build: use C++17 in depends #20471

In packages where we are passing -std=c++11 switch to -std=c++17, or, -std=c++1z in the case of Qt.

This PR also contains a commit that improves debug output when building Qt for debugging (DEBUG=1).

Now we'll get output like this:

g++ -c -pipe -ffunction-sections -O2 -fPIC -std=c++11 -fno-exceptions <lots more> ../../corelib/kernel/qcoreapplication.cpp

rather than just:

compiling ../../corelib/kernel/qcoreapplication.cpp

Note that when you look at the DEBUG output for these changes when building Qt, you'll see objects being compiled with a mix of C++11 and C++17. The breakdown is roughly:

1. qmake built with -std=c++11:

Creating qmake...
make[1]: Entering directory '<trim>/qt/5.9.8-4110fa99945/qtbase/qmake'
g++ -c -o project.o   -std=c++11 -ffunction-sections -O2 -g <trim> <trim>/qt/5.9.8-4110fa99945/qtbase/qmake/project.cpp

# when qmake, Qt also builds some of it's corelib, such as corelib/global/qmalloc.cpp
g++ -c -o qmalloc.o   -std=c++11 -ffunction-sections -O2 -g <trim> <trim>/qt/5.9.8-4110fa99945/qtbase/src/corelib/global/qmalloc.cpp

2. qmake is run, and passed our build options, including -c++std:

make[1]: Entering directory '<trim>/qt/5.9.8-4110fa99945/qtbase'
<trim>qt/5.9.8-4110fa99945/qtbase/bin/qmake -o Makefile qtbase.pro -- -bindir <trim>/native/bin -c++std c++1z -confirm-license <trim>

3. After some cleaning and configuring, we actually start to build Qt, as well as it's tools and internal libs:

Building qt...
make[1]: Entering directory '<trim>/qt/5.9.8-4110fa99945/qtbase/src'

# build libpng, zlib etc
gcc -c -m64 -pipe -pipe -O1 <trim> -o .obj/png.o png.c

# build libQt5Bootstrap, using C++11, which again compiles qmalloc.cpp
make[2]: Entering directory '<trim>/qt/5.9.8-4110fa99945/qtbase/src/tools/bootstrap'
g++ -c -pipe -ffunction-sections -O2 -fPIC -std=c++11 <trim> -o .obj/qmalloc.o ../../corelib/global/qmalloc.cpp

# build a bunch of tools like moc, rcc, uic, qfloat16-tables, qdbuscpp2xml, using C++11
g++ -c -pipe -O2 -std=c++11 -fno-exceptions -Wall -W <trim> -o .obj/rcc.o rcc.cpp

# from here, Qt is compiled with -std=c++1z, including qmalloc.cpp, for the third and final time:
g++ -c -include .pch/Qt5Core <trim> -g -Og -fPIC -std=c++1z -fvisibility=hidden <trim> -o .obj/qmalloc.o global/qmalloc.cpp

4. Finally, build tools like lrelease, lupdate, etc, but back to using -std=c++11

make[1]: Entering directory '<trim>/qt/5.9.8-4110fa99945/qttools/src/linguist/lrelease'
g++ -c -pipe -O2 -std=c++11 -fno-exceptions -Wall -W <trim> -o .obj/translator.o ../shared/translator.cpp

If you dump the debug info from the built Qt libs, they should also tell you that they were compiled with C++17:

objdump -g bitcoin/depends/x86_64-pc-linux-gnu/lib/libQt5Core.a
GNU C++17 9.3.0 -m64 -mtune=generic -march=x86-64 -g -O1 -Og -std=c++17 -fPIC -fvisibility=hidden -fvisibility-inlines-hidden -fasynchronous-unwind-tables -fstack-protector-strong -fstack-clash-protection -fcf-protection

Is this needed for anything or does this change anything about the binaries? I presume it is needed for qt5.15?

I think it's safer with regard to ABI to compile them with the same standard as the application, definitely for boost. We've seen some problems in that regard in the past.

And especially building with an older C++XX than the application itself could cause some things to be missing that are conditional on the C++XX version.

Note that when you look at the DEBUG output for these changes when building Qt, you'll see objects being compiled with a mix of C++11 and C++17. The breakdown is roughly:

Glad you got it to work eventually to build the library with C++17. It doesn't matter that the intermediate tools are built with C++11. It's a bit strange though that it seems to resist being built with C++17 at every step.

Would this be solved by upgrading Qt?

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #19716 (build: Qt 5.15.x by fanquake)
• #19245 ([WIP DONOTMERGE] Replace boost::filesystem with std::filesystem (in c++17) by kiminuo)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

Concept ACK for the reasons given by @laanwj

Would this be solved by upgrading Qt?

It doesn't look like it. #19716 (Qt 5.15.2) still behaves in the same way when built in c++1z mode.

It doesn't look like it. #19716 (Qt 5.15.2) still behaves in the same way when built in c++1z mode.

OK, good to know. That should be in for 0.22 anyway. I suppose for that version we can also just use c++17 in not need the 1z temporary name.

Code review ACK https://github.com/bitcoin/bitcoin/pull/20471/commits/2f5dfe4a7ff12b6b57427374142cdf7e266b73bc

I suppose for that version we can also just use c++17 in not need the 1z temporary name.

Yep, we will be able to. 5.15.x understands -c++std c++17.

Concept ACK.

ACK 2f5dfe4a7ff12b6b57427374142cdf7e266b73bc, I have reviewed the code and it looks OK, I agree it can be merged.

cr ACK 2f5dfe4a7ff12b6b57427374142cdf7e266b73bc: patch looks correct

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Guix builds

Code review ACK 2f5dfe4a7ff12b6b57427374142cdf7e266b73bc

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Gitian builds