support if-constexpr macros on MSVC

Summary: change the macros to work on MSVC

Reviewed By: yfeldblum

Differential Revision: D14942488

fbshipit-source-id: 12d6814230bd9b92b54603cd711d03cd8b5f8086

folly/experimental/pushmi/detail/if_constexpr.h

@@ -42,47 +42,64 @@
  *  }
  */
 
+#if defined(__clang__) || (defined(__GNUC__) && __GNUC__ >= 5)
+#define PUSHMI_CEXP_NOT_ON_WINDOWS 1
+#else
+#define PUSHMI_CEXP_NOT_ON_WINDOWS 0
+#endif
+
  #ifndef __has_attribute
- #define PUSHMI_HAS_ATTRIBUTE(x) 0
+ #define PUSHMI_CEXP_HAS_ATTRIBUTE(x) 0
  #else
- #define PUSHMI_HAS_ATTRIBUTE(x) __has_attribute(x)
+ #define PUSHMI_CEXP_HAS_ATTRIBUTE(x) __has_attribute(x)
  #endif
 
  #ifndef __has_cpp_attribute
- #define PUSHMI_HAS_CPP_ATTRIBUTE(x) 0
+ #define PUSHMI_CEXP_HAS_CPP_ATTRIBUTE(x) 0
  #else
- #define PUSHMI_HAS_CPP_ATTRIBUTE(x) __has_cpp_attribute(x)
+ #define PUSHMI_CEXP_HAS_CPP_ATTRIBUTE(x) __has_cpp_attribute(x)
  #endif
 
-#if PUSHMI_HAS_CPP_ATTRIBUTE(maybe_unused)
-#define PUSHMI_MAYBE_UNUSED [[maybe_unused]]
-#elif PUSHMI_HAS_ATTRIBUTE(__unused__) || __GNUC__
-#define PUSHMI_MAYBE_UNUSED __attribute__((__unused__))
+#if PUSHMI_CEXP_HAS_CPP_ATTRIBUTE(maybe_unused)
+#define PUSHMI_CEXP_MAYBE_UNUSED [[maybe_unused]]
+#elif PUSHMI_CEXP_HAS_ATTRIBUTE(__unused__) || __GNUC__
+#define PUSHMI_CEXP_MAYBE_UNUSED __attribute__((__unused__))
 #else
-#define PUSHMI_MAYBE_UNUSED
+#define PUSHMI_CEXP_MAYBE_UNUSED
 #endif
 
 // disable buggy compatibility warning about "requires" and "concept" being
 // C++20 keywords.
-#if defined(__clang__) || (defined(__GNUC__) && __GNUC__ >= 5)
-#define PUSHMI_PP_IGNORE_SHADOW_BEGIN \
+#if PUSHMI_CEXP_NOT_ON_WINDOWS
+#define PUSHMI_CEXP_IGNORE_SHADOW_BEGIN \
     _Pragma("GCC diagnostic push") \
     _Pragma("GCC diagnostic ignored \"-Wshadow\"") \
     _Pragma("GCC diagnostic ignored \"-Wshadow-local\"") \
     /**/
-#define PUSHMI_PP_IGNORE_SHADOW_END \
+#define PUSHMI_CEXP_IGNORE_SHADOW_END \
     _Pragma("GCC diagnostic pop")
 #else
 
-#define PUSHMI_PP_IGNORE_SHADOW_BEGIN
-#define PUSHMI_PP_IGNORE_SHADOW_END
+#define PUSHMI_CEXP_IGNORE_SHADOW_BEGIN
+#define PUSHMI_CEXP_IGNORE_SHADOW_END
 #endif
 
-#define PUSHMI_COMMA ,
+#define PUSHMI_CEXP_COMMA ,
 
-#define PUSHMI_EVAL(F, ...) F(__VA_ARGS__)
+#if PUSHMI_CEXP_NOT_ON_WINDOWS
+#define PUSHMI_CEXP_EVAL(X, ...) X(__VA_ARGS__)
 
-#define PUSHMI_STRIP(...) __VA_ARGS__
+#define PUSHMI_CEXP_STRIP(...) __VA_ARGS__
+#else
+
+// https://stackoverflow.com/questions/5134523/msvc-doesnt-expand-va-args-correctly
+#define PUSHMI_CEXP_VA_ARGS_EXPANDER_INNER(X) X
+#define PUSHMI_CEXP_VA_ARGS_EXPANDER(X) PUSHMI_CEXP_VA_ARGS_EXPANDER_INNER(X)
+
+#define PUSHMI_CEXP_EVAL(X, ...) PUSHMI_CEXP_VA_ARGS_EXPANDER(X(__VA_ARGS__))
+
+#define PUSHMI_CEXP_STRIP(...) PUSHMI_CEXP_VA_ARGS_EXPANDER(__VA_ARGS__)
+#endif
 
 namespace folly {
 namespace pushmi {
@@ -105,22 +122,22 @@ struct id_fn {
 #define PUSHMI_IF_CONSTEXPR(LIST)                         \
   if constexpr (::folly::pushmi::detail::id_fn id = {}) { \
   } else if constexpr                                     \
-  PUSHMI_EVAL(PUSHMI_IF_CONSTEXPR_ELSE_, PUSHMI_IF_CONSTEXPR_IF_ LIST)
+  PUSHMI_CEXP_EVAL(PUSHMI_IF_CONSTEXPR_ELSE_, PUSHMI_IF_CONSTEXPR_IF_ LIST)
 
 #define PUSHMI_IF_CONSTEXPR_RETURN(LIST)\
-  PUSHMI_PP_IGNORE_SHADOW_BEGIN \
+  PUSHMI_CEXP_IGNORE_SHADOW_BEGIN \
   PUSHMI_IF_CONSTEXPR(LIST)\
-  PUSHMI_PP_IGNORE_SHADOW_END \
+  PUSHMI_CEXP_IGNORE_SHADOW_END \
   /**/
 
 #define PUSHMI_IF_CONSTEXPR_IF_(...) \
-  (__VA_ARGS__) PUSHMI_COMMA PUSHMI_IF_CONSTEXPR_THEN_
+  (__VA_ARGS__) PUSHMI_CEXP_COMMA PUSHMI_IF_CONSTEXPR_THEN_
 
 #define PUSHMI_IF_CONSTEXPR_THEN_(...) \
-  ({__VA_ARGS__}) PUSHMI_COMMA
+  ({__VA_ARGS__}) PUSHMI_CEXP_COMMA
 
 #define PUSHMI_IF_CONSTEXPR_ELSE_(A, B, C) \
-  A PUSHMI_STRIP B PUSHMI_IF_CONSTEXPR_ ## C
+  A PUSHMI_CEXP_STRIP B PUSHMI_IF_CONSTEXPR_ ## C
 
 #define PUSHMI_IF_CONSTEXPR_else(...) \
   else {__VA_ARGS__}
@@ -130,15 +147,15 @@ struct id_fn {
 #include <type_traits>
 
 #define PUSHMI_IF_CONSTEXPR(LIST)\
-  PUSHMI_PP_IGNORE_SHADOW_BEGIN \
-  PUSHMI_EVAL(PUSHMI_IF_CONSTEXPR_ELSE_, PUSHMI_IF_CONSTEXPR_IF_ LIST)\
-  PUSHMI_PP_IGNORE_SHADOW_END \
+  PUSHMI_CEXP_IGNORE_SHADOW_BEGIN \
+  PUSHMI_CEXP_EVAL(PUSHMI_IF_CONSTEXPR_ELSE_, PUSHMI_IF_CONSTEXPR_IF_ LIST)\
+  PUSHMI_CEXP_IGNORE_SHADOW_END \
   /**/
 
 #define PUSHMI_IF_CONSTEXPR_RETURN(LIST)\
-  PUSHMI_PP_IGNORE_SHADOW_BEGIN \
-  return PUSHMI_EVAL(PUSHMI_IF_CONSTEXPR_ELSE_, PUSHMI_IF_CONSTEXPR_IF_ LIST)\
-  PUSHMI_PP_IGNORE_SHADOW_END \
+  PUSHMI_CEXP_IGNORE_SHADOW_BEGIN \
+  return PUSHMI_CEXP_EVAL(PUSHMI_IF_CONSTEXPR_ELSE_, PUSHMI_IF_CONSTEXPR_IF_ LIST)\
+  PUSHMI_CEXP_IGNORE_SHADOW_END \
   /**/
 
 #define PUSHMI_IF_CONSTEXPR_IF_(...)                                                  \
@@ -146,7 +163,7 @@ struct id_fn {
   /**/
 
 #define PUSHMI_IF_CONSTEXPR_THEN_(...) \
-  ([&](PUSHMI_MAYBE_UNUSED auto id)mutable->decltype(auto){__VA_ARGS__})) PUSHMI_COMMA \
+  ([&](PUSHMI_CEXP_MAYBE_UNUSED auto id)mutable->decltype(auto){__VA_ARGS__})) PUSHMI_CEXP_COMMA \
   /**/
 
 #define PUSHMI_IF_CONSTEXPR_ELSE_(A, B) \
@@ -154,7 +171,7 @@ struct id_fn {
   /**/
 
 #define PUSHMI_IF_CONSTEXPR_else(...) \
-  ([&](PUSHMI_MAYBE_UNUSED auto id)mutable->decltype(auto){__VA_ARGS__});\
+  ([&](PUSHMI_CEXP_MAYBE_UNUSED auto id)mutable->decltype(auto){__VA_ARGS__});\
   /**/
 
 namespace folly {