Fix type application for classes with generic constructors (#17354)

Fixes #17212

Note I removed the problematic asset after all. It is hard to maintain
it, since this function may be called from both explicit application,
and from type inference code paths. And these two cases may have
different min/max type argument count (see tests and comments for
examples).

Author: ilevkivskyi

mypy/applytype.py

@@ -101,8 +101,7 @@ def apply_generic_arguments(
     bound or constraints, instead of giving an error.
     """
     tvars = callable.variables
-    min_arg_count = sum(not tv.has_default() for tv in tvars)
-    assert min_arg_count <= len(orig_types) <= len(tvars)
+    assert len(orig_types) <= len(tvars)
     # Check that inferred type variable values are compatible with allowed
     # values and bounds.  Also, promote subtype values to allowed values.
     # Create a map from type variable id to target type.
@@ -156,7 +155,7 @@ def apply_generic_arguments(
         type_is = None
 
     # The callable may retain some type vars if only some were applied.
-    # TODO: move apply_poly() logic from checkexpr.py here when new inference
+    # TODO: move apply_poly() logic here when new inference
     # becomes universally used (i.e. in all passes + in unification).
     # With this new logic we can actually *add* some new free variables.
     remaining_tvars: list[TypeVarLikeType] = []

mypy/checkexpr.py

@@ -4781,7 +4781,11 @@ class C(Generic[T, Unpack[Ts]]): ...
         We simply group the arguments that need to go into Ts variable into a TupleType,
         similar to how it is done in other places using split_with_prefix_and_suffix().
         """
-        vars = t.variables
+        if t.is_type_obj():
+            # Type arguments must map to class type variables, ignoring constructor vars.
+            vars = t.type_object().defn.type_vars
+        else:
+            vars = list(t.variables)
         args = flatten_nested_tuples(args)
 
         # TODO: this logic is duplicated with semanal_typeargs.
@@ -4799,6 +4803,7 @@ class C(Generic[T, Unpack[Ts]]): ...
 
         if not vars or not any(isinstance(v, TypeVarTupleType) for v in vars):
             return list(args)
+        # TODO: in future we may want to support type application to variadic functions.
         assert t.is_type_obj()
         info = t.type_object()
         # We reuse the logic from semanal phase to reduce code duplication.
@@ -4832,10 +4837,23 @@ def apply_type_arguments_to_callable(
         tp = get_proper_type(tp)
 
         if isinstance(tp, CallableType):
-            min_arg_count = sum(not v.has_default() for v in tp.variables)
-            has_type_var_tuple = any(isinstance(v, TypeVarTupleType) for v in tp.variables)
+            if tp.is_type_obj():
+                # If we have a class object in runtime context, then the available type
+                # variables are those of the class, we don't include additional variables
+                # of the constructor. So that with
+                #     class C(Generic[T]):
+                #         def __init__(self, f: Callable[[S], T], x: S) -> None
+                # C[int] is valid
+                # C[int, str] is invalid (although C as a callable has 2 type variables)
+                # Note: various logic below and in applytype.py relies on the fact that
+                # class type variables appear *before* constructor variables.
+                type_vars = tp.type_object().defn.type_vars
+            else:
+                type_vars = list(tp.variables)
+            min_arg_count = sum(not v.has_default() for v in type_vars)
+            has_type_var_tuple = any(isinstance(v, TypeVarTupleType) for v in type_vars)
             if (
-                len(args) < min_arg_count or len(args) > len(tp.variables)
+                len(args) < min_arg_count or len(args) > len(type_vars)
             ) and not has_type_var_tuple:
                 if tp.is_type_obj() and tp.type_object().fullname == "builtins.tuple":
                     # e.g. expression tuple[X, Y]
@@ -4854,19 +4872,24 @@ def apply_type_arguments_to_callable(
                         bound_args=tp.bound_args,
                     )
                 self.msg.incompatible_type_application(
-                    min_arg_count, len(tp.variables), len(args), ctx
+                    min_arg_count, len(type_vars), len(args), ctx
                 )
                 return AnyType(TypeOfAny.from_error)
             return self.apply_generic_arguments(tp, self.split_for_callable(tp, args, ctx), ctx)
         if isinstance(tp, Overloaded):
             for it in tp.items:
-                min_arg_count = sum(not v.has_default() for v in it.variables)
-                has_type_var_tuple = any(isinstance(v, TypeVarTupleType) for v in it.variables)
+                if tp.is_type_obj():
+                    # Same as above.
+                    type_vars = tp.type_object().defn.type_vars
+                else:
+                    type_vars = list(it.variables)
+                min_arg_count = sum(not v.has_default() for v in type_vars)
+                has_type_var_tuple = any(isinstance(v, TypeVarTupleType) for v in type_vars)
                 if (
-                    len(args) < min_arg_count or len(args) > len(it.variables)
+                    len(args) < min_arg_count or len(args) > len(type_vars)
                 ) and not has_type_var_tuple:
                     self.msg.incompatible_type_application(
-                        min_arg_count, len(it.variables), len(args), ctx
+                        min_arg_count, len(type_vars), len(args), ctx
                     )
                     return AnyType(TypeOfAny.from_error)
             return Overloaded(

mypy/typeanal.py

@@ -2376,6 +2376,12 @@ def validate_instance(t: Instance, fail: MsgCallback, empty_tuple_index: bool) -
         if not t.args:
             if not (empty_tuple_index and len(t.type.type_vars) == 1):
                 # The Any arguments should be set by the caller.
+                if empty_tuple_index and min_tv_count:
+                    fail(
+                        f"At least {min_tv_count} type argument(s) expected, none given",
+                        t,
+                        code=codes.TYPE_ARG,
+                    )
                 return False
         elif not correct:
             fail(

test-data/unit/check-generics.test

@@ -3443,6 +3443,19 @@ h: Callable[[Unpack[Us]], Foo[int]]
 reveal_type(dec(h))  # N: Revealed type is "def (builtins.int) -> __main__.Foo[builtins.int]"
 [builtins fixtures/list.pyi]
 
+[case testTypeApplicationGenericConstructor]
+from typing import Generic, TypeVar, Callable
+
+T = TypeVar("T")
+S = TypeVar("S")
+class C(Generic[T]):
+    def __init__(self, f: Callable[[S], T], x: S) -> None:
+        self.x = f(x)
+
+reveal_type(C[int])  # N: Revealed type is "def [S] (f: def (S`-1) -> builtins.int, x: S`-1) -> __main__.C[builtins.int]"
+Alias = C[int]
+C[int, str]  # E: Type application has too many types (1 expected)
+
 [case testHigherOrderGenericPartial]
 from typing import TypeVar, Callable
 

test-data/unit/check-typevar-tuple.test

@@ -2378,3 +2378,30 @@ def a2(x: Array[int, str]) -> None:
     reveal_type(func(x, 2, "Hello", True))   # E: Cannot infer type argument 1 of "func" \
                                              # N: Revealed type is "builtins.tuple[Any, ...]"
 [builtins fixtures/tuple.pyi]
+
+[case testTypeVarTupleTypeApplicationOverload]
+from typing import Generic, TypeVar, TypeVarTuple, Unpack, overload, Callable
+
+T = TypeVar("T")
+T1 = TypeVar("T1")
+T2 = TypeVar("T2")
+T3 = TypeVar("T3")
+Ts = TypeVarTuple("Ts")
+
+class C(Generic[T, Unpack[Ts]]):
+    @overload
+    def __init__(self, f: Callable[[Unpack[Ts]], T]) -> None: ...
+    @overload
+    def __init__(self, f: Callable[[T1, T2, T3, Unpack[Ts]], T], a: T1, b: T2, c: T3) -> None: ...
+    def __init__(self, f, *args, **kwargs) -> None:
+        ...
+
+reveal_type(C[int, str])  # N: Revealed type is "Overload(def (f: def (builtins.str) -> builtins.int) -> __main__.C[builtins.int, builtins.str], def [T1, T2, T3] (f: def (T1`-1, T2`-2, T3`-3, builtins.str) -> builtins.int, a: T1`-1, b: T2`-2, c: T3`-3) -> __main__.C[builtins.int, builtins.str])"
+Alias = C[int, str]
+
+def f(x: int, y: int, z: int, t: int) -> str: ...
+x = C(f, 0, 0, "hm")  # E: Argument 1 to "C" has incompatible type "Callable[[int, int, int, int], str]"; expected "Callable[[int, int, str, int], str]"
+reveal_type(x)  # N: Revealed type is "__main__.C[builtins.str, builtins.int]"
+reveal_type(C(f))  # N: Revealed type is "__main__.C[builtins.str, builtins.int, builtins.int, builtins.int, builtins.int]"
+C[()]  # E: At least 1 type argument(s) expected, none given
+[builtins fixtures/tuple.pyi]