diff --git a/Schema.Cpp.Test/ExemplarSemanticTypeTests.cs b/Schema.Cpp.Test/ExemplarSemanticTypeTests.cs
index 26a9bf9..d3c676e 100644
--- a/Schema.Cpp.Test/ExemplarSemanticTypeTests.cs
+++ b/Schema.Cpp.Test/ExemplarSemanticTypeTests.cs
@@ -55,6 +55,7 @@ public sealed class ExemplarSemanticTypeTests
 
 		#pragma once
 
+		#include <compare>
 		#include <cstdint>
 		#include <type_traits>
 
diff --git a/Schema.Cpp/CppFileBuilder.cs b/Schema.Cpp/CppFileBuilder.cs
index f333f0d..6d74192 100644
--- a/Schema.Cpp/CppFileBuilder.cs
+++ b/Schema.Cpp/CppFileBuilder.cs
@@ -147,6 +147,13 @@ public SourceFile SemanticType(SchemaSemanticType semanticType)
 		}
 
 		declaration.Members.Add(Comparison(name, "==", "bool"));
+
+		// A defaulted <=> answers with one of the ordering types, and those live in <compare>.
+		// libstdc++ happens to have declared them by the time this header is read and libc++ does
+		// not, so without this the generated type compiles under GCC and is refused by Clang -
+		// which is exactly the kind of difference a generator must not leave to whoever includes
+		// it.
+		mapper.Require("<compare>");
 		declaration.Members.Add(Comparison(name, "<=>", "auto"));
 		declaration.Members.Add(new FieldDeclaration(ValueField, UnderlyingAlias) { Visibility = Visibility.Private });