Annotate absl::FunctionRef as a view type via [[gsl::Pointer]] and absl_internal_is_view

This also catches bug-prone code in our own tests.

PiperOrigin-RevId: 813887857
Change-Id: Ibadcc6f9bf0ed4f9c46393a6f6fb2bcbcba5581a

absl/functional/function_ref.h

diff --git a/absl/functional/function_ref.h b/absl/functional/function_ref.h
index fe52853..3882eb0 100644
--- a/absl/functional/function_ref.h
+++ b/absl/functional/function_ref.h
@@ -85,7 +85,7 @@
 //   bool Visitor(absl::FunctionRef<void(my_proto&, absl::string_view)>
 //                  callback);
 template <typename R, typename... Args>
-class FunctionRef<R(Args...)> {
+class ABSL_ATTRIBUTE_VIEW FunctionRef<R(Args...)> {
  protected:
   // Used to disable constructors for objects that are not compatible with the
   // signature of this FunctionRef.
@@ -158,6 +158,8 @@
   }
 #endif
 
+  using absl_internal_is_view = std::true_type;
+
   // Call the underlying object.
   R operator()(Args... args) const {
     return invoker_(ptr_, std::forward<Args>(args)...);
@@ -171,7 +173,8 @@
 // Allow const qualified function signatures. Since FunctionRef requires
 // constness anyway we can just make this a no-op.
 template <typename R, typename... Args>
-class FunctionRef<R(Args...) const> : private FunctionRef<R(Args...)> {
+class ABSL_ATTRIBUTE_VIEW
+    FunctionRef<R(Args...) const> : private FunctionRef<R(Args...)> {
   using Base = FunctionRef<R(Args...)>;
 
   template <typename F, typename... U>
@@ -215,6 +218,8 @@
       : Base(arg, obj) {}
 #endif
 
+  using absl_internal_is_view = std::true_type;
+
   using Base::operator();
 };
 

absl/functional/function_ref_test.cc

diff --git a/absl/functional/function_ref_test.cc b/absl/functional/function_ref_test.cc
index a0c6745..97aa3d5 100644
--- a/absl/functional/function_ref_test.cc
+++ b/absl/functional/function_ref_test.cc
@@ -407,7 +407,7 @@
   // producing a copy rather than another indirection.
   absl::FunctionRef<int()> a = +[]() { return 1; };
   absl::FunctionRef<int() const> b = a;
-  a = []() { return 2; };
+  a = +[]() { return 2; };
   EXPECT_EQ(b(), 2);
 }