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@xcodereleases note for clang users: looks like Apple Clang 17 (shipped with Xcode 16.3) is based on LLVM 19, with some flags set by default.

tested (in zsh) with:

args=( -std=c++23 -x c++ -E -dM );
cmd(){ compiler=$1; echo '' | $compiler $args -; };
llvm_clang=$(brew --prefix llvm@19)/bin/clang++;
apple_clang=/usr/bin/clang++
$apple_clang -v 2>&1 | grep version;
$llvm_clang -v 2>&1 | grep version;
difft --override='*:C++' <( cmd $llvm_clang ) <( cmd $apple_clang )

@xcodereleases if you run that, you'll see the following line is "deleted" (i.e. not present in output from Apple Clang):

#define __cpp_sized_deallocation 201309L

...apparently because Apple Clang sets the flag -fno-sized-deallocation by default.

@xcodereleases it may also be interesting to look at some of the other flags that apple sets that are not set by the stock LLVM version:

args=( -std=c++23 -x c++ -v -c );
cmd(){ compiler=$1; echo '' | $compiler $args - 2>&1 | sed 's/ -/\n -/g' ; };
llvm_clang=$(brew --prefix llvm@19)/bin/clang++;
apple_clang=/usr/bin/clang++
$apple_clang -v 2>&1 | grep version;
$llvm_clang -v 2>&1 | grep version;
difft <( cmd $llvm_clang ) <( cmd $apple_clang )