I just came across this build failure while working on an upcoming patch that added a new file:

In file included from /Users/antoine/Builds/Debug/DerivedSources/WebCore/unified-sources/UnifiedSource140.cpp:1:
In file included from /Users/antoine/Code/safari/OpenSource/Source/WebCore/css/typedom/MainThreadStylePropertyMapReadOnly.cpp:29:
In file included from /Users/antoine/Code/safari/OpenSource/Source/WebCore/css/CSSPendingSubstitutionValue.h:33:
/Users/antoine/Code/safari/OpenSource/Source/WebCore/css/CSSVariableReferenceValue.h:110:51: error: member access into incomplete type 'WebCore::CSSVariableData'
    auto resolvedTokens = resolveTokenRange(m_data->tokenRange(), builderState);
/Users/antoine/Code/safari/OpenSource/Source/WebCore/css/CSSVariableReferenceValue.h:41:7: note: forward declaration of 'WebCore::CSSVariableData'
class CSSVariableData;
/Users/antoine/Code/safari/OpenSource/Source/WebCore/css/CSSVariableReferenceValue.h:114:56: error: member access into incomplete type 'WebCore::CSSVariableData'
    if (!m_cacheDependencyData || m_cacheDependencyData->tokens() != *resolvedTokens) {
/Users/antoine/Code/safari/OpenSource/Source/WebCore/css/CSSVariableReferenceValue.h:41:7: note: forward declaration of 'WebCore::CSSVariableData'
class CSSVariableData;
/Users/antoine/Code/safari/OpenSource/Source/WebCore/css/CSSVariableReferenceValue.h:115:33: error: incomplete type 'WebCore::CSSVariableData' named in nested name specifier
        m_cacheDependencyData = CSSVariableData::create(*resolvedTokens, context());
/Users/antoine/Code/safari/OpenSource/Source/WebCore/css/CSSVariableReferenceValue.h:41:7: note: forward declaration of 'WebCore::CSSVariableData'
class CSSVariableData;
3 errors generated.

It looks like merely forward-declaring CSSVariableData in CSSVariableReferenceValue.h is not enough.