[C++20] -Wdeprecated-enum-enum-conversion raises many warnings with -std=gnu++20

[stac47]

I am compiling Crypto++ with GCC 11 and -std=gnu++20 and the compilation raises many warnings like the following one:

asn.h:494:37: warning: bitwise operation between different enumeration types 'CryptoPP::ASNTag' and
'CryptoPP::ASNIdFlag' is deprecated [-Wdeprecated-enum-enum-conversion]
  494 |         enum {DefaultTag = SEQUENCE | CONSTRUCTED};
      |                            ~~~~~~~~~^~~~~~~~~~~~~

From the GCC documentation (https://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Dialect-Options.html#C_002b_002b-Dialect-Options)

-Wno-deprecated-enum-enum-conversion (C++ and Objective-C++ only)

    Disable the warning about the case when the usual arithmetic conversions are applied on operands where
    one is of enumeration type and the other is of a different enumeration type. This conversion was
    deprecated in C++20. For example:

    enum E1 { e };
    enum E2 { f };
    int k = f - e;

    -Wdeprecated-enum-enum-conversion is enabled by default with -std=gnu++20. In pre-C++20 dialects, this
    warning can be enabled by -Wenum-conversion.

Requested information:

• OS: Ubuntu 20.04
• Crypto++ version 8.5
• Build system: gnu make
• Compile command (I don't think the LTO options are of any interest):

  g++ -fPIC -pthread -pipe -DNDEBUG -O3 -flto -ffat-lto-objects -fuse-linker-plugin -std=gnu++20 -fno-working-directory -ggdb3 -c cryptlib.cpp

• Link command

  g++ -o cryptest.exe -fPIC -pthread -pipe -DNDEBUG -O3 -flto -ffat-lto-objects -fuse-linker-plugin -std=gnu++20 -fno-working-directory -ggdb3 adhoc.o test.o bench1.o bench2.o bench3.o datatest.o dlltest.o fipsalgt.o validat0.o validat1.o validat2.o validat3.o validat4.o validat5.o validat6.o validat7.o validat8.o validat9.o validat10.o regtest1.o regtest2.o regtest3.o regtest4.o ./libcryptopp.a -DNDEBUG -O3 -Wl,-flto -ffat-lto-objects -fuse-linker-plugin -fno-working-directory -ggdb3

[noloader]

Thanks @stac47,

I can duplicate similar problems on Fedora 33 with Clang. F33 GCC is 10.2, which does not tickle the issue.

A lot of those values are placed in enums to creates a const class value that does not take up space; as opposed to, say, a static int, which would require a memory allocation.

(That's about a useless warning in my naive opinion, but that's a different matter).

What do you recommend?

[noloader]

Cleared at Commit de45ebeec15c and Commit f7e6af6344f2.

Thanks again for the report.