Deserialization should be done after coercion

apischema/deserialization/__init__.py

@@ -647,7 +647,11 @@ def factory(constraints: Optional[Constraints], _) -> DeserializationMethod:
                     if fact.cls is not NoneType
                 )
                 return OptionalMethod(value_method, self.coercer)
-            elif len(method_by_cls) == len(alt_factories):
+            elif len(method_by_cls) == len(alt_factories) and not any(
+                isinstance(x, CoercerMethod) for x in alt_methods
+            ):
+                # Coercion induces a different type in data than type to deserialize.
+                # Prefer UnionMethod in this case.
                 return UnionByTypeMethod(method_by_cls)
             else:
                 return UnionMethod(alt_methods)

tests/integration/test_deserialize_with_coercion.py

@@ -0,0 +1,33 @@
+from __future__ import annotations
+
+import json
+from dataclasses import dataclass
+from typing import Any, Mapping, Sequence, Union
+
+from apischema import deserialize
+
+
+def _coerce_json(cls, data):
+    if not isinstance(data, cls) and isinstance(data, str):
+        return json.loads(data)
+    else:
+        return data
+
+
+@dataclass
+class MyClass:
+    my_property: Union[Sequence[str], Mapping[str, Any]]
+
+
+def test_coerce_json():
+    key = "test"
+    value = 2
+    ret = deserialize(
+        MyClass,
+        {
+            "my_property": f'{{"{key}": {value}}}',
+        },
+        coerce=_coerce_json,
+    )
+    assert isinstance(ret, MyClass)
+    assert isinstance(ret.my_property, dict) and ret.my_property[key] == value

tests/integration/test_object_fields_overriding.py

@@ -15,7 +15,8 @@ class Foo:
 
 
 @pytest.mark.skipif(
-    sys.version_info < (3, 8), reason="dataclasses.replace bug with InitVar"
+    (sys.version_info < (3, 8) or ((3, 9) < sys.version_info < (3, 9, 5))),
+    reason="dataclasses.replace bug with InitVar",
 )
 def test_object_fields_overriding():
     set_object_fields(Foo, [])