docs: public static Symbol.MethodSymbol getFunctionalInterfaceMethod(ExpressionTree tree, Types types) {
      Preconditions.checkArgument(
          (tree instanceof LambdaExpressionTree) || (tree instanceof MemberReferenceTree));
      Type funcInterfaceType = ((JCTree.JCFunctionalExpression) tree).type;
      return (Symbol.MethodSymbol) types.findDescriptorSymbol(funcInterfaceType.tsym);
    }