Mutex: Fix Condition pointer-to-member cast to respect const qualifier

Previously, `absl::Condition` incorrectly used the same (non-`const`)
pointer-to-method type when wrapping both `const` and non-`const` methods.

Unfortunately, this is undefined behavior according to `[expr.reinterpret.cast]`
in the C++ standard:

> The effect of calling a function through a pointer to a function type that is
> not the same as the type used in the definition of the function is undefined.

This fixes the UB.

PiperOrigin-RevId: 591981682
Change-Id: Iaca955346699417232383d3a1800ea9b82ea5761

1 files changed, 9 insertions, 0 deletions

diff --git a/absl/synchronization/mutex_test.cc b/absl/synchronization/mutex_test.cc
index 35b43334..a8d75827 100644
--- a/absl/synchronization/mutex_test.cc
+++ b/absl/synchronization/mutex_test.cc
@@ -977,6 +977,15 @@ TEST(Mutex, FunctionPointerConditionWithDerivedToBaseConversion) {
                                      const Derived *>::value));
 }
 
+struct Constable {
+  bool WotsAllThisThen() const { return true; }
+};
+
+TEST(Mutex, FunctionPointerConditionWithConstMethod) {
+  const Constable chapman;
+  EXPECT_TRUE(absl::Condition(&chapman, &Constable::WotsAllThisThen).Eval());
+}
+
 struct True {
   template <class... Args>
   bool operator()(Args...) const {