Merge bitcoin/bitcoin#24164: build: Bump minimum required clang/libc++ to 8.0

fae20e6b50 Revert "Avoid the use of P0083R3 std::set::merge" (MarcoFalke)
fab53b5fd4 ci/doc: Set minimum required clang/libc++ version to 8.0 (MarcoFalke)

Pull request description:

  This is not for 23.0, but for 24.0. It comes with the following benefits:

  * Can use C++17 P0083R3 std::set::merge from libc++ 8.0
  * No longer need to provide support for clang-7, which already fails to compile on some architectures (https://github.com/bitcoin/bitcoin/issues/21294#issuecomment-998098483)

  This should be fine, given that all supported operating systems ship with at least clang-10:

  * CentOS 8: clang-12
  * Stretch: https://packages.debian.org/stretch/clang-11
  * Buster: https://packages.debian.org/buster-backports/clang-11
  * Bionic: https://packages.ubuntu.com/bionic-updates/clang-10
  * Focal: https://packages.ubuntu.com/focal/clang-10

ACKs for top commit:
  fanquake:
    ACK fae20e6b50 - I think this is fine to do. I would be surprised if in another 6 months time someone was stuck on a system we supported, needing to compile Core, and only had access to Clang 7 or older. As mentioned in the PR description, all systems we currently support, already support multiple newer versions of Clang.
  hebasto:
    ACK fae20e6b50, I have reviewed the code and it looks OK, I agree it can be merged.

Tree-SHA512: 3b4c6c130ff40dd7e84934af076863415e5dd661d823c72e3e3832566c65be6e877a7ef9164bbcf394bcea4b897fc29a48db0f231c22ace0e2c9b5638659a628

ci/test/00_setup_env_native_nowallet_libbitcoinkernel.sh

@@ -8,7 +8,7 @@ export LC_ALL=C.UTF-8
 
 export CONTAINER_NAME=ci_native_nowallet_libbitcoinkernel
 export DOCKER_NAME_TAG=ubuntu:18.04  # Use bionic to have one config run the tests in python3.6, see doc/dependencies.md
-export PACKAGES="python3-zmq clang-7 llvm-7 libc++abi-7-dev libc++-7-dev"  # Use clang-7 to test C++17 compatibility, see doc/dependencies.md
+export PACKAGES="python3-zmq clang-8 llvm-8 libc++abi-8-dev libc++-8-dev"  # Use clang-8 to test C++17 compatibility, see doc/dependencies.md
-export DEP_OPTS="NO_WALLET=1 CC=clang-7 CXX='clang++-7 -stdlib=libc++'"
+export DEP_OPTS="NO_WALLET=1 CC=clang-8 CXX='clang++-8 -stdlib=libc++'"
 export GOAL="install"
-export BITCOIN_CONFIG="--enable-reduce-exports CC=clang-7 CXX='clang++-7 -stdlib=libc++' --enable-experimental-util-chainstate"
+export BITCOIN_CONFIG="--enable-reduce-exports CC=clang-8 CXX='clang++-8 -stdlib=libc++' --enable-experimental-util-chainstate"

doc/dependencies.md

@@ -7,7 +7,7 @@ These are the dependencies currently used by Bitcoin Core. You can find instruct
 | --- | --- | --- | --- | --- | --- |
 | Berkeley DB | [4.8.30](https://www.oracle.com/technetwork/database/database-technologies/berkeleydb/downloads/index.html) | 4.8.x | No |  |  |
 | Boost | [1.77.0](https://www.boost.org/users/download/) | [1.64.0](https://github.com/bitcoin/bitcoin/pull/22320) | No |  |  |
-| Clang<sup>[ \* ](#note1)</sup> |  | [7.0](https://releases.llvm.org/download.html) (C++17 & std::filesystem support) |  |  |  |
+| Clang |  | [8.0](https://releases.llvm.org/download.html) (C++17 & std::filesystem support) |  |  |  |
 | Fontconfig | [2.12.6](https://www.freedesktop.org/software/fontconfig/release/) |  | No | Yes |  |
 | FreeType | [2.11.0](https://download.savannah.gnu.org/releases/freetype) |  | No |  | [Yes](https://github.com/bitcoin/bitcoin/blob/master/depends/packages/qt.mk) (Android only) |
 | GCC |  | [8.1](https://gcc.gnu.org/) (C++17 & std::filesystem support) |  |  |  |
@@ -28,8 +28,6 @@ These are the dependencies currently used by Bitcoin Core. You can find instruct
 | ZeroMQ | [4.3.1](https://github.com/zeromq/libzmq/releases) | 4.0.0 | No |  |  |
 | zlib |  |  |  |  | [Yes](https://github.com/bitcoin/bitcoin/blob/master/depends/packages/qt.mk) |
 
-<a name="note1">Note \*</a> : When compiling with `-stdlib=libc++`, the minimum supported libc++ version is 7.0.
-
 Controlling dependencies
 ------------------------
 Some dependencies are not needed in all configurations. The following are some factors that affect the dependency list.

src/script/standard.cpp

@@ -398,13 +398,7 @@ void TaprootSpendData::Merge(TaprootSpendData other)
         merkle_root = other.merkle_root;
     }
     for (auto& [key, control_blocks] : other.scripts) {
-        // Once P0083R3 is supported by all our targeted platforms,
+        scripts[key].merge(std::move(control_blocks));
-        // this loop body can be replaced with:
-        // scripts[key].merge(std::move(control_blocks));
-        auto& target = scripts[key];
-        for (auto& control_block: control_blocks) {
-            target.insert(std::move(control_block));
-        }
     }
 }