absl: Deprecate ABSL_HAVE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION

Always define it as `1`. CTAD is a C++17 feature, and Abseil does not support older versions of the language.

PiperOrigin-RevId: 729144923
Change-Id: I13572569004d7f0b56a2c11f264b7a3de15927b9

Author: dinord

absl/base/config.h

@@ -274,7 +274,6 @@ static_assert(ABSL_INTERNAL_INLINE_NAMESPACE_STR[0] != 'h' ||
 #define ABSL_HAVE_STD_IS_TRIVIALLY_COPYABLE 1
 #endif
 
-
 // ABSL_HAVE_THREAD_LOCAL
 //
 // DEPRECATED - `thread_local` is available on all supported platforms.
@@ -813,10 +812,12 @@ static_assert(ABSL_INTERNAL_INLINE_NAMESPACE_STR[0] != 'h' ||
 
 // ABSL_HAVE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION
 //
-// Class template argument deduction is a language feature added in C++17.
+// Deprecated: always defined to 1.
+// Class template argument deduction is a language feature added in C++17,
+// which means all versions of C++ supported by Abseil have it.
 #ifdef ABSL_HAVE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION
 #error "ABSL_HAVE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION cannot be directly set."
-#elif defined(__cpp_deduction_guides)
+#else
 #define ABSL_HAVE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION 1
 #endif
 

absl/base/no_destructor.h

@@ -199,12 +199,10 @@ class NoDestructor {
       impl_;
 };
 
-#ifdef ABSL_HAVE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION
 // Provide 'Class Template Argument Deduction': the type of NoDestructor's T
 // will be the same type as the argument passed to NoDestructor's constructor.
 template <typename T>
 NoDestructor(T) -> NoDestructor<T>;
-#endif  // ABSL_HAVE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION
 
 ABSL_NAMESPACE_END
 }  // namespace absl

absl/base/no_destructor_test.cc

@@ -196,14 +196,10 @@ TEST(NoDestructorTest, StaticPattern) {
   EXPECT_EQ(0, Int());  // should get zero-initialized
 }
 
-#ifdef ABSL_HAVE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION
-// This would fail to compile if Class Template Argument Deduction was not
-// provided for absl::NoDestructor.
 TEST(NoDestructorTest, ClassTemplateArgumentDeduction) {
   absl::NoDestructor i(1);
   static_assert(std::is_same<decltype(i), absl::NoDestructor<int>>::value,
                 "Expected deduced type to be int.");
 }
-#endif
 
 }  // namespace

absl/cleanup/cleanup.h

@@ -115,10 +115,8 @@ class ABSL_MUST_USE_RESULT Cleanup final {
 // `absl::Cleanup c = /* callback */;`
 //
 // C++17 type deduction API for creating an instance of `absl::Cleanup`
-#if defined(ABSL_HAVE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION)
 template <typename Callback>
 Cleanup(Callback callback) -> Cleanup<cleanup_internal::Tag, Callback>;
-#endif  // defined(ABSL_HAVE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION)
 
 // `auto c = absl::MakeCleanup(/* callback */);`
 //

absl/cleanup/cleanup_test.cc

@@ -116,7 +116,6 @@ TYPED_TEST(CleanupTest, FactoryProducesCorrectType) {
   }
 }
 
-#if defined(ABSL_HAVE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION)
 TYPED_TEST(CleanupTest, CTADProducesCorrectType) {
   {
     auto callback = TypeParam::AsCallback([] {});
@@ -186,7 +185,6 @@ TYPED_TEST(CleanupTest, FactoryAndCTADProduceSameType) {
         IsSame<decltype(factory_cleanup), decltype(deduction_cleanup)>(), "");
   }
 }
-#endif  // defined(ABSL_HAVE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION)
 
 TYPED_TEST(CleanupTest, BasicUsage) {
   bool called = false;

absl/log/internal/structured.h

@@ -139,8 +139,6 @@ class ABSL_MUST_USE_RESULT AsStructuredValueImpl final {
   }
 };
 
-#ifdef ABSL_HAVE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION
-
 // Template deduction guide so `AsStructuredValueImpl(42, data)` works
 // without specifying the template type.
 template <typename T>
@@ -155,8 +153,6 @@ AsStructuredValueImpl(
     typename AsStructuredValueImpl<T>::ValueFormatter value_formatter)
     -> AsStructuredValueImpl<T>;
 
-#endif  // ABSL_HAVE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION
-
 }  // namespace log_internal
 ABSL_NAMESPACE_END
 }  // namespace absl