Make joins of callables respect positional parameter names

mypy/join.py

@@ -13,6 +13,7 @@
     is_subtype, is_equivalent, is_subtype_ignoring_tvars, is_proper_subtype,
     is_protocol_implementation
 )
+from mypy.nodes import ARG_NAMED, ARG_NAMED_OPT
 
 from mypy import experiments
 
@@ -348,7 +349,6 @@ def is_similar_callables(t: CallableType, s: CallableType) -> bool:
     """Return True if t and s have identical numbers of
     arguments, default arguments and varargs.
     """
-
     return (len(t.arg_types) == len(s.arg_types) and t.min_args == s.min_args and
             t.is_var_arg == s.is_var_arg)
 
@@ -366,6 +366,7 @@ def join_similar_callables(t: CallableType, s: CallableType) -> CallableType:
     else:
         fallback = s.fallback
     return t.copy_modified(arg_types=arg_types,
+                           arg_names=combine_arg_names(t, s),
                            ret_type=join_types(t.ret_type, s.ret_type),
                            fallback=fallback,
                            name=None)
@@ -383,11 +384,42 @@ def combine_similar_callables(t: CallableType, s: CallableType) -> CallableType:
     else:
         fallback = s.fallback
     return t.copy_modified(arg_types=arg_types,
+                           arg_names=combine_arg_names(t, s),
                            ret_type=join_types(t.ret_type, s.ret_type),
                            fallback=fallback,
                            name=None)
 
 
+def combine_arg_names(t: CallableType, s: CallableType) -> List[Optional[str]]:
+    """Produces a list of argument names compatible with both callables.
+
+    For example, suppose 't' and 's' have the following signatures:
+
+    - t: (a: int, b: str, X: str) -> None
+    - s: (a: int, b: str, Y: str) -> None
+
+    This function would return ["a", "b", None]. This information
+    is then used above to compute the join of t and s, which results
+    in a signature of (a: int, b: str, str) -> None.
+
+    Note that the third argument's name is omitted and 't' and 's'
+    are both valid subtypes of this inferred signature.
+
+    Precondition: is_similar_types(t, s) is true.
+    """
+    num_args = len(t.arg_types)
+    new_names = []
+    named = (ARG_NAMED, ARG_NAMED_OPT)
+    for i in range(num_args):
+        t_name = t.arg_names[i]
+        s_name = s.arg_names[i]
+        if t_name == s_name or t.arg_kinds[i] in named or s.arg_kinds[i] in named:
+            new_names.append(t_name)
+        else:
+            new_names.append(None)
+    return new_names
+
+
 def object_from_instance(instance: Instance) -> Instance:
     """Construct the type 'builtins.object' from an instance type."""
     # Use the fact that 'object' is always the last class in the mro.

test-data/unit/check-inference.test

@@ -856,13 +856,59 @@ s = s_s() # E: Incompatible types in assignment (expression has type "Set[str]",
 [builtins fixtures/set.pyi]
 
 [case testSetWithStarExpr]
-
 s = {1, 2, *(3, 4)}
 t = {1, 2, *s}
 reveal_type(s)  # E: Revealed type is 'builtins.set[builtins.int*]'
 reveal_type(t)  # E: Revealed type is 'builtins.set[builtins.int*]'
 [builtins fixtures/set.pyi]
 
+[case testListLiteralWithFunctionsErasesNames]
+def f1(x: int) -> int: ...
+def g1(y: int) -> int: ...
+def h1(x: int) -> int: ...
+
+list_1 = [f1, g1]
+list_2 = [f1, h1]
+reveal_type(list_1)  # E: Revealed type is 'builtins.list[def (builtins.int) -> builtins.int]'
+reveal_type(list_2)  # E: Revealed type is 'builtins.list[def (x: builtins.int) -> builtins.int]'
+
+def f2(x: int, z: str) -> int: ...
+def g2(y: int, z: str) -> int: ...
+def h2(x: int, z: str) -> int: ...
+
+list_3 = [f2, g2]
+list_4 = [f2, h2]
+reveal_type(list_3)  # E: Revealed type is 'builtins.list[def (builtins.int, z: builtins.str) -> builtins.int]'
+reveal_type(list_4)  # E: Revealed type is 'builtins.list[def (x: builtins.int, z: builtins.str) -> builtins.int]'
+[builtins fixtures/list.pyi]
+
+[case testListLiteralWithSimilarFunctionsErasesName]
+from typing import Union
+
+class A: ...
+class B(A): ...
+class C: ...
+class D: ...
+
+def f(x: Union[A, C], y: B) -> A: ...
+def g(z: Union[B, D], y: A) -> B: ...
+def h(x: Union[B, D], y: A) -> B: ...
+
+list_1 = [f, g]
+list_2 = [f, h]
+reveal_type(list_1)  # E: Revealed type is 'builtins.list[def (__main__.B, y: __main__.B) -> __main__.A]'
+reveal_type(list_2)  # E: Revealed type is 'builtins.list[def (x: __main__.B, y: __main__.B) -> __main__.A]'
+[builtins fixtures/list.pyi]
+
+[case testListLiteralWithNameOnlyArgsDoesNotEraseNames]
+def f(*, x: int) -> int: ...
+def g(*, y: int) -> int: ...
+def h(*, x: int) -> int: ...
+
+list_1 = [f, g]  # E: List item 0 has incompatible type "Callable[[NamedArg(int, 'x')], int]"; expected "Callable[[NamedArg(int, 'y')], int]"
+list_2 = [f, h]
+[builtins fixtures/list.pyi]
+
 
 -- For statements
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