Fix compile failure on MacOS X

[zedinosaur]

std::shared_ptr::unique() is deprecated in C++17 and removed in C++20.

Inspired by PR#1362.

Snippet of the failure:

In file included from ../../src/terminal/terminal.h:42:
../../src/terminal/terminalframebuffer.h:431:23: error: 'unique' is deprecated [-Werror,-Wdeprecated-declarations]
  431 |     if ( !mutable_row.unique() ) {
      |                       ^
/Applications/Xcode_16.4.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c++/v1/__memory/shared_ptr.h:752:3: note: 'unique' has been explicitly marked deprecated here
  752 |   _LIBCPP_DEPRECATED_IN_CXX17 _LIBCPP_HIDE_FROM_ABI bool unique() const _NOEXCEPT { return use_count() == 1; }
      |   ^
/Applications/Xcode_16.4.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c++/v1/__config:731:41: note: expanded from macro '_LIBCPP_DEPRECATED_IN_CXX17'
  731 | #    define _LIBCPP_DEPRECATED_IN_CXX17 _LIBCPP_DEPRECATED
      |                                         ^
/Applications/Xcode_16.4.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c++/v1/__config:696:49: note: expanded from macro '_LIBCPP_DEPRECATED'
  696 | #      define _LIBCPP_DEPRECATED __attribute__((__deprecated__))
      |                                                 ^
1 error generated.

[eminence]

Isn't this a duplicate of #1362 ?

[zedinosaur]

It is sort of a duplicate of #1362.

This still isn't compiling due lots of failures that look like this:

In file included from transportfragment.cc:37:
In file included from ../../src/protobufs/transportinstruction.pb.h:20:
In file included from /opt/homebrew/Cellar/protobuf/33.4_1/include/google/protobuf/io/coded_stream.h:109:
In file included from /opt/homebrew/Cellar/abseil/20260107.0/include/absl/log/absl_log.h:36:
In file included from /opt/homebrew/Cellar/abseil/20260107.0/include/absl/log/internal/log_impl.h:22:
/opt/homebrew/Cellar/abseil/20260107.0/include/absl/log/internal/log_message.h:76:26: error: type nullability specifier '_Nonnull' is a Clang extension [-Werror,-Wnullability-extension]
   76 |   LogMessage(const char* absl_nonnull file, int line,
      |                          ^
/opt/homebrew/Cellar/abseil/20260107.0/include/absl/base/nullability.h:240:22: note: expanded from macro 'absl_nonnull'
  240 | #define absl_nonnull _Nonnull
      |                      ^

Which is odd since I would assume this was being compiled with clang on macos.

[zedinosaur]

More problems:

c++ -std=gnu++17 -DHAVE_CONFIG_H -I. -I../../src/include  -I../../ -I/opt/homebrew/Cellar/protobuf/33.4_1/include -DPROTOBUF_USE_DLLS -I/opt/homebrew/Cellar/abseil/20260107.1/include -Wno-float-conversion -Wno-implicit-float-conversion -Wno-implicit-int-float-conversion -Wno-unknown-warning-option -Wno-unused-command-line-argument -DNOMINMAX  -Wall -Werror -Wno-c++17-extensions -Wextra -pedantic -Wno-long-long -Weffc++ -Wmissing-declarations -Wno-unused-parameter -Wno-nullability-extension -fno-strict-overflow -D_FORTIFY_SOURCE=2 -fstack-protector-all -Wstack-protector --param ssp-buffer-size=1 -fPIE -pipe  -g -O2 -MT transportfragment.o -MD -MP -MF .deps/transportfragment.Tpo -c -o transportfragment.o transportfragment.cc
In file included from transportfragment.cc:37:
In file included from ../../src/protobufs/transportinstruction.pb.h:20:
In file included from /opt/homebrew/Cellar/protobuf/33.4_1/include/google/protobuf/io/coded_stream.h:113:
In file included from /opt/homebrew/Cellar/abseil/20260107.1/include/absl/strings/cord.h:80:
In file included from /opt/homebrew/Cellar/abseil/20260107.1/include/absl/crc/internal/crc_cord_state.h:23:
In file included from /opt/homebrew/Cellar/abseil/20260107.1/include/absl/crc/crc32c.h:32:
In file included from /opt/homebrew/Cellar/abseil/20260107.1/include/absl/strings/str_format.h:83:
In file included from /opt/homebrew/Cellar/abseil/20260107.1/include/absl/strings/internal/str_format/arg.h:36:
In file included from /opt/homebrew/Cellar/abseil/20260107.1/include/absl/numeric/int128.h:41:
/opt/homebrew/Cellar/abseil/20260107.1/include/absl/types/compare.h:78:22: error: 'enable_if' is a clang extension [-Werror,-Wgcc-compat]
   78 |       __attribute__((enable_if(n == 0, "Only literal `0` is allowed."))) {}
      |                      ^

and

In file included from transportfragment.cc:37:
In file included from ../../src/protobufs/transportinstruction.pb.h:28:
In file included from /opt/homebrew/Cellar/protobuf/33.4_1/include/google/protobuf/extension_set.h:36:
In file included from /opt/homebrew/Cellar/abseil/20260107.1/include/absl/container/btree_map.h:67:
/opt/homebrew/Cellar/abseil/20260107.1/include/absl/container/internal/btree_container.h:524:30: error: passing no argument for the '...' parameter of a variadic macro is a C++20 extension [-Werror,-Wvariadic-macro-arguments-omitted]
  524 |                   false, true);
      |                              ^

This looks like these warnings are coming from the -pedantic flag. There's some indication that abseil can't really be compiled with -pedantic (abseil/abseil-cpp#157) since it uses a number of extensions conditionally and intentionally. Maybe it should be removed and more targeted warning flags enabled?

[zedinosaur]

Closing this in favor of #1374 instead.