Allow redefining a function via import

Author: JukkaL

mypy/semanal.py

@@ -829,7 +829,7 @@ def process_import_over_existing_name(self,
                                           module_symbol: SymbolTableNode,
                                           import_node: ImportBase) -> bool:
         if (existing_symbol.kind in (LDEF, GDEF, MDEF) and
-                isinstance(existing_symbol.node, Var)):
+                isinstance(existing_symbol.node, (Var, FuncDef))):
             # This is a valid import over an existing definition in the file. Construct a dummy
             # assignment that we'll use to type check the import.
             lvalue = NameExpr(imported_id)

mypy/test/data/check-modules.test

@@ -512,7 +512,7 @@ except:
 x = ''
 y = ''
 
-[case testOverrideImportedFunctionViaImport]
+[case testRedefineImportedFunctionViaImport]
 try:
     from m import f, g
 except:
@@ -532,6 +532,17 @@ x = 1
 [file n.py]
 x = ''
 
+[case testRedefineFunctionViaImport]
+def f(x): pass
+def g(x): pass
+try:
+    from m import f, g # E: Incompatible import of "g" (imported name has type Callable[[Any, Any], Any], local name has type Callable[[Any], Any])
+except:
+    pass
+[file m.py]
+def f(x): pass
+def g(x, y): pass
+
 
 -- Test cases that simulate 'mypy -m modname'
 --