diff --git a/mypy/checkexpr.py b/mypy/checkexpr.py
index 2e93a598fb7e..5c6db86964ac 100644
--- a/mypy/checkexpr.py
+++ b/mypy/checkexpr.py
@@ -2213,7 +2213,7 @@ def check_overload_call(
                     # we don't want to introduce internal inconsistencies.
                     unioned_result = (
                         make_simplified_union(list(returns), context.line, context.column),
-                        self.combine_function_signatures(inferred_types),
+                        self.combine_function_signatures(get_proper_types(inferred_types)),
                     )
 
         # Step 3: We try checking each branch one-by-one.
@@ -2554,7 +2554,7 @@ def type_overrides_set(
             for expr in exprs:
                 del self.type_overrides[expr]
 
-    def combine_function_signatures(self, types: Sequence[Type]) -> AnyType | CallableType:
+    def combine_function_signatures(self, types: list[ProperType]) -> AnyType | CallableType:
         """Accepts a list of function signatures and attempts to combine them together into a
         new CallableType consisting of the union of all of the given arguments and return types.
 
@@ -2562,10 +2562,9 @@ def combine_function_signatures(self, types: Sequence[Type]) -> AnyType | Callab
         an ambiguity because of Any in arguments).
         """
         assert types, "Trying to merge no callables"
-        types = get_proper_types(types)
         if not all(isinstance(c, CallableType) for c in types):
             return AnyType(TypeOfAny.special_form)
-        callables = cast(Sequence[CallableType], types)
+        callables = cast("list[CallableType]", types)
         if len(callables) == 1:
             return callables[0]
 
@@ -3463,7 +3462,7 @@ def check_op(
             # we call 'combine_function_signature' instead of just unioning the inferred
             # callable types.
             results_final = make_simplified_union(all_results)
-            inferred_final = self.combine_function_signatures(all_inferred)
+            inferred_final = self.combine_function_signatures(get_proper_types(all_inferred))
             return results_final, inferred_final
         else:
             return self.check_method_call_by_name(
diff --git a/mypy/types.py b/mypy/types.py
index fa1502f48fa4..354a740567dc 100644
--- a/mypy/types.py
+++ b/mypy/types.py
@@ -2888,23 +2888,35 @@ def get_proper_type(typ: Type | None) -> ProperType | None:
         typ = typ.type_guard
     while isinstance(typ, TypeAliasType):
         typ = typ._expand_once()
-    assert isinstance(typ, ProperType), typ
     # TODO: store the name of original type alias on this type, so we can show it in errors.
-    return typ
+    return cast(ProperType, typ)
 
 
 @overload
-def get_proper_types(it: Iterable[Type]) -> list[ProperType]:  # type: ignore[misc]
+def get_proper_types(types: list[Type] | tuple[Type, ...]) -> list[ProperType]:  # type: ignore[misc]
     ...
 
 
 @overload
-def get_proper_types(it: Iterable[Type | None]) -> list[ProperType | None]:
+def get_proper_types(
+    types: list[Type | None] | tuple[Type | None, ...]
+) -> list[ProperType | None]:
     ...
 
 
-def get_proper_types(it: Iterable[Type | None]) -> list[ProperType] | list[ProperType | None]:
-    return [get_proper_type(t) for t in it]
+def get_proper_types(
+    types: list[Type] | list[Type | None] | tuple[Type | None, ...]
+) -> list[ProperType] | list[ProperType | None]:
+    if isinstance(types, list):
+        typelist = types
+        # Optimize for the common case so that we don't need to allocate anything
+        if not any(
+            isinstance(t, (TypeAliasType, TypeGuardedType)) for t in typelist  # type: ignore[misc]
+        ):
+            return cast("list[ProperType]", typelist)
+        return [get_proper_type(t) for t in typelist]
+    else:
+        return [get_proper_type(t) for t in types]
 
 
 # We split off the type visitor base classes to another module