diff --git a/flang/include/flang/Evaluate/type.h b/flang/include/flang/Evaluate/type.h
index 6ab09f60789d2..a57d8107b7e33 100644
--- a/flang/include/flang/Evaluate/type.h
+++ b/flang/include/flang/Evaluate/type.h
@@ -142,7 +142,7 @@ class DynamicType {
     return charLengthParamValue_;
   }
   constexpr std::optional<std::int64_t> knownLength() const {
-#if !__clang__ && __GNUC__ == 7
+#if defined(_GLIBCXX_RELEASE) && _GLIBCXX_RELEASE == 7
     if (knownLength_ < 0) {
       return std::nullopt;
     }
@@ -222,7 +222,7 @@ class DynamicType {
   TypeCategory category_{TypeCategory::Derived}; // overridable default
   int kind_{0};
   const semantics::ParamValue *charLengthParamValue_{nullptr};
-#if !__clang__ && __GNUC__ == 7
+#if defined(_GLIBCXX_RELEASE) && _GLIBCXX_RELEASE == 7
   // GCC 7's optional<> lacks a constexpr operator=
   std::int64_t knownLength_{-1};
 #else