I created an issue for that: https://github.com/microsoft/vscode-cpptools/issues/13738

Quick fix

If you right click on assign and go to the definition, you'll see:

    /**
     *  @brief  Assigns a given value to a %vector.
     *  @param  __n  Number of elements to be assigned.
     *  @param  __val  Value to be assigned.
     *
     * ....
     */
    _GLIBCXX20_CONSTEXPR
    void assign(size_type __n, const value_type& __val) { _M_fill_assign(__n, __val); }

Put void on the same line as the macro and it'll work.

    /**
     * ....
     */
    _GLIBCXX20_CONSTEXPR void 
    assign(...){...}