Raise error when flatten can not be used

Only dataclass and optional dataclass are supported at the moment

serde/compat.py

@@ -513,6 +513,25 @@ def is_bare_opt(typ: Any) -> bool:
     return not type_args(typ) and typ is Optional
 
 
+@cache
+def is_opt_dataclass(typ: Any) -> bool:
+    """
+    Test if the type is optional dataclass.
+
+    >>> is_opt_dataclass(Optional[int])
+    False
+    >>> @dataclasses.dataclass
+    ... class Foo:
+    ...     pass
+    >>> is_opt_dataclass(Foo)
+    False
+    >>> is_opt_dataclass(Optional[Foo])
+    False
+    """
+    args = get_args(typ)
+    return is_opt(typ) and len(args) > 0 and is_dataclass(args[0])
+
+
 @cache
 def is_list(typ: type[Any]) -> bool:
     """

serde/core.py

@@ -41,6 +41,7 @@
     is_new_type_primitive,
     is_any,
     is_opt,
+    is_opt_dataclass,
     is_set,
     is_tuple,
     is_union,
@@ -620,6 +621,8 @@ def from_dataclass(cls, f: dataclasses.Field[T], parent: Optional[Any] = None) -
         flatten = f.metadata.get("serde_flatten")
         if flatten is True:
             flatten = FlattenOpts()
+        if flatten and not (dataclasses.is_dataclass(f.type) or is_opt_dataclass(f.type)):
+            raise SerdeError(f"pyserde does not support flatten attribute for {typename(f.type)}")
 
         kw_only = bool(f.kw_only) if sys.version_info >= (3, 10) else False
 

tests/test_flatten.py

@@ -6,7 +6,7 @@
 
 import pytest
 
-from serde import field, serde
+from serde import field, serde, SerdeError
 from serde.json import from_json, to_json
 
 from .common import all_formats
@@ -68,3 +68,16 @@ class Foo:
 
     f = Foo(a=10, b="foo", bar=Bar(c=100.0, d=True))
     assert de(Foo, se(f)) == f
+
+
+@pytest.mark.parametrize("se,de", all_formats)
+def test_flatten_not_supported(se: Any, de: Any) -> None:
+    @serde
+    class Bar:
+        pass
+
+    with pytest.raises(SerdeError):
+
+        @serde
+        class Foo:
+            bar: list[Bar] = field(flatten=True)