Flexible array members are not standard C++

[gilles-peskine-arm]

Since Mbed TLS 3.6.0, one of the types declared in an Mbed TLS header is a struct with a flexible array member: typedef struct psa_key_production_parameters_s psa_key_production_parameters_t.

struct psa_key_production_parameters_s {
    /* Future versions may add other fields in this structure. */
    uint32_t flags;
    uint8_t data[];
};

This is standard C99, but not standard C++ (any version), although many compilers accept it.

3.6.0 % echo '#include <psa/crypto.h>' | g++ -c -I include -std=c++2a -x c++ -
3.6.0 % echo '#include <psa/crypto.h>' | g++ -c -I include -std=c++2a -pedantic -x c++ -
In file included from include/psa/crypto.h:4828,
                 from <stdin>:1:
include/psa/crypto_struct.h:229:13: warning: ISO C++ forbids flexible array member ‘data’ [-Wpedantic]
  229 |     uint8_t data[];
      |             ^~~~
3.6.0 % echo '#include <psa/crypto.h>' | clang++ -c -I include -std=c++2a -x c++ -
3.6.0 % echo '#include <psa/crypto.h>' | clang++ -c -I include -std=c++2a -pedantic -x c++ -
In file included from <stdin>:1:
In file included from include/psa/crypto.h:4828:
include/psa/crypto_struct.h:229:13: warning: flexible array members are a C99 feature [-Wc99-extensions]
    uint8_t data[];
            ^
1 warning generated.

First reported here.

Importance:

• Although we only advertise Mbed TLS as a C library, it is generally accepted that the headers of a C library can be consumed by a C++ compiler, and we do have a test that checks that C++ builds work (currently using GCC in non-pedantic mode, but that's originally not by design, just because we didn't think of adding -pedantic; currently partly by design because of other non-strict-C++-compliance that we'd also like to fix).
• This evidently impacts many users, given that it was one of the first things reported after the 3.6 release.
• Many projects like to invoke their compiler in pedantic mode. Disabling pedantic for the sake of the Mbed TLS headers would also potentially hide other desired errors.
• Most C++ compilers do include a C99 compiler and are capable of mixing C99 and C++ features. However this is not the case for all of them. On Godbolt, I could only find one compiler that doesn't accept a flexible array member with its default command line: EDG C++, which claims C++17 and C90 but not C99 support.

Goal of this issue: find a solution so that at least users who don't call psa_generate_key_ext, and so don't actually need struct psa_key_production_parameters_s, can compile their C++ application with a strict C++ compiler that doesn't support flexible array members. This must be done without an API change for users who are building with a C99 compiler.

Validation: g++ -std=c++17 -pedantic and clang++ -std=c++17 -pedantic, ideally on the CI, but at least manually if we haven't fixed #7087.