From 2ad51aab5d3164cc88d53c36ac832b932594550b Mon Sep 17 00:00:00 2001
From: Martin Imre <martinimre25@gmail.com>
Date: Sun, 13 Jul 2025 09:41:38 +0200
Subject: [PATCH 1/2] feat(overloads): Expand finite sum types into a union of
 possible types

---
 mypy/checkexpr.py                     | 10 +++++++
 mypy/typeops.py                       |  5 ++++
 test-data/unit/check-overloading.test | 41 +++++++++++++++++++++++++++
 3 files changed, 56 insertions(+)

diff --git a/mypy/checkexpr.py b/mypy/checkexpr.py
index 8223ccfe4ca0..2cf9ae5d023f 100644
--- a/mypy/checkexpr.py
+++ b/mypy/checkexpr.py
@@ -2709,6 +2709,16 @@ def check_overload_call(
         # Normalize unpacked kwargs before checking the call.
         callee = callee.with_unpacked_kwargs()
         arg_types = self.infer_arg_types_in_empty_context(args)
+
+        # Expand finite sum types into unions
+        # See https://github.com/python/mypy/issues/14764#issuecomment-3054510950
+        # And https://typing.python.org/en/latest/spec/overload.html#argument-type-expansion
+        arg_types = [
+                try_expanding_sum_type_to_union(arg_type,  arg_type.type.fullname)
+                if isinstance(arg_type, Instance) else arg_type
+                for arg_type in arg_types
+                ]
+
         # Step 1: Filter call targets to remove ones where the argument counts don't match
         plausible_targets = self.plausible_overload_call_targets(
             arg_types, arg_kinds, arg_names, callee
diff --git a/mypy/typeops.py b/mypy/typeops.py
index 9aa08b40a991..b2174d28457d 100644
--- a/mypy/typeops.py
+++ b/mypy/typeops.py
@@ -1050,7 +1050,12 @@ class Status(Enum):
         ]
         return make_simplified_union(items, contract_literals=False)
 
+
     if isinstance(typ, Instance) and typ.type.fullname == target_fullname:
+        if isinstance(typ.last_known_value, LiteralType):
+            # fallback for Literal[True] and Literal[False]
+            return typ
+
         if typ.type.fullname == "builtins.bool":
             items = [LiteralType(True, typ), LiteralType(False, typ)]
             return make_simplified_union(items, contract_literals=False)
diff --git a/test-data/unit/check-overloading.test b/test-data/unit/check-overloading.test
index e427d5b21d40..21cf6bec5400 100644
--- a/test-data/unit/check-overloading.test
+++ b/test-data/unit/check-overloading.test
@@ -5266,6 +5266,47 @@ tmp/lib.pyi:3: error: Name "func" already defined on line 1
 tmp/lib.pyi:3: error: Name "overload" is not defined
 main:3: note: Revealed type is "Any"
 
+[case testOverloadCheckExpandsBools]
+from typing import Literal, overload, Union
+
+@overload
+def foo(x: Literal[False]) -> None: ...
+@overload
+def foo(x: Literal[True]) -> int: ...
+
+def foo(x: bool) -> Union[None, int]: ...
+
+reveal_type(foo(True)) # N: Revealed type is "builtins.int"
+reveal_type(foo(False)) # N: Revealed type is "None"
+x: bool
+reveal_type(foo(x)) # N: Revealed type is "Union[builtins.int, None]"
+
+[case testOverloadCheckExpandsEnums]
+from typing import Literal, overload, Union
+import enum
+
+class Color(enum.Enum):
+    RED = 1
+    BLUE = 2
+    YELLOW = 3
+
+@overload
+def foo(x: Literal[Color.RED]) -> None: ...
+@overload
+def foo(x: Literal[Color.BLUE]) -> int: ...
+@overload
+def foo(x: Literal[Color.YELLOW]) -> str: ...
+
+def foo(x: Color) -> Union[None, int, str]: ...
+
+reveal_type(foo(Color.RED)) # N: Revealed type is "None"
+reveal_type(foo(Color.BLUE)) # N: Revealed type is "builtins.int"
+reveal_type(foo(Color.YELLOW)) # N: Revealed type is "builtins.str"
+
+x: Color
+reveal_type(foo(x)) # N: Revealed type is "Union[None, builtins.int, builtins.str]"
+[builtins fixtures/tuple.pyi]
+
 [case testLiteralSubtypeOverlap]
 from typing import Literal, overload
 

From 752bbd289c5749582818b36eb13ce33b078df83a Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Sun, 13 Jul 2025 07:48:18 +0000
Subject: [PATCH 2/2] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 mypy/checkexpr.py | 11 +++++++----
 mypy/typeops.py   |  1 -
 2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/mypy/checkexpr.py b/mypy/checkexpr.py
index 2cf9ae5d023f..616c47955468 100644
--- a/mypy/checkexpr.py
+++ b/mypy/checkexpr.py
@@ -2714,10 +2714,13 @@ def check_overload_call(
         # See https://github.com/python/mypy/issues/14764#issuecomment-3054510950
         # And https://typing.python.org/en/latest/spec/overload.html#argument-type-expansion
         arg_types = [
-                try_expanding_sum_type_to_union(arg_type,  arg_type.type.fullname)
-                if isinstance(arg_type, Instance) else arg_type
-                for arg_type in arg_types
-                ]
+            (
+                try_expanding_sum_type_to_union(arg_type, arg_type.type.fullname)
+                if isinstance(arg_type, Instance)
+                else arg_type
+            )
+            for arg_type in arg_types
+        ]
 
         # Step 1: Filter call targets to remove ones where the argument counts don't match
         plausible_targets = self.plausible_overload_call_targets(
diff --git a/mypy/typeops.py b/mypy/typeops.py
index b2174d28457d..764357eff395 100644
--- a/mypy/typeops.py
+++ b/mypy/typeops.py
@@ -1050,7 +1050,6 @@ class Status(Enum):
         ]
         return make_simplified_union(items, contract_literals=False)
 
-
     if isinstance(typ, Instance) and typ.type.fullname == target_fullname:
         if isinstance(typ.last_known_value, LiteralType):
             # fallback for Literal[True] and Literal[False]