importing gsl::span if std::span is not available (#1167)

CMakeLists.txt

@@ -102,7 +102,9 @@ else()
   )
 endif()
 
-option(WITH_STL "Whether to use Standard Library for C++latest features" OFF)
+option(WITH_STL "Whether to use Standard Library for C++ latest features" OFF)
+option(WITH_GSL
+       "Whether to use Guidelines Support Library for C++ latest features" OFF)
 
 option(WITH_ABSEIL "Whether to use Abseil for C++latest features" OFF)
 
@@ -137,18 +139,6 @@ if(WITH_STL)
   # (absl::variant or std::variant) in variant unit test code is consistent with
   # the global project build definitions.
   add_definitions(-DHAVE_CPP_STDLIB)
-  add_definitions(-DHAVE_GSL)
-
-  # Guidelines Support Library path. Used if we are not on not get C++20.
-  #
-  # TODO: respect WITH_ABSEIL as alternate implementation of std::span
-  find_package(Microsoft.GSL QUIET)
-  if(TARGET Microsoft.GSL::GSL)
-    list(APPEND CORE_RUNTIME_LIBS Microsoft.GSL::GSL)
-  else()
-    set(GSL_DIR third_party/ms-gsl)
-    include_directories(${GSL_DIR}/include)
-  endif()
 
   # Optimize for speed to reduce the hops
   if(CMAKE_CXX_COMPILER_ID STREQUAL "MSVC")
@@ -165,6 +155,20 @@ if(WITH_STL)
   endif()
 endif()
 
+if(WITH_GSL)
+  add_definitions(-DHAVE_GSL)
+
+  # Guidelines Support Library path. Used if we are not on not get C++20.
+  #
+  find_package(Microsoft.GSL QUIET)
+  if(TARGET Microsoft.GSL::GSL)
+    list(APPEND CORE_RUNTIME_LIBS Microsoft.GSL::GSL)
+  else()
+    set(GSL_DIR third_party/ms-gsl)
+    include_directories(${GSL_DIR}/include)
+  endif()
+endif()
+
 option(WITH_OTLP "Whether to include the OpenTelemetry Protocol in the SDK" OFF)
 option(WITH_ZIPKIN "Whether to include the Zipkin exporter in the SDK" OFF)
 

api/CMakeLists.txt

@@ -47,8 +47,7 @@ endif()
 
 if(WITH_STL)
   message("Building with standard library types...")
-  target_compile_definitions(opentelemetry_api INTERFACE HAVE_CPP_STDLIB
-                                                         HAVE_GSL)
+  target_compile_definitions(opentelemetry_api INTERFACE HAVE_CPP_STDLIB)
 else()
   message("Building with nostd types...")
 endif()

api/include/opentelemetry/std/span.h

@@ -11,7 +11,7 @@
 #if defined __has_include
 #  if __has_include(<version>)  // Check for __cpp_{feature}
 #    include <version>
-#    if defined(__cpp_lib_span)
+#    if defined(__cpp_lib_span) && __cplusplus > 201703L
 #      define HAVE_SPAN
 #    endif
 #  endif
@@ -21,7 +21,7 @@
 #      define HAVE_SPAN
 #    endif
 #    // Check for other compiler span implementation
-#    if !defined(_MSVC_LANG) && __has_include(<span>)
+#    if !defined(_MSVC_LANG) && __has_include(<span>) && __cplusplus > 201703L
 // This works as long as compiler standard is set to C++20
 #      define HAVE_SPAN
 #    endif

sdk/include/opentelemetry/sdk/instrumentationlibrary/instrumentation_library.h

@@ -3,6 +3,7 @@
 
 #pragma once
 
+#include <string>
 #include "opentelemetry/nostd/string_view.h"
 #include "opentelemetry/nostd/unique_ptr.h"
 #include "opentelemetry/version.h"