commit 87f49da7a84ed9bbce4b8ef263d85b9bd5c42eac
Author: Christoph Moench-Tegeder <cmt@FreeBSD.org>

    Partially de-constexpr'ify LanguageId code
    
    This is to deal with our (FreeBSD) libc++, where in some cases
    std:pair<> is not a "literal type" (suitable for constexpr), even
    if the same std:pair<> is just fine on all other relevant systems.
    
    This removes just enough constexpr annotations to make the code
    compile.
    
    Sample compiler error message:
    
    /wrkdirs/usr/ports/www/firefox/work/firefox-151.0/js/src/util/LanguageId.h:234:66: error: constexpr function's return type 'mozilla::Maybe<std::pair<LanguageId>
      234 |   static constexpr mozilla::Maybe<std::pair<LanguageId, size_t>> from(
          |                                                                  ^
    /wrkdirs/usr/ports/www/firefox/work/.build/dist/include/mozilla/Maybe.h:367:7: note: 'Maybe<std::pair<js::LanguageId, unsigned long>>' is not literal because i>
      367 |     : private detail::MaybeStorage<T>,
          |       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

diff --git js/src/builtin/intl/LocaleNegotiation.cpp js/src/builtin/intl/LocaleNegotiation.cpp
index 0aff7274eda1..5c25e53b8ea1 100644
--- js/src/builtin/intl/LocaleNegotiation.cpp
+++ js/src/builtin/intl/LocaleNegotiation.cpp
@@ -1124,13 +1124,13 @@ struct OldStyleLanguageTagMapping {
   LanguageId oldStyle;
   LanguageId modernStyle;
 
-  consteval OldStyleLanguageTagMapping(std::string_view oldStyle,
+  OldStyleLanguageTagMapping(std::string_view oldStyle,
                                        std::string_view modernStyle)
       : oldStyle(LanguageId::fromValidBcp49(oldStyle)),
         modernStyle(LanguageId::fromValidBcp49(modernStyle)) {}
 };
 
-static constexpr OldStyleLanguageTagMapping oldStyleLanguageTagMappings[] = {
+static OldStyleLanguageTagMapping oldStyleLanguageTagMappings[] = {
     {"pa-PK", "pa-Arab-PK"}, {"zh-CN", "zh-Hans-CN"}, {"zh-HK", "zh-Hant-HK"},
     {"zh-SG", "zh-Hans-SG"}, {"zh-TW", "zh-Hant-TW"},
 };
diff --git js/src/builtin/intl/SharedIntlData.cpp js/src/builtin/intl/SharedIntlData.cpp
index 581c9d1e6ebd..9dd3a50acb1c 100644
--- js/src/builtin/intl/SharedIntlData.cpp
+++ js/src/builtin/intl/SharedIntlData.cpp
@@ -431,11 +431,11 @@ bool js::intl::SharedIntlData::getAvailableLocales(
   // directly support it (but does support it through fallback, e.g. supporting
   // "en-GB" indirectly using "en" support).
   {
-    static constexpr auto lastDitch = LastDitchLocale();
-    static_assert(std::string_view{lastDitch.toString()} == "en-GB");
+    static auto lastDitch = LastDitchLocale();
+    // static_assert(std::string_view{lastDitch.toString()} == "en-GB");
 
 #ifdef DEBUG
-    static constexpr auto lastDitchParent = lastDitch.parentLocale();
+    static auto lastDitchParent = lastDitch.parentLocale();
     static_assert(std::string_view{lastDitchParent.toString()} == "en");
 
     MOZ_ASSERT(locales.has(lastDitchParent),
diff --git js/src/util/LanguageId.h js/src/util/LanguageId.h
index 7c193c3e2e07..bf391ef39717 100644
--- js/src/util/LanguageId.h
+++ js/src/util/LanguageId.h
@@ -231,7 +231,7 @@ class LanguageId final {
 
  private:
   template <char... separators, typename CharT>
-  static constexpr mozilla::Maybe<std::pair<LanguageId, size_t>> from(
+  static /* constexpr */ mozilla::Maybe<std::pair<LanguageId, size_t>> from(
       std::basic_string_view<CharT> localeId) {
     // Return true iff |sv| starts with a subtag of length |len|.
     auto hasSubtag = [](std::basic_string_view<CharT> sv, size_t len) {
@@ -313,7 +313,7 @@ class LanguageId final {
    *
    * Subtags in ICU and Unicode locale identifiers are separated by "-" or "_".
    */
-  static constexpr auto fromId(std::string_view localeId) {
+  static auto fromId(std::string_view localeId) {
     return from<'-', '_'>(localeId);
   }
 
@@ -327,7 +327,7 @@ class LanguageId final {
    *
    * Subtags in ICU and Unicode locale identifiers are separated by "-" or "_".
    */
-  static constexpr auto fromId(mozilla::Span<const char> localeId) {
+  static auto fromId(mozilla::Span<const char> localeId) {
     return fromId(std::string_view{localeId.data(), localeId.size()});
   }
 
@@ -341,7 +341,7 @@ class LanguageId final {
    *
    * Subtags in BCP 47 locale identifiers are separated by "-".
    */
-  static constexpr auto fromBcp49(std::string_view localeId) {
+  static auto fromBcp49(std::string_view localeId) {
     return from<'-'>(localeId);
   }
 
@@ -355,7 +355,7 @@ class LanguageId final {
    *
    * Subtags in BCP 47 locale identifiers are separated by "-".
    */
-  static constexpr auto fromBcp49(std::u16string_view localeId) {
+  static auto fromBcp49(std::u16string_view localeId) {
     return from<u'-'>(localeId);
   }
 
@@ -370,7 +370,7 @@ class LanguageId final {
    * Subtags in BCP 47 locale identifiers are separated by "-".
    */
   template <typename CharT>
-  static constexpr auto fromBcp49(mozilla::Span<const CharT> localeId) {
+  static auto fromBcp49(mozilla::Span<const CharT> localeId) {
     return fromBcp49(std::basic_string_view{localeId.data(), localeId.size()});
   }
 
@@ -381,7 +381,7 @@ class LanguageId final {
    *
    * Subtags in BCP 47 locale identifiers are separated by "-".
    */
-  static consteval auto fromValidBcp49(std::string_view localeId) {
+  static auto fromValidBcp49(std::string_view localeId) {
     return fromBcp49(localeId)->first;
   }
 
@@ -408,8 +408,8 @@ class LanguageId final {
   /**
    * Return the language identifier for the undetermined locale "und".
    */
-  static constexpr auto und() {
-    constexpr LanguageId locale = fromValidBcp49("und");
+  static auto und() {
+    LanguageId locale = fromValidBcp49("und");
     return locale;
   }