include the otel header-only part directly for ease of use

ext/opentelemetry-cpp-api-only/include/opentelemetry/std/span.h

@@ -0,0 +1,71 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+#pragma once
+
+// IWYU pragma: private, include "opentelemetry/nostd/span.h"
+
+#include "opentelemetry/version.h"
+
+// Standard library implementation requires at least C++17 compiler.
+// Older C++14 compilers may provide support for __has_include as a
+// conforming extension.
+#if defined __has_include
+#  if __has_include(<version>)  // Check for __cpp_{feature}
+#    include <version>
+#    if defined(__cpp_lib_span) && __cplusplus > 201703L
+#      define OPENTELEMETRY_HAVE_SPAN
+#    endif
+#  endif
+#  if !defined(OPENTELEMETRY_HAVE_SPAN)
+#    // Check for Visual Studio span
+#    if defined(_MSVC_LANG) && _HAS_CXX20
+#      define OPENTELEMETRY_HAVE_SPAN
+#    endif
+#    // Check for other compiler span implementation
+#    if !defined(_MSVC_LANG) && __has_include(<span>) && __cplusplus > 201703L
+// This works as long as compiler standard is set to C++20
+#      define OPENTELEMETRY_HAVE_SPAN
+#    endif
+#  endif
+#  if !__has_include(<gsl/gsl>)
+#    undef HAVE_GSL
+#  endif
+#endif
+
+#if !defined(OPENTELEMETRY_HAVE_SPAN)
+#  if defined(HAVE_GSL)
+#    include <type_traits>
+// Guidelines Support Library provides an implementation of std::span
+#    include <gsl/gsl>
+OPENTELEMETRY_BEGIN_NAMESPACE
+namespace nostd
+{
+using gsl::dynamic_extent;
+template <class ElementType, std::size_t Extent = gsl::dynamic_extent>
+using span = gsl::span<ElementType, Extent>;
+}  // namespace nostd
+OPENTELEMETRY_END_NAMESPACE
+#    define OPENTELEMETRY_HAVE_SPAN
+#  else
+// No `gsl::span`, no `std::span`, fallback to `nostd::span`
+#  endif
+
+#else  // OPENTELEMETRY_HAVE_SPAN
+// Using std::span (https://wg21.link/P0122R7) from Standard Library available in C++20 :
+// - GCC libstdc++ 10+
+// - Clang libc++ 7
+// - MSVC Standard Library 19.26*
+// - Apple Clang 10.0.0*
+#  include <limits>
+#  include <span>
+OPENTELEMETRY_BEGIN_NAMESPACE
+namespace nostd
+{
+constexpr std::size_t dynamic_extent = (std::numeric_limits<std::size_t>::max)();
+
+template <class ElementType, std::size_t Extent = nostd::dynamic_extent>
+using span = std::span<ElementType, Extent>;
+}  // namespace nostd
+OPENTELEMETRY_END_NAMESPACE
+#endif  // if OPENTELEMETRY_HAVE_SPAN