CXX-3088 fix build with GCC 4.8.5 (#1194)

* declare `npos` as `constexpr` to fix old gcc

Resolves error: "redeclaration [...] differs in 'constexpr'"

* use C++11 syntax for user defined literals to fix old gcc

Resolves error: "missing space between '""' and suffix identifier"

* add test with gcc 4.8.5

* do not add `-Wno-expansion-to-defined` for RHEL-7

gcc 4.8.5 does not recognize.

* add `-Wno-unused-parameter` on RHEL 7 compile

Resolves warnings (causing errors) addressed in CDRIVER-5673. Upgrading C driver to get fix results in test failures due to removal of "rangePreview" algorithm.

Author: kevinAlbs

.evergreen/compile.sh

@@ -148,7 +148,13 @@ darwin*)
   ;;
 linux*)
   cc_flags+=("${cc_flags_init[@]}")
-  cxx_flags+=("${cxx_flags_init[@]}" -Wno-expansion-to-defined -Wno-missing-field-initializers)
+  cxx_flags+=("${cxx_flags_init[@]}" -Wno-missing-field-initializers)
+
+  if [[ "${distro_id:?}" != rhel7* ]]; then
+    cxx_flags+=("-Wno-expansion-to-defined")
+  else
+    cxx_flags+=("-Wno-unused-parameter") # TODO: remove once C driver is upgraded to include fix of CDRIVER-5673.
+  fi
   ;;
 *)
   echo "unrecognized operating system ${OSTYPE:?}" 1>&2

.mci.yml

@@ -2209,3 +2209,13 @@ buildvariants:
       display_name: silk
       tasks:
         - name: .silk
+
+    - name: rhel79-compile
+      display_name: "RHEL 7.9 (gcc 4.8.5)"
+      expansions:
+          build_type: "Release"
+          BSONCXX_POLYFILL: impls
+      run_on:
+          - rhel79-small
+      tasks:
+          - name: compile_without_tests

src/bsoncxx/include/bsoncxx/v_noabi/bsoncxx/json.hpp

@@ -82,7 +82,7 @@ BSONCXX_API document::value BSONCXX_CALL from_json(stdx::string_view json);
 ///
 /// @throws bsoncxx::v_noabi::exception with error details if the conversion failed.
 ///
-BSONCXX_API document::value BSONCXX_CALL operator""_bson(const char* json, size_t len);
+BSONCXX_API document::value BSONCXX_CALL operator"" _bson(const char* json, size_t len);
 
 }  // namespace v_noabi
 }  // namespace bsoncxx
@@ -92,7 +92,7 @@ namespace bsoncxx {
 using ::bsoncxx::v_noabi::from_json;
 using ::bsoncxx::v_noabi::to_json;
 
-using ::bsoncxx::v_noabi::operator""_bson;
+using ::bsoncxx::v_noabi::operator"" _bson;
 
 }  // namespace bsoncxx
 

src/bsoncxx/include/bsoncxx/v_noabi/bsoncxx/stdx/string_view.hpp

@@ -438,7 +438,7 @@ class basic_string_view : bsoncxx::detail::equality_operators, bsoncxx::detail::
 // Required to define this here for compatibility with C++14 and older. Can be removed in C++17 or
 // newer.
 template <typename C, typename Tr>
-const std::size_t basic_string_view<C, Tr>::npos;
+constexpr std::size_t basic_string_view<C, Tr>::npos;
 
 using string_view = basic_string_view<char>;
 

src/bsoncxx/lib/bsoncxx/v_noabi/bsoncxx/json.cpp

@@ -99,7 +99,7 @@ document::value BSONCXX_CALL from_json(stdx::string_view json) {
     return document::value{buf, length, bson_free_deleter};
 }
 
-document::value BSONCXX_CALL operator""_bson(const char* str, size_t len) {
+document::value BSONCXX_CALL operator"" _bson(const char* str, size_t len) {
     return from_json(stdx::string_view{str, len});
 }