Correctly handle runtime type applications of variadic types (#16240)

This adds some missing pieces to runtime type application handling for
both `TypeVarTuple` and `ParamSpec`. Everything is straightforward
(maybe a bit hacky, but we already import `typeanal` in `checkexpr` for
similar purposes, e.g. type aliases in runtime context).

Fixes #14799

Author: ilevkivskyi

mypy/checkexpr.py

@@ -28,7 +28,7 @@
 from mypy.maptype import map_instance_to_supertype
 from mypy.meet import is_overlapping_types, narrow_declared_type
 from mypy.message_registry import ErrorMessage
-from mypy.messages import MessageBuilder
+from mypy.messages import MessageBuilder, format_type
 from mypy.nodes import (
     ARG_NAMED,
     ARG_POS,
@@ -116,10 +116,12 @@
 from mypy.type_visitor import TypeTranslator
 from mypy.typeanal import (
     check_for_explicit_any,
+    fix_instance,
     has_any_from_unimported_type,
     instantiate_type_alias,
     make_optional_type,
     set_any_tvars,
+    validate_instance,
 )
 from mypy.typeops import (
     callable_type,
@@ -166,10 +168,12 @@
     TypeVarLikeType,
     TypeVarTupleType,
     TypeVarType,
+    UnboundType,
     UninhabitedType,
     UnionType,
     UnpackType,
     find_unpack_in_list,
+    flatten_nested_tuples,
     flatten_nested_unions,
     get_proper_type,
     get_proper_types,
@@ -4637,15 +4641,35 @@ class C(Generic[T, Unpack[Ts]]): ...
         similar to how it is done in other places using split_with_prefix_and_suffix().
         """
         vars = t.variables
+        args = flatten_nested_tuples(args)
+
+        # TODO: this logic is duplicated with semanal_typeargs.
+        for tv, arg in zip(t.variables, args):
+            if isinstance(tv, ParamSpecType):
+                if not isinstance(
+                    get_proper_type(arg), (Parameters, ParamSpecType, AnyType, UnboundType)
+                ):
+                    self.chk.fail(
+                        "Can only replace ParamSpec with a parameter types list or"
+                        f" another ParamSpec, got {format_type(arg, self.chk.options)}",
+                        ctx,
+                    )
+                    return [AnyType(TypeOfAny.from_error)] * len(vars)
+
         if not vars or not any(isinstance(v, TypeVarTupleType) for v in vars):
             return list(args)
+        assert t.is_type_obj()
+        info = t.type_object()
+        # We reuse the logic from semanal phase to reduce code duplication.
+        fake = Instance(info, args, line=ctx.line, column=ctx.column)
+        if not validate_instance(fake, self.chk.fail):
+            fix_instance(
+                fake, self.chk.fail, self.chk.note, disallow_any=False, options=self.chk.options
+            )
+            args = list(fake.args)
 
         prefix = next(i for (i, v) in enumerate(vars) if isinstance(v, TypeVarTupleType))
         suffix = len(vars) - prefix - 1
-        if len(args) < len(vars) - 1:
-            self.msg.incompatible_type_application(len(vars), len(args), ctx)
-            return [AnyType(TypeOfAny.from_error)] * len(vars)
-
         tvt = vars[prefix]
         assert isinstance(tvt, TypeVarTupleType)
         start, middle, end = split_with_prefix_and_suffix(tuple(args), prefix, suffix)

test-data/unit/check-parameter-specification.test

@@ -1977,6 +1977,19 @@ g(cb, y='a', x=0)  # E: Argument "y" to "g" has incompatible type "str"; expecte
                    # E: Argument "x" to "g" has incompatible type "int"; expected "str"
 [builtins fixtures/paramspec.pyi]
 
+[case testParamSpecBadRuntimeTypeApplication]
+from typing import ParamSpec, TypeVar, Generic, Callable
+
+R = TypeVar("R")
+P = ParamSpec("P")
+class C(Generic[P, R]):
+    x: Callable[P, R]
+
+bad = C[int, str]()  # E: Can only replace ParamSpec with a parameter types list or another ParamSpec, got "int"
+reveal_type(bad)  # N: Revealed type is "__main__.C[Any, Any]"
+reveal_type(bad.x)  # N: Revealed type is "def (*Any, **Any) -> Any"
+[builtins fixtures/paramspec.pyi]
+
 [case testParamSpecNoCrashOnUnificationAlias]
 import mod
 [file mod.pyi]

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

@@ -1845,3 +1845,23 @@ def foo2(func: Callable[[Unpack[Args]], T], *args: Unpack[Args2]) -> T:
 def foo3(func: Callable[[int, Unpack[Args2]], T], *args: Unpack[Args2]) -> T:
    return submit(func, 1, *args)
 [builtins fixtures/tuple.pyi]
+
+[case testTypeVarTupleRuntimeTypeApplication]
+from typing import Generic, TypeVar, Tuple
+from typing_extensions import Unpack, TypeVarTuple
+
+T = TypeVar("T")
+S = TypeVar("S")
+Ts = TypeVarTuple("Ts")
+class C(Generic[T, Unpack[Ts], S]): ...
+
+Ints = Tuple[int, int]
+x = C[Unpack[Ints]]()
+reveal_type(x)  # N: Revealed type is "__main__.C[builtins.int, builtins.int]"
+
+y = C[Unpack[Tuple[int, ...]]]()
+reveal_type(y)  # N: Revealed type is "__main__.C[builtins.int, Unpack[builtins.tuple[builtins.int, ...]], builtins.int]"
+
+z = C[int]()  # E: Bad number of arguments, expected: at least 2, given: 1
+reveal_type(z)  # N: Revealed type is "__main__.C[Any, Unpack[builtins.tuple[Any, ...]], Any]"
+[builtins fixtures/tuple.pyi]