diff --git a/src/core/include/mp-units/bits/external/hacks.h b/src/core/include/mp-units/bits/external/hacks.h
index 34479a34..4bd4d0a4 100644
--- a/src/core/include/mp-units/bits/external/hacks.h
+++ b/src/core/include/mp-units/bits/external/hacks.h
@@ -107,7 +107,7 @@
 
 #endif
 
-#if MP_UNITS_COMP_CLANG < 17
+#if (defined MP_UNITS_COMP_CLANG && MP_UNITS_COMP_CLANG < 17) || (defined MP_UNITS_COMP_GCC && MP_UNITS_COMP_GCC < 12)
 
 #define MP_UNITS_CONSTEVAL constexpr
 
diff --git a/src/core/include/mp-units/bits/reference_concepts.h b/src/core/include/mp-units/bits/reference_concepts.h
index e0e3d6a4..849d0427 100644
--- a/src/core/include/mp-units/bits/reference_concepts.h
+++ b/src/core/include/mp-units/bits/reference_concepts.h
@@ -32,11 +32,12 @@ struct reference;
 
 namespace detail {
 
+// do not refactor below to a variable template - GCC-11 does not like it
 template<typename T>
-inline constexpr bool is_specialization_of_reference = false;
+struct is_specialization_of_reference : std::false_type {};
 
 template<auto Q, auto U>
-inline constexpr bool is_specialization_of_reference<reference<Q, U>> = true;
+struct is_specialization_of_reference<reference<Q, U>> : std::true_type {};
 
 }  // namespace detail
 
@@ -46,7 +47,7 @@ inline constexpr bool is_specialization_of_reference<reference<Q, U>> = true;
  * Satisfied by all specializations of @c reference.
  */
 template<typename T>
-concept Reference = AssociatedUnit<T> || detail::is_specialization_of_reference<T>;
+concept Reference = AssociatedUnit<T> || detail::is_specialization_of_reference<T>::value;
 
 [[nodiscard]] consteval QuantitySpec auto get_quantity_spec(AssociatedUnit auto u);
 
diff --git a/src/core/include/mp-units/bits/unit_concepts.h b/src/core/include/mp-units/bits/unit_concepts.h
index ba727a20..7fcdec2a 100644
--- a/src/core/include/mp-units/bits/unit_concepts.h
+++ b/src/core/include/mp-units/bits/unit_concepts.h
@@ -31,8 +31,9 @@ namespace mp_units {
 
 namespace detail {
 
+// do not refactor below to a variable template - GCC-11 does not like it
 template<typename T>
-inline constexpr bool is_unit = false;
+struct is_unit : std::false_type {};
 
 }  // namespace detail
 
@@ -42,7 +43,7 @@ inline constexpr bool is_unit = false;
  * Satisfied by all unit types provided by the library.
  */
 template<typename T>
-concept Unit = detail::is_unit<T>;
+concept Unit = detail::is_unit<T>::value;
 
 template<Magnitude auto M, Unit U>
 struct scaled_unit;
@@ -140,8 +141,8 @@ template<basic_symbol_text Symbol, Magnitude auto M, PrefixableUnit auto U>
 inline constexpr bool is_specialization_of_prefixed_unit<prefixed_unit<Symbol, M, U>> = true;
 
 template<typename T>
-  requires requires(T* t) { is_unit_impl(t); }
-inline constexpr bool is_unit<T> = !is_specialization_of_named_unit<T> && !is_specialization_of_prefixed_unit<T>;
+  requires requires(T* t) { is_unit_impl(t); } && (!is_specialization_of_named_unit<T>) && (!is_specialization_of_prefixed_unit<T>)
+struct is_unit<T> : std::true_type {};
 
 template<Unit U>
 [[nodiscard]] consteval bool has_associated_quantity(U);
diff --git a/src/core/include/mp-units/bits/value_cast.h b/src/core/include/mp-units/bits/value_cast.h
index 27c6d268..3fcf5745 100644
--- a/src/core/include/mp-units/bits/value_cast.h
+++ b/src/core/include/mp-units/bits/value_cast.h
@@ -46,7 +46,7 @@ template<Unit auto ToU, typename Q>
 {
   using q_type = std::remove_reference_t<Q>;
   constexpr auto r = [] {
-    if constexpr (detail::is_specialization_of_reference<std::remove_const_t<decltype(q_type::reference)>> ||
+    if constexpr (detail::is_specialization_of_reference<std::remove_const_t<decltype(q_type::reference)>>::value ||
                   !AssociatedUnit<std::remove_const_t<decltype(ToU)>>)
       return reference<q_type::quantity_spec, ToU>{};
     else
diff --git a/src/core/include/mp-units/quantity_spec.h b/src/core/include/mp-units/quantity_spec.h
index c94d1af3..ad381cc1 100644
--- a/src/core/include/mp-units/quantity_spec.h
+++ b/src/core/include/mp-units/quantity_spec.h
@@ -115,7 +115,7 @@ struct quantity_spec_interface {
   }
 #else
   template<typename Self_ = Self, UnitOf<Self_{}> U>
-  [[nodiscard]] consteval Reference auto operator[](U u) const
+  [[nodiscard]] MP_UNITS_CONSTEVAL Reference auto operator[](U u) const
   {
     if constexpr (detail::QuantityKindSpec<Self_>)
       return u;
@@ -294,7 +294,7 @@ struct quantity_spec<Self, QS, Args...> : std::remove_const_t<decltype(QS)> {
 
 #ifndef __cpp_explicit_this_parameter
   template<typename Self_ = Self, UnitOf<Self_{}> U>
-  [[nodiscard]] consteval Reference auto operator[](U u) const
+  [[nodiscard]] MP_UNITS_CONSTEVAL Reference auto operator[](U u) const
   {
     if constexpr (detail::QuantityKindSpec<Self>)
       return u;
diff --git a/test/unit_test/static/unit_symbol_test.cpp b/test/unit_test/static/unit_symbol_test.cpp
index f1130610..a642caaf 100644
--- a/test/unit_test/static/unit_symbol_test.cpp
+++ b/test/unit_test/static/unit_symbol_test.cpp
@@ -30,7 +30,7 @@ using namespace mp_units;
 using namespace mp_units::si;
 using namespace mp_units::iec80000;
 
-#ifdef __cpp_lib_constexpr_string
+#if __cpp_lib_constexpr_string && (!defined MP_UNITS_COMP_GCC || MP_UNITS_COMP_GCC > 11)
 
 using enum text_encoding;
 using enum unit_symbol_solidus;