Better handling of covariance, contravariance, and TypeVar in from_type()

[Zac-HD]

hypothesis/hypothesis-python/src/hypothesis/searchstrategy/types.py

Lines 118 to 126 in 2c6b6f3

	if isinstance(thing, typing.TypeVar):
	if getattr(thing, "__bound__", None) is not None:
	return st.from_type(thing.__bound__)
	if getattr(thing, "__constraints__", None):
	return st.shared(
	st.sampled_from(thing.__constraints__), key="typevar-with-constraint"
	).flatmap(st.from_type)
	# Constraints may be None or () on various Python versions.
	return st.text() # An arbitrary type for the typevar

Type variables are tricky!

• We should handle constraints before bounds. They're mutually exclusive, but easier to handle the "neither" case that way.
• The shared() strategy should be keyed on f"typevar-{thing.__name__}"
• The bounds case should also use shared()! Otherwise it's not really a typevar...
• We can use a (shared() of) from_type(getattr(thing, "__bound__", object)) to generalise the text() hack.

A larger problem is that from_type() currently always treats things as covariant (i.e. returns a strategy for subtypes if the exact type is unknown). We should probably provide an argument for in/co/contra variance and default to invariance, which will be an annoying deprecation pathway. Plus use that from the TypeVar handling, obviously.

(this should be retagged from enhancement to bug if it's seen in the wild)