Use eval_type_backport on Python 3.9 if it's installed to resolve `in…

…t | None` etc.

This uses the same module that pydantic does, and it allows people to use the
new pipe syntax if they have to support Python3.9 too -- very useful for
libraries.

(Also it works better with many type checkers which seem to mistakenly think
that with `from __future__ import annotations` means `int| None` will work,
but it doesn't out of the box.)

msgspec/_utils.py

@@ -45,6 +45,28 @@ def _forward_ref(value):
 
     def _eval_type(t, globalns, localns):
         return typing._eval_type(t, globalns, localns, ())
+elif sys.version_info < (3, 10):
+
+    def _eval_type(t, globalns, localns):
+        try:
+            return typing._eval_type(t, globalns, localns)
+        except TypeError as e:
+            try:
+                from eval_type_backport import eval_type_backport
+            except ImportError:
+                raise TypeError(
+                    f"Unable to evaluate type annotation {t.__forward_arg__!r}. If you are making use "
+                    "of the new typing syntax (unions using `|` since Python 3.10 or builtins subscripting "
+                    "since Python 3.9), you should either replace the use of new syntax with the existing "
+                    "`typing` constructs or install the `eval_type_backport` package."
+                ) from e
+
+            return eval_type_backport(
+                t,
+                globalns,
+                localns,
+                try_default=False,
+            )
 else:
     _eval_type = typing._eval_type
 

tests/test_utils.py

@@ -1,12 +1,16 @@
 from __future__ import annotations
 
-from typing import Generic, List, Set, TypeVar
+import contextlib
+import sys
+from typing import Generic, List, Optional, Set, TypeVar
 
 import pytest
 from utils import temp_module
 
 from msgspec._utils import get_class_annotations
 
+PY310 = sys.version_info[:2] >= (3, 10)
+
 T = TypeVar("T")
 S = TypeVar("S")
 U = TypeVar("U")
@@ -201,3 +205,25 @@ class Sub(Base[Invalid]):
             pass
 
         assert get_class_annotations(Sub) == {"x": Invalid}
+
+    @pytest.mark.skipif(PY310, reason="<3.10 only")
+    @pytest.mark.parametrize(
+        ("matcher"),
+        [
+            # Installed, should give us the result to check
+            pytest.param(contextlib.nullcontext(), id="installed"),
+            # Not installed, sohuld throw this error
+            pytest.param(
+                pytest.raises(
+                    TypeError, match=r"or install the `eval_type_backport` package."
+                ),
+                id="not_installed",
+            ),
+        ],
+    )
+    def test_union_backcompat(self, matcher):
+        class X:
+            opt_int: int | None = None
+
+        with matcher:
+            assert get_class_annotations(X) == {"opt_int": Optional[int]}