisonExpr, CallExpr, IndexExpr, EllipsisExpr,
     ClassDef, MypyFile, Decorator, AssignmentStmt,
-    IfStmt, ImportAll, ImportFrom, Import, FuncDef, FuncBase, TempNode,
+    IfStmt, ReturnStmt, ImportAll, ImportFrom, Import, FuncDef, FuncBase, TempNode,
     ARG_POS, ARG_STAR, ARG_STAR2, ARG_NAMED, ARG_NAMED_OPT,
 )
 from mypy.stubgenc import parse_all_signatures, find_unique_signatures, generate_stub_for_c_module
@@ -475,7 +475,7 @@ def visit_func_def(self, o: FuncDef) -> None:
         retname = None
         if isinstance(o.type, CallableType):
             retname = self.print_annotation(o.type.ret_type)
-        elif o.name() == '__init__':
+        elif o.name() == '__init__' or not has_return_statement(o):
             retname = 'None'
         retfield = ''
         if retname is not None:
@@ -814,6 +814,19 @@ def visit_assignment_stmt(self, o: AssignmentStmt) -> None:
     return results
 
 
+def has_return_statement(fdef: FuncBase) -> bool:
+    class ReturnSeeker(mypy.traverser.TraverserVisitor):
+        def __init__(self) -> None:
+            self.found = False
+
+        def visit_return_stmt(self, o: ReturnStmt) -> None:
+            self.found = True
+
+    seeker = ReturnSeeker()
+    fdef.accept(seeker)
+    return seeker.found
+
+
 def get_qualified_name(o: Expression) -> str:
     if isinstance(o, NameExpr):
         return o.name