Adapt o3tl::span to P1872R0

..."span should have size_type, not index_type"
(<http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1872r0.pdf>), as
implemented by libc++ since <https://github.com/llvm/llvm-project/commit/
1466335cf4b2854a0be1defcf279fe50772bad6f> "[libc++][P1872] span should have
size_type, not index_type."

All uses of index_type had been added to mitigate the previous std::span change
from signed (ptrdiff_t) to unsigned (size_t) index_type, see
6ef8420fdbf8dff16de13147c5ab833bc5e01121 "Adapt o3tl::span to updated C++2a
std::span".  There is no easy solution to transparently support all three
std::span variants currently out there (signed index_type, unsigned index_type,
unsigned size_type), without causing compilation failures due to
CPPUNIT_ASSERT_EQUAL with arguments of different types, or compiler warnings
about mixed signed/unsigned comparisons.  So rule out the oldest std::span
variant (signed index_type) in configure.ac (so that o3tl::span will use its
own hand-rolled code in that case) and simplify the uses of index_type to
std::size_t (as had already been mentioned in
6ef8420fdbf8dff16de13147c5ab833bc5e01121).

Change-Id: I6ddf424ffb7941da3f69ad66fd29ecd35f09afae
Reviewed-on: https://gerrit.libreoffice.org/84652
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

5 files changed, 33 insertions, 10 deletions

diff --git a/config_host/config_global.h.in b/config_host/config_global.h.in
index adb36c39ab8b..5b04594c12f5 100644
--- a/config_host/config_global.h.in
+++ b/config_host/config_global.h.in
@@ -29,6 +29,9 @@ Any change in this header will cause a rebuild of almost everything.
 // constexpr", and <https://wg21.link/P1143R2> "Adding the constinit keyword":
 #define HAVE_CPP_CONSTINIT_SORTED_VECTOR 0
 
+// Useable C++2a <span>:
+#define HAVE_CPP_SPAN 0
+
 /* GCC bug <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87150> "move ctor wrongly chosen in return
    stmt (derived vs. base)": */
 #define HAVE_GCC_BUG_87150 0
diff --git a/configure.ac b/configure.ac
index 651e9d60c2d4..194a3cc80fa5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -6701,6 +6701,22 @@ AC_COMPILE_IFELSE([AC_LANG_SOURCE([
 CXXFLAGS=$save_CXXFLAGS
 AC_LANG_POP([C++])
 
+AC_MSG_CHECKING([whether $CXX_BASE supports C++2a <span> with unsigned size_type])
+AC_LANG_PUSH([C++])
+save_CXXFLAGS=$CXXFLAGS
+CXXFLAGS="$CXXFLAGS $CXXFLAGS_CXX11"
+AC_COMPILE_IFELSE([AC_LANG_SOURCE([
+        #include <span>
+        #include <type_traits>
+        // Don't check size_type directly, as it was called index_type before P1872R0:
+        void f(std::span<int> s) { static_assert(std::is_unsigned_v<decltype(s.size())>); };
+    ])], [
+        AC_DEFINE([HAVE_CPP_SPAN],[1])
+        AC_MSG_RESULT([yes])
+    ], [AC_MSG_RESULT([no])])
+CXXFLAGS=$save_CXXFLAGS
+AC_LANG_POP([C++])
+
 AC_MSG_CHECKING([whether $CXX_BASE has GCC bug 87150])
 AC_LANG_PUSH([C++])
 save_CXXFLAGS=$CXXFLAGS
diff --git a/include/o3tl/span.hxx b/include/o3tl/span.hxx
index 1618b86df897..b19d2d847ac7 100644
--- a/include/o3tl/span.hxx
+++ b/include/o3tl/span.hxx
@@ -12,7 +12,9 @@
 
 #include <sal/config.h>
 
-#if __has_include(<span>)
+#include <config_global.h>
+
+#if HAVE_CPP_SPAN
 
 #include <span>
 
@@ -40,7 +42,7 @@ public:
     using iterator = pointer;
     using const_reverse_iterator = std::reverse_iterator<const_iterator>;
     using reverse_iterator = std::reverse_iterator<iterator>;
-    using index_type = std::size_t;
+    using size_type = std::size_t;
     using difference_type = std::ptrdiff_t;
 
     constexpr span() noexcept : data_(nullptr), size_(0) {}
@@ -48,7 +50,7 @@ public:
     template<std::size_t N>
     constexpr span (T (&a)[N]) noexcept : data_(a), size_(N) {}
 
-    constexpr span (T *a, index_type len) noexcept
+    constexpr span (T *a, size_type len) noexcept
         : data_(a), size_(len)
     {
         // not terribly sure about this, might need to strengthen it
@@ -72,9 +74,9 @@ public:
     { return rbegin(); }
     constexpr const_reverse_iterator crend() const noexcept { return rend(); }
 
-    constexpr index_type size() const noexcept { return size_; }
+    constexpr size_type size() const noexcept { return size_; }
 
-    constexpr reference operator [](index_type pos) const {
+    constexpr reference operator [](size_type pos) const {
         assert(pos < size());
         return data_[pos];
     }
@@ -83,7 +85,7 @@ public:
 
 private:
     pointer    data_;
-    index_type size_;
+    size_type size_;
 };
 
 } // namespace o3tl
diff --git a/o3tl/qa/test-span.cxx b/o3tl/qa/test-span.cxx
index 7ec67fa7fd91..3cb78ace1db2 100644
--- a/o3tl/qa/test-span.cxx
+++ b/o3tl/qa/test-span.cxx
@@ -9,6 +9,7 @@
 
 #include <sal/config.h>
 
+#include <cstddef>
 #include <utility>
 
 #include <cppunit/TestAssert.h>
@@ -42,7 +43,7 @@ private:
         CPPUNIT_ASSERT_EQUAL(3, *v.crbegin());
         CPPUNIT_ASSERT_EQUAL(
             o3tl::span<int>::difference_type(3), v.crend() - v.crbegin());
-        CPPUNIT_ASSERT_EQUAL(o3tl::span<int>::index_type(3), v.size());
+        CPPUNIT_ASSERT_EQUAL(std::size_t(3), v.size());
         CPPUNIT_ASSERT(!v.empty());
         CPPUNIT_ASSERT_EQUAL(2, v[1]);
         CPPUNIT_ASSERT_EQUAL(1, *v.data());
@@ -52,8 +53,8 @@ private:
             o3tl::span<int const> v1( d1 );
             o3tl::span<int const> v2( d2 );
             std::swap(v1, v2);
-            CPPUNIT_ASSERT_EQUAL(o3tl::span<int>::index_type(4), v1.size());
-            CPPUNIT_ASSERT_EQUAL(o3tl::span<int>::index_type(2), v2.size());
+            CPPUNIT_ASSERT_EQUAL(std::size_t(4), v1.size());
+            CPPUNIT_ASSERT_EQUAL(std::size_t(2), v2.size());
         }
     }
 };
diff --git a/sfx2/source/control/dispatch.cxx b/sfx2/source/control/dispatch.cxx
index 9edf43c4ecd6..9b6519ca1814 100644
--- a/sfx2/source/control/dispatch.cxx
+++ b/sfx2/source/control/dispatch.cxx
@@ -20,6 +20,7 @@
 #include <config_feature_desktop.h>
 
 #include <algorithm>
+#include <cstddef>
 #include <deque>
 #include <vector>
 
@@ -1453,7 +1454,7 @@ void SfxDispatcher::SetSlotFilter(SfxSlotFilterState nEnable,
 {
 #ifdef DBG_UTIL
     // Check Array
-    for ( o3tl::span<sal_uInt16 const>::index_type n = 1; n < pSIDs.size(); ++n )
+    for ( std::size_t n = 1; n < pSIDs.size(); ++n )
         DBG_ASSERT( pSIDs[n] > pSIDs[n-1], "SetSlotFilter: SIDs not sorted" );
 #endif