code: func NewFunctionInternal(ctx sessionctx.Context, funcName string, retType *types.FieldType, args ...Expression) Expression {
  	expr, err := NewFunction(ctx, funcName, retType, args...)
  	terror.Log(err)
  	return expr
  }
docs: // NewFunctionInternal is similar to NewFunction, but do not returns error, should only be used internally.