autodoc shows undecorated signature for decorated functions

Describe the bug
autodoc shows undecorated signature for decorated functions

To Reproduce

def wrapper(fn):
    @functools.wraps(fn)
    def func():
        return fn(1, 2, 3)

    return func


@wrapper
def foo(x, y, z):
    return x + y + z

.. automodule:: example
   :members:
   :undoc-members:

This is converted to foo(x, y, z).

Expected behavior
The function foo should be shown as foo(). That is what users see.

Your project
No

Screenshots
No

Environment info

• OS: Mac
• Python version: 3.8.2
• Sphinx version: 3.1.0dev
• Sphinx extensions: sphinx.ext.autodoc
• Extra tools: No

Additional context
No

Start by reproducing the example with Sphinx 3.1.0dev and the sphinx.ext.autodoc automodule directive. Trace how autodoc determines the displayed signature for the decorated function, then verify that the generated documentation shows foo() rather than foo(x, y, z).