Don't pass None as the full name to a plugin (#3582)

We would sometimes pass `None` as the full name of a function to a
plugin, which could cause a crash within a plugin that doesn't expect
it.

Author: JukkaL

mypy/checkexpr.py

@@ -375,7 +375,7 @@ def apply_function_plugin(self,
                               formal_to_actual: List[List[int]],
                               args: List[Expression],
                               num_formals: int,
-                              fullname: Optional[str],
+                              fullname: str,
                               object_type: Optional[Type],
                               context: Context) -> Type:
         """Use special case logic to infer the return type of a specific named function/method.
@@ -529,8 +529,10 @@ def check_call(self, callee: Type, args: List[Expression],
                 # Store the inferred callable type.
                 self.chk.store_type(callable_node, callee)
 
-            if ((object_type is None and self.plugin.get_function_hook(callable_name))
-                    or (object_type is not None and self.plugin.get_method_hook(callable_name))):
+            if (callable_name
+                    and ((object_type is None and self.plugin.get_function_hook(callable_name))
+                         or (object_type is not None
+                             and self.plugin.get_method_hook(callable_name)))):
                 ret_type = self.apply_function_plugin(
                     arg_types, callee.ret_type, arg_kinds, formal_to_actual,
                     args, len(callee.arg_types), callable_name, object_type, context)

test-data/unit/check-custom-plugin.test

@@ -11,6 +11,17 @@ reveal_type(f())  # E: Revealed type is 'builtins.int'
 [[mypy]
 plugins=<ROOT>/test-data/unit/plugins/fnplugin.py
 
+[case testFunctionPluginFullnameIsNotNone]
+# flags: --config-file tmp/mypy.ini
+from typing import Callable, TypeVar
+f: Callable[[], None]
+T = TypeVar('T')
+def g(x: T) -> T: return x  # This strips out the name of a callable
+g(f)()
+[file mypy.ini]
+[[mypy]
+plugins=<ROOT>/test-data/unit/plugins/fnplugin.py
+
 [case testTwoPlugins]
 # flags: --config-file tmp/mypy.ini
 def f(): ...

test-data/unit/plugins/fnplugin.py

@@ -4,6 +4,7 @@ class MyPlugin(Plugin):
     def get_function_hook(self, fullname):
         if fullname == '__main__.f':
             return my_hook
+        assert fullname is not None
         return None
 
 def my_hook(ctx):