Use TypeVar defaults to fix instances

Author: cdce8p

mypy/messages.py

@@ -2910,12 +2910,15 @@ def for_function(callee: CallableType) -> str:
     return ""
 
 
-def wrong_type_arg_count(n: int, act: str, name: str) -> str:
-    s = f"{n} type arguments"
-    if n == 0:
-        s = "no type arguments"
-    elif n == 1:
-        s = "1 type argument"
+def wrong_type_arg_count(low: int, high: int, act: str, name: str) -> str:
+    if low == high:
+        s = f"{low} type arguments"
+        if low == 0:
+            s = "no type arguments"
+        elif low == 1:
+            s = "1 type argument"
+    else:
+        s = f"between {low} and {high} type arguments"
     if act == "0":
         act = "none"
     return f'"{name}" expects {s}, but {act} given'

mypy/typeanal.py

@@ -9,6 +9,7 @@
 
 from mypy import errorcodes as codes, message_registry, nodes
 from mypy.errorcodes import ErrorCode
+from mypy.expandtype import expand_type
 from mypy.messages import MessageBuilder, format_type_bare, quote_type_string, wrong_type_arg_count
 from mypy.nodes import (
     ARG_NAMED,
@@ -74,6 +75,7 @@
     TypeOfAny,
     TypeQuery,
     TypeType,
+    TypeVarId,
     TypeVarLikeType,
     TypeVarTupleType,
     TypeVarType,
@@ -1676,25 +1678,39 @@ def fix_instance(
 
     Also emit a suitable error if this is not due to implicit Any's.
     """
-    if len(t.args) == 0:
-        if use_generic_error:
-            fullname: str | None = None
-        else:
-            fullname = t.type.fullname
-        any_type = get_omitted_any(disallow_any, fail, note, t, options, fullname, unexpanded_type)
-        t.args = (any_type,) * len(t.type.type_vars)
-        return
-    # Invalid number of type parameters.
-    fail(
-        wrong_type_arg_count(len(t.type.type_vars), str(len(t.args)), t.type.name),
-        t,
-        code=codes.TYPE_ARG,
-    )
-    # Construct the correct number of type arguments, as
-    # otherwise the type checker may crash as it expects
-    # things to be right.
-    t.args = tuple(AnyType(TypeOfAny.from_error) for _ in t.type.type_vars)
-    t.invalid = True
+    arg_count = len(t.args)
+    max_tv_count = len(t.type.type_vars)
+    args: list[Type] = [*(t.args[:max_tv_count])]
+    any_type: AnyType | None = None
+    env: dict[TypeVarId, Type] = {}
+    for tv, arg in itertools.zip_longest(t.type.defn.type_vars, t.args, fillvalue=None):
+        if tv is None:
+            continue
+        if arg is None:
+            if tv.has_default():
+                arg = tv.default
+            else:
+                if any_type is None:
+                    fullname = None if use_generic_error else t.type.fullname
+                    any_type = get_omitted_any(
+                        disallow_any, fail, note, t, options, fullname, unexpanded_type
+                    )
+                arg = any_type
+            args.append(arg)
+        env[tv.id] = arg
+    t.args = tuple(args)
+    fixed = expand_type(t, env)
+    assert isinstance(fixed, Instance)
+    t.args = fixed.args
+
+    min_tv_count = sum(tv.has_default() is False for tv in t.type.defn.type_vars)
+    if arg_count != 0 and not (min_tv_count <= arg_count <= max_tv_count):
+        fail(
+            wrong_type_arg_count(min_tv_count, max_tv_count, str(arg_count), t.type.name),
+            t,
+            code=codes.TYPE_ARG,
+        )
+        t.invalid = True
 
 
 def expand_type_alias(
@@ -1753,7 +1769,7 @@ def expand_type_alias(
         if use_standard_error:
             # This is used if type alias is an internal representation of another type,
             # for example a generic TypedDict or NamedTuple.
-            msg = wrong_type_arg_count(exp_len, str(act_len), node.name)
+            msg = wrong_type_arg_count(exp_len, exp_len, str(act_len), node.name)
         else:
             msg = f"Bad number of arguments for type alias, expected: {exp_len}, given: {act_len}"
         fail(msg, ctx, code=codes.TYPE_ARG)

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

@@ -116,3 +116,63 @@ def func_c1(x: Union[int, Callable[[Unpack[Ts1]], None]]) -> Tuple[Unpack[Ts1]]:
 # reveal_type(func_c1(callback1))  # Revealed type is "builtins.tuple[str]"  # TODO
 # reveal_type(func_c1(2))  # Revealed type is "builtins.tuple[builtins.int, builtins.str]"  # TODO
 [builtins fixtures/tuple.pyi]
+
+[case testTypeVarDefaultsClass1]
+from typing import Generic, TypeVar
+
+T1 = TypeVar("T1")
+T2 = TypeVar("T2", default=int)
+T3 = TypeVar("T3", default=str)
+
+class ClassA1(Generic[T2, T3]): ...
+
+def func_a1(
+    a: ClassA1,
+    b: ClassA1[float],
+    c: ClassA1[float, float],
+) -> None:
+    reveal_type(a)  # N: Revealed type is "__main__.ClassA1[builtins.int, builtins.str]"
+    reveal_type(b)  # N: Revealed type is "__main__.ClassA1[builtins.float, builtins.str]"
+    reveal_type(c)  # N: Revealed type is "__main__.ClassA1[builtins.float, builtins.float]"
+
+class ClassA2(Generic[T1, T2, T3]): ...
+
+def func_a2(
+    a: ClassA2,
+    b: ClassA2[float],
+    c: ClassA2[float, float],
+    d: ClassA2[float, float, float],
+) -> None:
+    reveal_type(a)  # N: Revealed type is "__main__.ClassA2[Any, builtins.int, builtins.str]"
+    reveal_type(b)  # N: Revealed type is "__main__.ClassA2[builtins.float, builtins.int, builtins.str]"
+    reveal_type(c)  # N: Revealed type is "__main__.ClassA2[builtins.float, builtins.float, builtins.str]"
+    reveal_type(d)  # N: Revealed type is "__main__.ClassA2[builtins.float, builtins.float, builtins.float]"
+
+[case testTypeVarDefaultsClass2]
+from typing import Generic, ParamSpec
+
+P1 = ParamSpec("P1")
+P2 = ParamSpec("P2", default=(int, str))
+P3 = ParamSpec("P3", default=...)
+
+class ClassB1(Generic[P2, P3]): ...
+
+def func_b1(
+    a: ClassB1,
+    b: ClassB1[[float]],
+    c: ClassB1[[float], [float]],
+) -> None:
+    reveal_type(a)  # N: Revealed type is "__main__.ClassB1[[builtins.int, builtins.str], ...]"
+    reveal_type(b)  # N: Revealed type is "__main__.ClassB1[[builtins.float], ...]"
+    reveal_type(c)  # N: Revealed type is "__main__.ClassB1[[builtins.float], [builtins.float]]"
+
+class ClassB2(Generic[P1, P2]): ...
+
+def func_b2(
+    a: ClassB2,
+    b: ClassB2[[float]],
+    c: ClassB2[[float], [float]],
+) -> None:
+    reveal_type(a)  # N: Revealed type is "__main__.ClassB2[Any, [builtins.int, builtins.str]]"
+    reveal_type(b)  # N: Revealed type is "__main__.ClassB2[[builtins.float], [builtins.int, builtins.str]]"
+    reveal_type(c)  # N: Revealed type is "__main__.ClassB2[[builtins.float], [builtins.float]]"