span: Make Span template deduction guides work in SFINAE context

Also add test to make sure this doesn't get broken in the future.

This was breaking vector<bool> serialization in multiprocess code because
template current deduction guides would make it appear like vector<bool> could
be converted to a span, but then the actual conversion to span would fail.

src/test/span_tests.cpp

@@ -0,0 +1,73 @@
+// Copyright (c) 2023 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <span.h>
+
+#include <boost/test/unit_test.hpp>
+#include <array>
+#include <set>
+#include <vector>
+
+namespace {
+struct Ignore
+{
+    template<typename T> Ignore(T&&) {}
+};
+template<typename T>
+bool Spannable(T&& value, decltype(Span{value})* enable = nullptr)
+{
+    return true;
+}
+bool Spannable(Ignore)
+{
+    return false;
+}
+
+#if defined(__clang__)
+#    pragma clang diagnostic push
+#    pragma clang diagnostic ignored "-Wunneeded-member-function"
+#    pragma clang diagnostic ignored "-Wunused-member-function"
+#endif
+struct SpannableYes
+{
+    int* data();
+    size_t size();
+};
+struct SpannableNo
+{
+    void* data();
+    size_t size();
+};
+#if defined(__clang__)
+#    pragma clang diagnostic pop
+#endif
+} // namespace
+
+BOOST_AUTO_TEST_SUITE(span_tests)
+
+// Make sure template Span template deduction guides accurately enable calls to
+// Span constructor overloads that work, and disable calls to constructor overloads that
+// don't work. This makes it is possible to use the Span constructor in a SFINAE
+// contexts like in the Spannable function above to detect whether types are or
+// aren't compatible with Spans at compile time.
+//
+// Previously there was a bug where writing a SFINAE check for vector<bool> was
+// not possible, because in libstdc++ vector<bool> has a data() memeber
+// returning void*, and the Span template guide ignored the data() return value,
+// so the template substitution would succeed, but the constructor would fail,
+// resulting in a fatal compile error, rather than a SFINAE error that could be
+// handled.
+BOOST_AUTO_TEST_CASE(span_constructor_sfinae)
+{
+    BOOST_CHECK(Spannable(std::vector<int>{}));
+    BOOST_CHECK(!Spannable(std::set<int>{}));
+    BOOST_CHECK(!Spannable(std::vector<bool>{}));
+    BOOST_CHECK(Spannable(std::array<int, 3>{}));
+    BOOST_CHECK(Spannable(Span<int>{}));
+    BOOST_CHECK(Spannable("char array"));
+    BOOST_CHECK(Spannable(SpannableYes{}));
+    BOOST_CHECK(!Spannable(SpannableNo{}));
+}
+
+BOOST_AUTO_TEST_SUITE_END()