--- a/js/src/frontend/TokenStream.cpp
+++ b/js/src/frontend/TokenStream.cpp
@@ -1257,20 +1257,17 @@ IsTokenSane(Token* tp)
 }
 #endif
 
 template<class AnyCharsAccess>
 bool
 TokenStreamChars<char16_t, AnyCharsAccess>::matchTrailForLeadSurrogate(char16_t lead,
                                                                        uint32_t* codePoint)
 {
-    static_assert(mozilla::IsBaseOf<TokenStreamChars<char16_t, AnyCharsAccess>,
-                                    TokenStreamSpecific<char16_t, AnyCharsAccess>>::value,
-                  "static_cast below presumes an inheritance relationship");
-    auto* ts = static_cast<TokenStreamSpecific<char16_t, AnyCharsAccess>*>(this);
+    TokenStreamSpecific* ts = asSpecific();
 
     int32_t maybeTrail = ts->getCharIgnoreEOL();
     if (!unicode::IsTrailSurrogate(maybeTrail)) {
         ts->ungetCharIgnoreEOL(maybeTrail);
         return false;
     }
 
     *codePoint = unicode::UTF16Decode(lead, maybeTrail);

--- a/js/src/frontend/TokenStream.h
+++ b/js/src/frontend/TokenStream.h
@@ -128,16 +128,17 @@
  * TokenStreamSpecific declarer to specify this is more explicit), we couldn't.
  */
 
 #include "mozilla/ArrayUtils.h"
 #include "mozilla/Assertions.h"
 #include "mozilla/Attributes.h"
 #include "mozilla/DebugOnly.h"
 #include "mozilla/PodOperations.h"
+#include "mozilla/TypeTraits.h"
 #include "mozilla/Unused.h"
 
 #include <stdarg.h>
 #include <stddef.h>
 #include <stdio.h>
 
 #include "jscntxt.h"
 #include "jspubtd.h"
@@ -946,18 +947,28 @@ class TokenStreamCharsBase
 };
 
 template<typename CharT, class AnyCharsAccess> class TokenStreamChars;
 
 template<class AnyCharsAccess>
 class TokenStreamChars<char16_t, AnyCharsAccess>
   : public TokenStreamCharsBase<char16_t>
 {
+    using Self = TokenStreamChars<char16_t, AnyCharsAccess>;
     using CharsBase = TokenStreamCharsBase<char16_t>;
 
+    using TokenStreamSpecific = frontend::TokenStreamSpecific<char16_t, AnyCharsAccess>;
+
+    TokenStreamSpecific* asSpecific() {
+        static_assert(mozilla::IsBaseOf<Self, TokenStreamSpecific>::value,
+                      "static_cast below presumes an inheritance relationship");
+
+        return static_cast<TokenStreamSpecific*>(this);
+    }
+
     bool matchTrailForLeadSurrogate(char16_t lead, uint32_t* codePoint);
 
   public:
     using CharsBase::CharsBase;
 
     TokenStreamAnyChars& anyChars() {
         return AnyCharsAccess::anyChars(this);
     }