This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
workInProgress.tag = FunctionComponent;
    if (__DEV__) {
      if (disableLegacyContext && Component.contextTypes) {
        console.error(
          '%s uses the legacy contextTypes API which is no longer supported. ' +
            'Use React.createContext() with React.useContext() instead.',
          getComponentNameFromType(Component) || 'Unknown',
        );
      }
      if (
        debugRenderPhaseSideEffectsForStrictMode &&
        workInProgress.mode & StrictLegacyMode
      ) {
        disableLogs();
        try {
          value = renderWithHooks(
            null,
            workInProgress,
            Component,
            props,
            context,
            renderLanes,
          );
        } finally {
          reenableLogs();
        }
      }
    }
    reconcileChildren(null, workInProgress, value, renderLanes);