Remove and refactor old overload selection logic (#5321)

This commit removes a few remnants of the old overload selection
algorithm in checkexpr.

Specifically, this commit:

1.  Modifies `erased_signature_similarity` so it returns a bool
    instead of an int.

2.  Simplifies how `erased_signature_similarity` handles types like
    `Type[X]`.

    Note: although this change relaxes and loosens the precision of
    `erased_signature_similarity`, it will not impact correctness:
    we now use this method exclusively as a heuristic to help us
    determine which overloads the user might have meant to select in
    our error messages.

    As a consequence, loosening this function actually led to a more
    helpful error message in one case.

3.  Remove the `match_signature_types` method. It didn't appear as if
    anybody was calling it.

Author: Michael0x2a

mypy/checkexpr.py

@@ -1575,12 +1575,9 @@ def union_overload_matches(self, types: Sequence[Type]) -> Union[AnyType, Callab
     def erased_signature_similarity(self, arg_types: List[Type], arg_kinds: List[int],
                                     arg_names: Optional[Sequence[Optional[str]]],
                                     callee: CallableType,
-                                    context: Context) -> int:
-        """Determine whether arguments could match the signature at runtime.
-
-        Return similarity level (0 = no match, 1 = can match, 2 = non-promotion match). See
-        overload_arg_similarity for a discussion of similarity levels.
-        """
+                                    context: Context) -> bool:
+        """Determine whether arguments could match the signature at runtime, after
+        erasing types."""
         formal_to_actual = map_actuals_to_formals(arg_kinds,
                                                   arg_names,
                                                   callee.arg_kinds,
@@ -1590,55 +1587,22 @@ def erased_signature_similarity(self, arg_types: List[Type], arg_kinds: List[int
         if not self.check_argument_count(callee, arg_types, arg_kinds, arg_names,
                                          formal_to_actual, None, None):
             # Too few or many arguments -> no match.
-            return 0
-
-        similarity = 2
+            return False
 
         def check_arg(caller_type: Type, original_caller_type: Type, caller_kind: int,
                       callee_type: Type, n: int, m: int, callee: CallableType,
                       context: Context, messages: MessageBuilder) -> None:
-            nonlocal similarity
-            similarity = min(similarity,
-                             overload_arg_similarity(caller_type, callee_type))
-            if similarity == 0:
+            if not arg_approximate_similarity(caller_type, callee_type):
                 # No match -- exit early since none of the remaining work can change
                 # the result.
                 raise Finished
 
         try:
             self.check_argument_types(arg_types, arg_kinds, callee, formal_to_actual,
                                       context=context, check_arg=check_arg)
+            return True
         except Finished:
-            pass
-
-        return similarity
-
-    def match_signature_types(self, arg_types: List[Type], arg_kinds: List[int],
-                              arg_names: Optional[Sequence[Optional[str]]], callee: CallableType,
-                              context: Context) -> bool:
-        """Determine whether arguments types match the signature.
-
-        Assume that argument counts are compatible.
-
-        Return True if arguments match.
-        """
-        formal_to_actual = map_actuals_to_formals(arg_kinds,
-                                                  arg_names,
-                                                  callee.arg_kinds,
-                                                  callee.arg_names,
-                                                  lambda i: arg_types[i])
-        ok = True
-
-        def check_arg(caller_type: Type, original_caller_type: Type, caller_kind: int,
-                      callee_type: Type, n: int, m: int, callee: CallableType,
-                      context: Context, messages: MessageBuilder) -> None:
-            nonlocal ok
-            if not is_subtype(caller_type, callee_type):
-                ok = False
-
-        self.check_argument_types(arg_types, arg_kinds, callee, formal_to_actual,
-                                  context=context, check_arg=check_arg)
-        return ok
+            return False
 
     def apply_generic_arguments(self, callable: CallableType, types: Sequence[Optional[Type]],
                                 context: Context) -> CallableType:
@@ -3399,101 +3363,68 @@ def visit_uninhabited_type(self, t: UninhabitedType) -> bool:
         return True
 
 
-def overload_arg_similarity(actual: Type, formal: Type) -> int:
-    """Return if caller argument (actual) is compatible with overloaded signature arg (formal).
+def arg_approximate_similarity(actual: Type, formal: Type) -> bool:
+    """Return if caller argument (actual) is roughly compatible with signature arg (formal).
 
-    Return a similarity level:
-      0: no match
-      1: actual is compatible, but only using type promotions (e.g. int vs float)
-      2: actual is compatible without type promotions (e.g. int vs object)
+    This function is deliberately loose and will report two types are similar
+    as long as their "shapes" are plausibly the same.
 
-    The distinction is important in cases where multiple overload items match. We want
-    give priority to higher similarity matches.
+    This is useful when we're doing error reporting: for example, if we're trying
+    to select an overload alternative and there's no exact match, we can use
+    this function to help us identify which alternative the user might have
+    *meant* to match.
     """
-    # Replace type variables with their upper bounds. Overloading
-    # resolution is based on runtime behavior which erases type
-    # parameters, so no need to handle type variables occurring within
-    # a type.
+
+    # Erase typevars: we'll consider them all to have the same "shape".
+
     if isinstance(actual, TypeVarType):
         actual = actual.erase_to_union_or_bound()
     if isinstance(formal, TypeVarType):
         formal = formal.erase_to_union_or_bound()
-    if (isinstance(actual, UninhabitedType) or isinstance(actual, AnyType) or
-            isinstance(formal, AnyType) or
-            (isinstance(actual, Instance) and actual.type.fallback_to_any)):
-        # These could match anything at runtime.
-        return 2
+
+    # Callable or Type[...]-ish types
+
+    def is_typetype_like(typ: Type) -> bool:
+        return (isinstance(typ, TypeType)
+                or (isinstance(typ, FunctionLike) and typ.is_type_obj())
+                or (isinstance(typ, Instance) and typ.type.fullname() == "builtins.type"))
+
     if isinstance(formal, CallableType):
-        if isinstance(actual, (CallableType, Overloaded)):
-            # TODO: do more sophisticated callable matching
-            return 2
-        if isinstance(actual, TypeType):
-            return 2 if is_subtype(actual, formal) else 0
-    if isinstance(actual, NoneTyp):
-        if not experiments.STRICT_OPTIONAL:
-            # NoneTyp matches anything if we're not doing strict Optional checking
-            return 2
-        else:
-            # NoneType is a subtype of object
-            if isinstance(formal, Instance) and formal.type.fullname() == "builtins.object":
-                return 2
+        if isinstance(actual, (CallableType, Overloaded, TypeType)):
+            return True
+    if is_typetype_like(actual) and is_typetype_like(formal):
+        return True
+
+    # Unions
+
     if isinstance(actual, UnionType):
-        return max(overload_arg_similarity(item, formal)
-                   for item in actual.relevant_items())
+        return any(arg_approximate_similarity(item, formal) for item in actual.relevant_items())
     if isinstance(formal, UnionType):
-        return max(overload_arg_similarity(actual, item)
-                   for item in formal.relevant_items())
-    if isinstance(formal, TypeType):
-        if isinstance(actual, TypeType):
-            # Since Type[T] is covariant, check if actual = Type[A] is
-            # a subtype of formal = Type[F].
-            return overload_arg_similarity(actual.item, formal.item)
-        elif isinstance(actual, FunctionLike) and actual.is_type_obj():
-            # Check if the actual is a constructor of some sort.
-            # Note that this is this unsound, since we don't check the __init__ signature.
-            return overload_arg_similarity(actual.items()[0].ret_type, formal.item)
-        else:
-            return 0
+        return any(arg_approximate_similarity(actual, item) for item in formal.relevant_items())
+
+    # TypedDicts
+
     if isinstance(actual, TypedDictType):
         if isinstance(formal, TypedDictType):
-            # Don't support overloading based on the keys or value types of a TypedDict since
-            # that would be complicated and probably only marginally useful.
-            return 2
-        return overload_arg_similarity(actual.fallback, formal)
+            return True
+        return arg_approximate_similarity(actual.fallback, formal)
+
+    # Instances
+    # For instances, we mostly defer to the existing is_subtype check.
+
     if isinstance(formal, Instance):
         if isinstance(actual, CallableType):
             actual = actual.fallback
         if isinstance(actual, Overloaded):
             actual = actual.items()[0].fallback
         if isinstance(actual, TupleType):
             actual = actual.fallback
-        if isinstance(actual, Instance):
-            # First perform a quick check (as an optimization) and fall back to generic
-            # subtyping algorithm if type promotions are possible (e.g., int vs. float).
-            if formal.type in actual.type.mro:
-                return 2
-            elif formal.type.is_protocol and is_subtype(actual, erasetype.erase_type(formal)):
-                return 2
-            elif actual.type._promote and is_subtype(actual, formal):
-                return 1
-            else:
-                return 0
-        elif isinstance(actual, TypeType):
-            item = actual.item
-            if formal.type.fullname() in {"builtins.object", "builtins.type"}:
-                return 2
-            elif isinstance(item, Instance) and item.type.metaclass_type:
-                # FIX: this does not handle e.g. Union of instances
-                return overload_arg_similarity(item.type.metaclass_type, formal)
-            else:
-                return 0
-        else:
-            return 0
-    if isinstance(actual, UnboundType) or isinstance(formal, UnboundType):
-        # Either actual or formal is the result of an error; shut up.
-        return 2
-    # Fall back to a conservative equality check for the remaining kinds of type.
-    return 2 if is_same_type(erasetype.erase_type(actual), erasetype.erase_type(formal)) else 0
+        if isinstance(actual, Instance) and formal.type in actual.type.mro:
+            # Try performing a quick check as an optimization
+            return True
+
+    # Fall back to a standard subtype check for the remaining kinds of type.
+    return is_subtype(erasetype.erase_type(actual), erasetype.erase_type(formal))
 
 
 def any_causes_overload_ambiguity(items: List[CallableType],

test-data/unit/check-classes.test

@@ -1223,11 +1223,8 @@ class D:
     def __get__(self, inst: Base, own: Type[Base]) -> str: pass
 [builtins fixtures/bool.pyi]
 [out]
-main:5: error: Revealed type is 'Any'
-main:5: error: No overload variant of "__get__" of "D" matches argument types "None", "Type[A]"
-main:5: note: Possible overload variants:
-main:5: note:     def __get__(self, inst: None, own: Type[Base]) -> D
-main:5: note:     def __get__(self, inst: Base, own: Type[Base]) -> str
+main:5: error: Revealed type is 'd.D'
+main:5: error: Argument 2 to "__get__" of "D" has incompatible type "Type[A]"; expected "Type[Base]"
 main:6: error: Revealed type is 'Any'
 main:6: error: No overload variant of "__get__" of "D" matches argument types "A", "Type[A]"
 main:6: note: Possible overload variants:
@@ -3016,16 +3013,10 @@ def f(a: Type[B]) -> None: pass
 @overload
 def f(a: int) -> None: pass
 
-f(A)  # E: No overload variant of "f" matches argument type "Type[A]" \
-      # N: Possible overload variants: \
-      # N:     def f(a: Type[B]) -> None \
-      # N:     def f(a: int) -> None
+f(A)  # E: Argument 1 to "f" has incompatible type "Type[A]"; expected "Type[B]"
 f(B)
 f(C)
-f(AType)  # E: No overload variant of "f" matches argument type "Type[A]" \
-          # N: Possible overload variants: \
-          # N:     def f(a: Type[B]) -> None \
-          # N:     def f(a: int) -> None
+f(AType)  # E: Argument 1 to "f" has incompatible type "Type[A]"; expected "Type[B]"
 f(BType)
 f(CType)
 [builtins fixtures/classmethod.pyi]

test-data/unit/check-overloading.test

@@ -4374,3 +4374,34 @@ def bar(x: T) -> T: ...
 @overload
 def bar(x: Any) -> int: ...
 [out]
+
+[case testBadOverloadProbableMatch]
+from typing import overload, List, Type
+
+class Other: pass
+
+@overload
+def multiple_plausible(x: int) -> int: ...
+@overload
+def multiple_plausible(x: str) -> str: ...
+def multiple_plausible(x): pass
+
+
+@overload
+def single_plausible(x: Type[int]) -> int: ...
+@overload
+def single_plausible(x: List[str]) -> str: ...
+def single_plausible(x): pass
+
+a = multiple_plausible(Other())  # E: No overload variant of "multiple_plausible" matches argument type "Other" \
+                                 # N: Possible overload variants: \
+                                 # N:     def multiple_plausible(x: int) -> int \
+                                 # N:     def multiple_plausible(x: str) -> str
+reveal_type(a)                   # E: Revealed type is 'Any'
+
+b = single_plausible(Other)      # E: Argument 1 to "single_plausible" has incompatible type "Type[Other]"; expected "Type[int]"
+reveal_type(b)                   # E: Revealed type is 'builtins.int'
+
+c = single_plausible([Other()])  # E: List item 0 has incompatible type "Other"; expected "str"
+reveal_type(c)                   # E: Revealed type is 'builtins.str'
+[builtins fixtures/list.pyi]