Certain type aliases break type formatting in automodule

[dfremont]

Describe the bug
If automodule is applied to a module which defines a documented complex type alias, the type annotations on all attributes later in the module will be ignored.

To Reproduce
Apply automodule to a module like the following:

import typing

Alias = typing.Union[int, float]
"""a type alias"""

class Foo:
    """blah"""

    bar: object
    """an attribute"""

The generated documentation will not list any type for the attribute bar. Removing the definition of Alias causes the type of bar to be correctly rendered.

Expected behavior
The presence of a type alias definition should not affect the display of type information for unrelated members.

Your project
Here is a tiny example project: example3.zip

Environment info

• OS: macOS 10.14.6
• Python version: 3.8.0
• Sphinx version: 3.2.1 (also reproduced on the current 3.x branch)
• Sphinx extensions: sphinx.ext.autodoc

Additional context
The reason for the bug is that autodoc.GenericAliasDocumenter.add_directive_header and autodoc.TypeVarDocumenter.add_directive_header use the line self.options.annotation = SUPPRESS to suppress the printing of the usual type information. Unfortunately, the self.options object can be shared across multiple documenters created as part of the same automodule directive, so mutating the options for one documenter affects all the others. One way to fix this is to change autodoc.Documenter.__init__ to make a copy of the Options object, using the line self.options = Options(directive.genopt). Making this change causes the bug described above to disappear.

Thanks to @altendky for helping me debug this problem.