Enable new type inference by default (#16345)

Fixes #15906

I am adding `--old-type-inference` so people can disable the flag if
they have issues (for few releases). IIRC there will be some fallback in
`mypy_primer`, but last time I checked it was all correct. Also I don't
remember if we need to update some tests, but we will see.

mypy/checker.py

@@ -4043,11 +4043,11 @@ def is_valid_defaultdict_partial_value_type(self, t: ProperType) -> bool:
             return True
         if len(t.args) == 1:
             arg = get_proper_type(t.args[0])
-            if self.options.new_type_inference:
-                allowed = isinstance(arg, (UninhabitedType, NoneType))
-            else:
+            if self.options.old_type_inference:
                 # Allow leaked TypeVars for legacy inference logic.
                 allowed = isinstance(arg, (UninhabitedType, NoneType, TypeVarType))
+            else:
+                allowed = isinstance(arg, (UninhabitedType, NoneType))
             if allowed:
                 return True
         return False

mypy/checkexpr.py

@@ -343,7 +343,7 @@ def __init__(
         # on whether current expression is a callee, to give better error messages
         # related to type context.
         self.is_callee = False
-        type_state.infer_polymorphic = self.chk.options.new_type_inference
+        type_state.infer_polymorphic = not self.chk.options.old_type_inference
 
     def reset(self) -> None:
         self.resolved_type = {}
@@ -2082,7 +2082,7 @@ def infer_function_type_arguments(
                 elif not first_arg or not is_subtype(self.named_type("builtins.str"), first_arg):
                     self.chk.fail(message_registry.KEYWORD_ARGUMENT_REQUIRES_STR_KEY_TYPE, context)
 
-            if self.chk.options.new_type_inference and any(
+            if not self.chk.options.old_type_inference and any(
                 a is None
                 or isinstance(get_proper_type(a), UninhabitedType)
                 or set(get_type_vars(a)) & set(callee_type.variables)
@@ -2181,7 +2181,11 @@ def infer_function_type_arguments_pass2(
                 lambda a: self.accept(args[a]),
             )
 
-        arg_types = self.infer_arg_types_in_context(callee_type, args, arg_kinds, formal_to_actual)
+        # Same as during first pass, disable type errors (we still have partial context).
+        with self.msg.filter_errors():
+            arg_types = self.infer_arg_types_in_context(
+                callee_type, args, arg_kinds, formal_to_actual
+            )
 
         inferred_args, _ = infer_function_type_arguments(
             callee_type,
@@ -5230,7 +5234,7 @@ def infer_lambda_type_using_context(
         # they must be considered as indeterminate. We use ErasedType since it
         # does not affect type inference results (it is for purposes like this
         # only).
-        if self.chk.options.new_type_inference:
+        if not self.chk.options.old_type_inference:
             # With new type inference we can preserve argument types even if they
             # are generic, since new inference algorithm can handle constraints
             # like S <: T (we still erase return type since it's ultimately unknown).

mypy/main.py

@@ -995,9 +995,13 @@ def add_invertible_flag(
         help="Use a custom typing module",
     )
     internals_group.add_argument(
-        "--new-type-inference",
+        "--old-type-inference",
         action="store_true",
-        help="Enable new experimental type inference algorithm",
+        help="Disable new experimental type inference algorithm",
+    )
+    # Deprecated reverse variant of the above.
+    internals_group.add_argument(
+        "--new-type-inference", action="store_true", help=argparse.SUPPRESS
     )
     parser.add_argument(
         "--enable-incomplete-feature",
@@ -1383,6 +1387,12 @@ def set_strict_flags() -> None:
     if options.logical_deps:
         options.cache_fine_grained = True
 
+    if options.new_type_inference:
+        print(
+            "Warning: --new-type-inference flag is deprecated;"
+            " new type inference algorithm is already enabled by default"
+        )
+
     if options.strict_concatenate and not strict_option_set:
         print("Warning: --strict-concatenate is deprecated; use --extra-checks instead")
 

mypy/options.py

@@ -62,7 +62,7 @@ class BuildType:
     | {
         "platform",
         "bazel",
-        "new_type_inference",
+        "old_type_inference",
         "plugins",
         "disable_bytearray_promotion",
         "disable_memoryview_promotion",
@@ -360,7 +360,9 @@ def __init__(self) -> None:
         # skip most errors after this many messages have been reported.
         # -1 means unlimited.
         self.many_errors_threshold = defaults.MANY_ERRORS_THRESHOLD
-        # Enable new experimental type inference algorithm.
+        # Disable new experimental type inference algorithm.
+        self.old_type_inference = False
+        # Deprecated reverse version of the above, do not use.
         self.new_type_inference = False
         # Export line-level, limited, fine-grained dependency information in cache data
         # (undocumented feature).

mypy_self_check.ini

@@ -8,7 +8,6 @@ always_false = MYPYC
 plugins = mypy.plugins.proper_plugin
 python_version = 3.8
 exclude = mypy/typeshed/|mypyc/test-data/|mypyc/lib-rt/
-new_type_inference = True
 enable_error_code = ignore-without-code,redundant-expr
 enable_incomplete_feature = PreciseTupleTypes
 show_error_code_links = True

test-data/unit/check-generics.test

@@ -2724,7 +2724,7 @@ def f(x: Callable[[G[T]], int]) -> T: ...
 class G(Generic[T]):
     def g(self, x: S) -> Union[S, T]: ...
 
-f(lambda x: x.g(0))  # E: Incompatible return value type (got "Union[int, T]", expected "int")
+reveal_type(f(lambda x: x.g(0)))  # N: Revealed type is "builtins.int"
 
 [case testDictStarInference]
 class B: ...
@@ -3059,14 +3059,19 @@ def dec5(f: Callable[[int], T]) -> Callable[[int], List[T]]:
         return [f(x)] * x
     return g
 
+I = TypeVar("I", bound=int)
+def dec4_bound(f: Callable[[I], List[T]]) -> Callable[[I], T]:
+    ...
+
 reveal_type(dec1(lambda x: x))  # N: Revealed type is "def [T] (T`3) -> builtins.list[T`3]"
 reveal_type(dec2(lambda x: x))  # N: Revealed type is "def [S] (S`4) -> builtins.list[S`4]"
 reveal_type(dec3(lambda x: x[0]))  # N: Revealed type is "def [S] (S`6) -> S`6"
 reveal_type(dec4(lambda x: [x]))  # N: Revealed type is "def [S] (S`9) -> S`9"
 reveal_type(dec1(lambda x: 1))  # N: Revealed type is "def (builtins.int) -> builtins.list[builtins.int]"
 reveal_type(dec5(lambda x: x))  # N: Revealed type is "def (builtins.int) -> builtins.list[builtins.int]"
 reveal_type(dec3(lambda x: x))  # N: Revealed type is "def [S] (S`16) -> builtins.list[S`16]"
-dec4(lambda x: x)  # E: Incompatible return value type (got "S", expected "List[object]")
+reveal_type(dec4(lambda x: x))  # N: Revealed type is "def [T] (builtins.list[T`19]) -> T`19"
+dec4_bound(lambda x: x)  # E: Value of type variable "I" of "dec4_bound" cannot be "List[T]"
 [builtins fixtures/list.pyi]
 
 [case testInferenceAgainstGenericParamSpecBasicInList]

test-data/unit/check-inference-context.test

@@ -1305,6 +1305,25 @@ def g(l: List[C], x: str) -> Optional[C]:
     return f(l, lambda c: reveal_type(c).x)  # N: Revealed type is "__main__.C"
 [builtins fixtures/list.pyi]
 
+[case testPartialTypeContextWithTwoLambdas]
+from typing import Any, Generic, TypeVar, Callable
+
+def int_to_any(x: int) -> Any: ...
+def any_to_int(x: Any) -> int: ...
+def any_to_str(x: Any) -> str: ...
+
+T = TypeVar("T")
+class W(Generic[T]):
+    def __init__(
+        self, serialize: Callable[[T], Any], deserialize: Callable[[Any], T]
+    ) -> None:
+        ...
+reveal_type(W(lambda x: int_to_any(x), lambda x: any_to_int(x)))  # N: Revealed type is "__main__.W[builtins.int]"
+W(
+    lambda x: int_to_any(x),  # E: Argument 1 to "int_to_any" has incompatible type "str"; expected "int"
+    lambda x: any_to_str(x)
+)
+
 [case testWideOuterContextEmpty]
 from typing import List, TypeVar
 

test-data/unit/cmdline.test

@@ -1472,6 +1472,14 @@ note: A user-defined top-level module with name "typing" is not supported
 Failed to find builtin module mypy_extensions, perhaps typeshed is broken?
 == Return code: 2
 
+[case testNewTypeInferenceFlagDeprecated]
+# cmd: mypy --new-type-inference a.py
+[file a.py]
+pass
+[out]
+Warning: --new-type-inference flag is deprecated; new type inference algorithm is already enabled by default
+== Return code: 0
+
 [case testNotesOnlyResultInExitSuccess]
 # cmd: mypy a.py
 [file a.py]