tweak RegexMatchCacheKey conversion to use variable template

Summary: Some versions of MSVC fail to compile with an argument passed to the function template.

Reviewed By: Gownta

Differential Revision: D64643990

fbshipit-source-id: f988a618b8c163c1bd930c091cc06076b0960ba7

folly/container/RegexMatchCache.h

@@ -391,16 +391,15 @@ class RegexMatchCacheKey {
 
   static data_type init(std::string_view regex) noexcept;
 
-  template <typename T, typename V = std::remove_cv_t<T>>
-  static constexpr bool is_span_compatible() noexcept {
-    return //
-        !std::is_volatile_v<T> && //
-        std::is_integral_v<V> && //
-        std::is_unsigned_v<V> && //
-        !std::is_same_v<bool, V> && //
-        !std::is_same_v<char, V> && //
-        alignof(V) <= data_align;
-  }
+  template <typename T, size_t E, typename V = std::remove_cv_t<T>>
+  static constexpr bool is_span_compatible_v = //
+      !std::is_volatile_v<T> && //
+      std::is_integral_v<V> && //
+      std::is_unsigned_v<V> && //
+      !std::is_same_v<bool, V> && //
+      !std::is_same_v<char, V> && //
+      alignof(V) <= data_align && //
+      (E == data_size / sizeof(T) || E == dynamic_extent);
 
  public:
   explicit RegexMatchCacheKey(std::string_view regex) noexcept
@@ -409,10 +408,7 @@ class RegexMatchCacheKey {
   template <
       typename T,
       std::size_t E,
-      std::enable_if_t<
-          is_span_compatible<T>() &&
-              (E == data_size / sizeof(T) || E == dynamic_extent),
-          int> = 0>
+      std::enable_if_t<is_span_compatible_v<T, E>, int> = 0>
   explicit operator span<T const, E>() const noexcept {
     return {reinterpret_cast<T const*>(data_.data()), E};
   }