dataclasses.replace: fall through to typeshed sig (#15962)

If the dataclasses plugin cannot determine a signature for
`dataclasses.replace`, it should not report an error. The underlying
typeshed signature will get a shot at verifying the type and reporting
an error, and it would enable the following pattern (without typing
`replace`'s kwargs, though)

mypy/plugins/dataclasses.py

@@ -972,25 +972,6 @@ def _has_direct_dataclass_transform_metaclass(info: TypeInfo) -> bool:
     )
 
 
-def _fail_not_dataclass(ctx: FunctionSigContext, t: Type, parent_t: Type) -> None:
-    t_name = format_type_bare(t, ctx.api.options)
-    if parent_t is t:
-        msg = (
-            f'Argument 1 to "replace" has a variable type "{t_name}" not bound to a dataclass'
-            if isinstance(t, TypeVarType)
-            else f'Argument 1 to "replace" has incompatible type "{t_name}"; expected a dataclass'
-        )
-    else:
-        pt_name = format_type_bare(parent_t, ctx.api.options)
-        msg = (
-            f'Argument 1 to "replace" has type "{pt_name}" whose item "{t_name}" is not bound to a dataclass'
-            if isinstance(t, TypeVarType)
-            else f'Argument 1 to "replace" has incompatible type "{pt_name}" whose item "{t_name}" is not a dataclass'
-        )
-
-    ctx.api.fail(msg, ctx.context)
-
-
 def _get_expanded_dataclasses_fields(
     ctx: FunctionSigContext, typ: ProperType, display_typ: ProperType, parent_typ: ProperType
 ) -> list[CallableType] | None:
@@ -999,9 +980,7 @@ def _get_expanded_dataclasses_fields(
     For generic classes, the field types are expanded.
     If the type contains Any or a non-dataclass, returns None; in the latter case, also reports an error.
     """
-    if isinstance(typ, AnyType):
-        return None
-    elif isinstance(typ, UnionType):
+    if isinstance(typ, UnionType):
         ret: list[CallableType] | None = []
         for item in typ.relevant_items():
             item = get_proper_type(item)
@@ -1018,14 +997,12 @@ def _get_expanded_dataclasses_fields(
     elif isinstance(typ, Instance):
         replace_sym = typ.type.get_method(_INTERNAL_REPLACE_SYM_NAME)
         if replace_sym is None:
-            _fail_not_dataclass(ctx, display_typ, parent_typ)
             return None
         replace_sig = replace_sym.type
         assert isinstance(replace_sig, ProperType)
         assert isinstance(replace_sig, CallableType)
         return [expand_type_by_instance(replace_sig, typ)]
     else:
-        _fail_not_dataclass(ctx, display_typ, parent_typ)
         return None
 
 

test-data/unit/check-dataclass-transform.test

@@ -853,7 +853,7 @@ class Person:
     name: str
 
 p = Person('John')
-y = replace(p, name='Bob')  # E: Argument 1 to "replace" has incompatible type "Person"; expected a dataclass
+y = replace(p, name='Bob')
 
 [typing fixtures/typing-full.pyi]
 [builtins fixtures/dataclasses.pyi]

test-data/unit/check-dataclasses.test

@@ -2122,6 +2122,8 @@ a2 = replace(a, x='42', q=42)  # E: Argument "x" to "replace" of "A" has incompa
 a2 = replace(a, q='42')  # E: Argument "q" to "replace" of "A" has incompatible type "str"; expected "int"
 reveal_type(a2)  # N: Revealed type is "__main__.A"
 
+[builtins fixtures/tuple.pyi]
+
 [case testReplaceUnion]
 from typing import Generic, Union, TypeVar
 from dataclasses import dataclass, replace, InitVar
@@ -2151,7 +2153,7 @@ _ = replace(a_or_b, x=42, y=True, z='42', init_var=42)  # E: Argument "z" to "re
 _ = replace(a_or_b, x=42, y=True, w={}, init_var=42)  # E: Argument "w" to "replace" of "Union[A[int], B]" has incompatible type "Dict[<nothing>, <nothing>]"; expected <nothing>
 _ = replace(a_or_b, y=42, init_var=42)  # E: Argument "y" to "replace" of "Union[A[int], B]" has incompatible type "int"; expected "bool"
 
-[builtins fixtures/dataclasses.pyi]
+[builtins fixtures/tuple.pyi]
 
 [case testReplaceUnionOfTypeVar]
 from typing import Generic, Union, TypeVar
@@ -2171,7 +2173,9 @@ TA = TypeVar('TA', bound=A)
 TB = TypeVar('TB', bound=B)
 
 def f(b_or_t: Union[TA, TB, int]) -> None:
-    a2 = replace(b_or_t)   # E: Argument 1 to "replace" has type "Union[TA, TB, int]" whose item "TB" is not bound to a dataclass  # E: Argument 1 to "replace" has incompatible type "Union[TA, TB, int]" whose item "int" is not a dataclass
+    a2 = replace(b_or_t)   # E: Value of type variable "_DataclassT" of "replace" cannot be "Union[TA, TB, int]"
+
+[builtins fixtures/tuple.pyi]
 
 [case testReplaceTypeVarBoundNotDataclass]
 from dataclasses import dataclass, replace
@@ -2183,16 +2187,18 @@ TNone = TypeVar('TNone', bound=None)
 TUnion = TypeVar('TUnion', bound=Union[str, int])
 
 def f1(t: TInt) -> None:
-    _ = replace(t, x=42)  # E: Argument 1 to "replace" has a variable type "TInt" not bound to a dataclass
+    _ = replace(t, x=42)  # E: Value of type variable "_DataclassT" of "replace" cannot be "TInt"
 
 def f2(t: TAny) -> TAny:
-    return replace(t, x='spam')  # E: Argument 1 to "replace" has a variable type "TAny" not bound to a dataclass
+    return replace(t, x='spam')  # E: Value of type variable "_DataclassT" of "replace" cannot be "TAny"
 
 def f3(t: TNone) -> TNone:
-    return replace(t, x='spam')  # E: Argument 1 to "replace" has a variable type "TNone" not bound to a dataclass
+    return replace(t, x='spam')  # E: Value of type variable "_DataclassT" of "replace" cannot be "TNone"
 
 def f4(t: TUnion) -> TUnion:
-    return replace(t, x='spam')  # E: Argument 1 to "replace" has incompatible type "TUnion" whose item "str" is not a dataclass  # E: Argument 1 to "replace" has incompatible type "TUnion" whose item "int" is not a dataclass
+    return replace(t, x='spam')  # E: Value of type variable "_DataclassT" of "replace" cannot be "TUnion"
+
+[builtins fixtures/tuple.pyi]
 
 [case testReplaceTypeVarBound]
 from dataclasses import dataclass, replace
@@ -2217,6 +2223,8 @@ def f(t: TA) -> TA:
 f(A(x=42))
 f(B(x=42))
 
+[builtins fixtures/tuple.pyi]
+
 [case testReplaceAny]
 from dataclasses import replace
 from typing import Any
@@ -2225,17 +2233,33 @@ a: Any
 a2 = replace(a)
 reveal_type(a2)  # N: Revealed type is "Any"
 
+[builtins fixtures/tuple.pyi]
+
 [case testReplaceNotDataclass]
 from dataclasses import replace
 
-replace(5)  # E: Argument 1 to "replace" has incompatible type "int"; expected a dataclass
+replace(5)  # E: Value of type variable "_DataclassT" of "replace" cannot be "int"
 
 class C:
     pass
 
-replace(C())  # E: Argument 1 to "replace" has incompatible type "C"; expected a dataclass
+replace(C())  # E: Value of type variable "_DataclassT" of "replace" cannot be "C"
 
-replace(None)  # E: Argument 1 to "replace" has incompatible type "None"; expected a dataclass
+replace(None)  # E: Value of type variable "_DataclassT" of "replace" cannot be "None"
+
+[builtins fixtures/tuple.pyi]
+
+[case testReplaceIsDataclass]
+from dataclasses import is_dataclass, replace
+
+def f(x: object) -> None:
+  _ = replace(x)  # E: Value of type variable "_DataclassT" of "replace" cannot be "object"
+  if is_dataclass(x):
+    _ = replace(x)  # E: Value of type variable "_DataclassT" of "replace" cannot be "Union[DataclassInstance, Type[DataclassInstance]]"
+    if not isinstance(x, type):
+      _ = replace(x)
+
+[builtins fixtures/tuple.pyi]
 
 [case testReplaceGeneric]
 from dataclasses import dataclass, replace, InitVar
@@ -2254,6 +2278,8 @@ reveal_type(a2)  # N: Revealed type is "__main__.A[builtins.int]"
 a2 = replace(a, x='42')  # E: Argument "x" to "replace" of "A[int]" has incompatible type "str"; expected "int"
 reveal_type(a2)  # N: Revealed type is "__main__.A[builtins.int]"
 
+[builtins fixtures/tuple.pyi]
+
 [case testPostInitCorrectSignature]
 from typing import Any, Generic, TypeVar, Callable, Self
 from dataclasses import dataclass, InitVar

test-data/unit/lib-stub/dataclasses.pyi

@@ -1,6 +1,14 @@
-from typing import Any, Callable, Generic, Mapping, Optional, TypeVar, overload, Type
+from typing import Any, Callable, Generic, Literal, Mapping, Optional, TypeVar, overload, Type, \
+    Protocol, ClassVar
+from typing_extensions import TypeGuard
+
+# DataclassInstance is in _typeshed.pyi normally, but alas we can't do the same for lib-stub
+# due to test-data/unit/lib-stub/builtins.pyi not having 'tuple'.
+class DataclassInstance(Protocol):
+    __dataclass_fields__: ClassVar[dict[str, Field[Any]]]
 
 _T = TypeVar('_T')
+_DataclassT = TypeVar("_DataclassT", bound=DataclassInstance)
 
 class InitVar(Generic[_T]):
     ...
@@ -33,4 +41,12 @@ def field(*,
 
 class Field(Generic[_T]): pass
 
-def replace(__obj: _T, **changes: Any) -> _T: ...
+@overload
+def is_dataclass(obj: DataclassInstance) -> Literal[True]: ...
+@overload
+def is_dataclass(obj: type) -> TypeGuard[type[DataclassInstance]]: ...
+@overload
+def is_dataclass(obj: object) -> TypeGuard[DataclassInstance | type[DataclassInstance]]: ...
+
+
+def replace(__obj: _DataclassT, **changes: Any) -> _DataclassT: ...