[cpp] move to C++17

Summary: Debian buster ships with gcc 7/8 which is all green according to: https://gcc.gnu.org/projects/cxx-status.html Depends on D8492 and D8495 edit: backported some changes from Core [[bitcoin/bitcoin#18591 | PR18591]] regarding implicit conversion between Span types Test Plan: ninja all check test_runner.py --extended Reviewers: #bitcoin_abc, Fabien, deadalnix Reviewed By: #bitcoin_abc, Fabien, deadalnix Subscribers: deadalnix, Fabien Differential Revision: https://reviews.bitcoinabc.org/D8493
Bitcoin-ABC · Nov 23, 2020 · 1541dc4 · 1541dc4
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diff --git a/arcanist/linter/CppCheckLinter.php b/arcanist/linter/CppCheckLinter.php
@@ -116,7 +116,7 @@ final class CppCheckLinter extends ArcanistExternalLinter {
     '-j2',
     '--enable=all',
     '--language=c++',
-    '--std=c++14',
+    '--std=c++17',
   );
 
   const CPPCHECK_DEFINITIONS = array(

diff --git a/doc/build-osx.md b/doc/build-osx.md
@@ -32,7 +32,7 @@ If you want to build the disk image with `ninja osx-dmg` (.dmg / optional), you
 Build Bitcoin ABC
 -----------------
 
-Before you start building, please make sure that your compiler supports C++14.
+Before you start building, please make sure that your compiler supports C++17.
 
 1. Clone the Bitcoin ABC source code and cd into `bitcoin-abc`
 

diff --git a/doc/build-unix.md b/doc/build-unix.md
@@ -5,7 +5,7 @@ Some notes on how to build Bitcoin ABC in Unix.
 To Build
 ---------------------
 
-Before you start building, please make sure that your compiler supports C++14.
+Before you start building, please make sure that your compiler supports C++17.
 
 It is recommended to create a build directory to build out-of-tree.
 

diff --git a/doc/build-windows.md b/doc/build-windows.md
@@ -15,7 +15,7 @@ Other options which may work, but which have not been extensively tested are (pl
 * On Windows, using a POSIX compatibility layer application such as [cygwin](http://www.cygwin.com/) or [msys2](http://www.msys2.org/).
 * On Windows, using a native compiler tool chain such as [Visual Studio](https://www.visualstudio.com).
 
-In any case please make sure that the compiler supports C++14.
+In any case please make sure that the compiler supports C++17.
 
 Installing Windows Subsystem for Linux
 ---------------------------------------

diff --git a/doc/dependencies.md b/doc/dependencies.md
@@ -7,12 +7,12 @@ These are the dependencies currently used by Bitcoin ABC. You can find instructi
 | --- | --- | --- | --- | --- | --- |
 | Berkeley DB | [5.3.28](http://www.oracle.com/technetwork/database/database-technologies/berkeleydb/downloads/index.html) | 5.3 | No |  |  |
 | Boost | [1.70.0](https://www.boost.org/users/download/) | 1.59.0 | No |  |  |
-| Clang |  | [3.4](https://releases.llvm.org/download.html) (C++14 support) |  |  |  |
+| Clang |  | [5](https://releases.llvm.org/download.html) (C++17 support) |  |  |  |
 | CMake |  | [3.16](https://cmake.org/download/) |  |  |  |
 | Expat | [2.2.7](https://libexpat.github.io/) |  | No | Yes |  |
 | fontconfig | [2.12.6](https://www.freedesktop.org/software/fontconfig/release/) |  | No | Yes |  |
 | FreeType | [2.7.1](http://download.savannah.gnu.org/releases/freetype) |  | No |  |  |
-| GCC |  | [5.0](https://gcc.gnu.org/) (C++14 support) |  |  |  |
+| GCC |  | [7](https://gcc.gnu.org/) (C++17 support) |  |  |  |
 | HarfBuzz-NG |  |  |  |  |  |
 | jemalloc | [5.2.1](https://github.com/jemalloc/jemalloc/releases) | 3.6.0 |  |  |  |
 | libevent | [2.1.11-stable](https://github.com/libevent/libevent/releases) | 2.0.22 | No |  |  |

diff --git a/doc/release-notes.md b/doc/release-notes.md
@@ -5,3 +5,5 @@ Bitcoin ABC version 0.22.8 is now available from:
   <https://download.bitcoinabc.org/0.22.8/>
 
 This release includes the following features and fixes:
+
+- Code updated to conform to the C++17 standard.
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
@@ -2,7 +2,8 @@
 
 project(bitcoind)
 
-set(CMAKE_CXX_STANDARD 14)
+set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
 # Default visibility is hidden on all targets.
 set(CMAKE_C_VISIBILITY_PRESET hidden)

diff --git a/src/compat/assumptions.h b/src/compat/assumptions.h
@@ -20,7 +20,7 @@
 #error "Bitcoin cannot be compiled without assertions."
 #endif
 
-// Assumption: We assume a C++14 (ISO/IEC 14882:2014) compiler (minimum
+// Assumption: We assume a C++17 (ISO/IEC 14882:2017) compiler (minimum
 //             requirement).
 // Example(s): We use std::make_unique() through the codebase.
 // Note:       MSVC does not report the expected __cplusplus value due to
@@ -29,7 +29,7 @@
 // N3936* §16.8 [cpp.predefined]/p1::
 // "The name __cplusplus is defined to the value 201402L when compiling a C++
 //  translation unit."
-static_assert(__cplusplus >= 201402L, "C++14 standard assumed");
+static_assert(__cplusplus >= 201703L, "C++17 standard assumed");
 #endif
 
 // Assumption: We assume the floating-point types to fulfill the requirements of

diff --git a/src/span.h b/src/span.h
@@ -25,6 +25,28 @@ template <typename C> class Span {
     constexpr Span(C *data, C *end) noexcept
         : m_data(data), m_size(end - data) {}
 
+    /** Implicit conversion of spans between compatible types.
+     *
+     *  Specifically, if a pointer to an array of type O can be implicitly
+     * converted to a pointer to an array of type C, then permit implicit
+     * conversion of Span<O> to Span<C>. This matches the behavior of the
+     * corresponding C++20 std::span constructor.
+     *
+     *  For example this means that a Span<T> can be converted into a Span<const
+     * T>.
+     */
+    template <typename O,
+              typename std::enable_if<
+                  std::is_convertible<O (*)[], C (*)[]>::value, int>::type = 0>
+    constexpr Span(const Span<O> &other) noexcept
+        : m_data(other.m_data), m_size(other.m_size) {}
+
+    /** Default copy constructor. */
+    constexpr Span(const Span &) noexcept = default;
+
+    /** Default assignment operator. */
+    Span &operator=(const Span &other) noexcept = default;
+
     constexpr C *data() const noexcept { return m_data; }
     constexpr C *begin() const noexcept { return m_data; }
     constexpr C *end() const noexcept { return m_data + m_size; }
@@ -69,6 +91,8 @@ template <typename C> class Span {
     friend constexpr bool operator>=(const Span &a, const Span &b) noexcept {
         return !(a < b);
     }
+
+    template <typename O> friend class Span;
 };
 
 /** Create a span to a container exposing data() and size().