FOLLY_CXX17_CONSTEXPR

Summary: A macro which expands to `constexpr` under C++17. Can be used with `if` and lambdas in cases where `constexpr` is an optional hint but not necessary for the program to compile.

Reviewed By: Mizuchi

Differential Revision: D32607012

fbshipit-source-id: 56a6d8679ab2add4f88c89642166f1ed4b956089

folly/Portability.h

@@ -541,17 +541,14 @@ constexpr auto kCpplibVer = 0;
 #define FOLLY_STORAGE_CONSTEXPR constexpr
 #endif
 
-//  FOLLY_LAMBDA_CONSTEXPR
+//  FOLLY_CXX17_CONSTEXPR
 //
-//  C++17 permits a lambda to be marked constexpr.
-//
-//  Example:
-//
-//    []() FOLLY_LAMBDA_CONSTEXPR { /* ... */ }
+//  C++17 permits more cases to be marked constexpr, including lambda bodies and
+//  the `if` keyword.
 #if FOLLY_CPLUSPLUS >= 201703L
-#define FOLLY_LAMBDA_CONSTEXPR constexpr
+#define FOLLY_CXX17_CONSTEXPR constexpr
 #else
-#define FOLLY_LAMBDA_CONSTEXPR
+#define FOLLY_CXX17_CONSTEXPR
 #endif
 
 #if __cplusplus >= 201703L

folly/functional/Invoke.h

@@ -491,7 +491,7 @@ struct invoke_traits : detail::invoke_traits_base<I> {
  */
 #define FOLLY_INVOKE_MEMBER(membername)                 \
   [](auto&& __folly_param_o, auto&&... __folly_param_a) \
-      FOLLY_LAMBDA_CONSTEXPR FOLLY_DETAIL_FORWARD_BODY( \
+      FOLLY_CXX17_CONSTEXPR FOLLY_DETAIL_FORWARD_BODY(  \
           FOLLY_DETAIL_FORWARD_REF(__folly_param_o)     \
               .membername(FOLLY_DETAIL_FORWARD_REF(__folly_param_a)...))