Improve CallableType join in simple cases (python#18406)

Fixes python#17479 , although as you can see in the test case the logic still
remains far from perfect

Author: hauntsaninja

mypy/join.py

@@ -377,24 +377,34 @@ def visit_instance(self, t: Instance) -> ProperType:
             return self.default(self.s)
 
     def visit_callable_type(self, t: CallableType) -> ProperType:
-        if isinstance(self.s, CallableType) and is_similar_callables(t, self.s):
-            if is_equivalent(t, self.s):
-                return combine_similar_callables(t, self.s)
-            result = join_similar_callables(t, self.s)
-            # We set the from_type_type flag to suppress error when a collection of
-            # concrete class objects gets inferred as their common abstract superclass.
-            if not (
-                (t.is_type_obj() and t.type_object().is_abstract)
-                or (self.s.is_type_obj() and self.s.type_object().is_abstract)
-            ):
-                result.from_type_type = True
-            if any(
-                isinstance(tp, (NoneType, UninhabitedType))
-                for tp in get_proper_types(result.arg_types)
-            ):
-                # We don't want to return unusable Callable, attempt fallback instead.
+        if isinstance(self.s, CallableType):
+            if is_similar_callables(t, self.s):
+                if is_equivalent(t, self.s):
+                    return combine_similar_callables(t, self.s)
+                result = join_similar_callables(t, self.s)
+                if any(
+                    isinstance(tp, (NoneType, UninhabitedType))
+                    for tp in get_proper_types(result.arg_types)
+                ):
+                    # We don't want to return unusable Callable, attempt fallback instead.
+                    return join_types(t.fallback, self.s)
+                # We set the from_type_type flag to suppress error when a collection of
+                # concrete class objects gets inferred as their common abstract superclass.
+                if not (
+                    (t.is_type_obj() and t.type_object().is_abstract)
+                    or (self.s.is_type_obj() and self.s.type_object().is_abstract)
+                ):
+                    result.from_type_type = True
+                return result
+            else:
+                s2, t2 = self.s, t
+                if t2.is_var_arg:
+                    s2, t2 = t2, s2
+                if is_subtype(s2, t2):
+                    return t2.copy_modified()
+                elif is_subtype(t2, s2):
+                    return s2.copy_modified()
                 return join_types(t.fallback, self.s)
-            return result
         elif isinstance(self.s, Overloaded):
             # Switch the order of arguments to that we'll get to visit_overloaded.
             return join_types(t, self.s)

test-data/unit/check-functions.test

@@ -3512,6 +3512,56 @@ class Qux(Bar):
         pass
 [builtins fixtures/tuple.pyi]
 
+[case testCallableJoinWithDefaults]
+from typing import Callable, TypeVar
+
+T = TypeVar("T")
+
+def join(t1: T, t2: T) -> T: ...
+
+def f1() -> None: ...
+def f2(i: int = 0) -> None: ...
+def f3(i: str = "") -> None: ...
+
+reveal_type(join(f1, f2))  # N: Revealed type is "def ()"
+reveal_type(join(f1, f3))  # N: Revealed type is "def ()"
+reveal_type(join(f2, f3))  # N: Revealed type is "builtins.function"  # TODO: this could be better
+[builtins fixtures/tuple.pyi]
+
+[case testCallableJoinWithDefaultsMultiple]
+from typing import TypeVar
+T = TypeVar("T")
+def join(t1: T, t2: T, t3: T) -> T: ...
+
+def f_1(common, a=None): ...
+def f_any(*_, **__): ...
+def f_3(common, b=None, x=None): ...
+
+fdict = {
+    "f_1": f_1,
+    "f_any": f_any,
+    "f_3": f_3,
+}
+reveal_type(fdict)  # N: Revealed type is "builtins.dict[builtins.str, def (common: Any, a: Any =) -> Any]"
+
+reveal_type(join(f_1, f_any, f_3))  # N: Revealed type is "def (common: Any, a: Any =) -> Any"
+
+[builtins fixtures/tuple.pyi]
+
+[case testCallableJoinWithType]
+from __future__ import annotations
+class Exc: ...
+class AttributeErr(Exc):
+    def __init__(self, *args: object) -> None: ...
+class FnfErr(Exc): ...
+
+x = [
+    FnfErr,
+    AttributeErr,
+]
+reveal_type(x)  # N: Revealed type is "builtins.list[builtins.type]"
+[builtins fixtures/type.pyi]
+
 [case testDistinctFormatting]
 from typing import Awaitable, Callable, ParamSpec