Error building when using MQTTProperties.h: ISO C++ prohibits anonymous structs

[jsrivaya]

Describe the bug ISO C++ prohibits anonymous structs. There are anonymous struct definitions in MQTTProperties header

/**
 * Structure to hold an MQTT version 5 property of any type
 */
typedef struct
{
  enum MQTTPropertyCodes identifier; /**<  The MQTT V5 property id. A multi-byte integer. */
  /** The value of the property, as a union of the different possible types. */
  union {
    unsigned char byte;       /**< holds the value of a byte property type */
    unsigned short integer2;  /**< holds the value of a 2 byte integer property type */
    unsigned int integer4;    /**< holds the value of a 4 byte integer property type */
    struct {
      MQTTLenString data;  /**< The value of a string property, or the name of a user property. */
      MQTTLenString value; /**< The value of a user property. */
    };
  } value;
} MQTTProperty;

It can be fixed by giving it a type and name

/**
 * Structure to hold an MQTT version 5 property of any type
 */
typedef struct
{
  enum MQTTPropertyCodes identifier; /**<  The MQTT V5 property id. A multi-byte integer. */
  /** The value of the property, as a union of the different possible types. */
  union {
    unsigned char byte;       /**< holds the value of a byte property type */
    unsigned short integer2;  /**< holds the value of a 2 byte integer property type */
    unsigned int integer4;    /**< holds the value of a 4 byte integer property type */
    struct string_type {
      MQTTLenString data;  /**< The value of a string property, or the name of a user property. */
      MQTTLenString value; /**< The value of a user property. */
    } string_type;
  } value;
} MQTTProperty;

It can be silent by using -fms-extensions flag, but this is probably not the right thing to do.

To Reproduce Take a library trace as outlined in the README, and/or have a program or describe the steps to reproduce the behavior: Build your application source using -Werror=pedantic

~/.conan/data/paho-mqtt-c/1.3.10/_/_/package/5d725c629ca6508685c6dcfeb3cae54a8ba144f2/include/MQTTProperties.h:105:5: error: ISO C++ prohibits anonymous structs [-Werror=pedantic]

Expected behavior I would expect ISO C++ and avoid this Warnings which can be errors for some of us

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Environment (please complete the following information):

• OS: [e.g. Linux]
• Version [e.g. 22]

Additional context Add any other context about the problem here.

[icraggs]

Unfortunately no-one has raised the issue before since 2017 when it was created.

To make the change you suggest now would be an interface change - application programs which used the header would have to change. I wouldn't want to do that without a major version update.

An alternative would be to change the union to a struct and remove the anonymous struct, but that would introduce a binary backward incompatiblity.