[3.16] backport: #11009 #10962

doc/changes/10962.md

@@ -0,0 +1,2 @@
+- Call the C++ compiler with `-std=c++11` when using OCaml >= 5.0
+  (#10962, @kit-ty-kate)

src/dune_rules/cxx_flags.ml

@@ -1,24 +1,29 @@
 open Import
 
-type phase =
-  | Compile
-  | Link
-
 type ccomp_type =
   | Gcc
   | Msvc
   | Clang
   | Other of string
 
-let base_cxx_flags ~for_ cc =
-  match cc, for_ with
-  | Gcc, Compile -> [ "-x"; "c++" ]
-  | Gcc, Link -> [ "-lstdc++"; "-shared-libgcc" ]
-  | Clang, Compile -> [ "-x"; "c++" ]
-  | Clang, Link -> [ "-lc++" ]
-  | Msvc, Compile -> [ "/TP" ]
-  | Msvc, Link -> []
-  | Other _, (Link | Compile) -> []
+type phase =
+  | Compile of Ocaml.Version.t
+  | Link
+
+let base_cxx_compile_flags version = function
+  | Gcc | Clang ->
+    "-x"
+    :: "c++"
+    :: (if Ocaml.Version.add_std_cxx_flag version then [ "-std=gnu++11" ] else [])
+  | Msvc -> [ "/TP" ]
+  | Other _ -> []
+;;
+
+let base_cxx_link_flags = function
+  | Gcc -> [ "-lstdc++"; "-shared-libgcc" ]
+  | Clang -> [ "-lc++" ]
+  | Msvc -> []
+  | Other _ -> []
 ;;
 
 let fdiagnostics_color = function
@@ -63,5 +68,8 @@ let ccomp_type (ctx : Build_context.t) =
 let get_flags ~for_ ctx =
   let open Action_builder.O in
   let+ ccomp_type = ccomp_type ctx in
-  base_cxx_flags ~for_ ccomp_type
+  (match for_ with
+   | Compile version -> base_cxx_compile_flags version
+   | Link -> base_cxx_link_flags)
+    ccomp_type
 ;;

src/dune_rules/cxx_flags.mli

@@ -4,7 +4,7 @@
 open Import
 
 type phase =
-  | Compile
+  | Compile of Ocaml.Version.t
   | Link
 
 (** The detected compiler *)

src/dune_rules/foreign_rules.ml

@@ -28,7 +28,11 @@ let default_context_flags (ctx : Build_context.t) ocaml_config ~project =
       in
       let cxx =
         let+ fdiagnostics_color = fdiagnostics_color
-        and+ db_flags = Cxx_flags.get_flags ~for_:Compile ctx in
+        and+ db_flags =
+          Cxx_flags.get_flags
+            ~for_:(Compile (Ocaml.Version.make (Ocaml_config.version ocaml_config)))
+            ctx
+        in
         List.concat [ db_flags; cxxflags; fdiagnostics_color ]
       in
       c, cxx

src/ocaml/version.ml

@@ -29,3 +29,4 @@ let supports_alerts version = version >= (4, 8, 0)
 let has_sandboxed_otherlibs version = version >= (5, 0, 0)
 let has_META_files version = version >= (5, 0, 0)
 let supports_bin_annot_occurrences version = version >= (5, 2, 0)
+let add_std_cxx_flag version = version >= (5, 0, 0)

src/ocaml/version.mli

@@ -76,3 +76,5 @@ val has_META_files : t -> bool
 
 (** Whether the compiler supports occurrences indexation *)
 val supports_bin_annot_occurrences : t -> bool
+
+val add_std_cxx_flag : t -> bool

test/blackbox-tests/test-cases/cpp-std-ocaml5.t/cpp11.cpp

@@ -0,0 +1,14 @@
+#include <iostream>
+#include <cstring>
+#include <caml/misc.h>
+#include <caml/mlvalues.h>
+
+// strdup is not part of the C standard library but is part of POSIX.
+// By default GCC and Clang both use the GNU variant of the C standard,
+// so we are using strdup here to ensure users have access to it by default.
+// If -std=c++11 is used instead of -std=gnu++11 this would fail to compile
+// on non-POSIX platforms such as Cygwin.
+extern "C" CAMLprim value cpp11(value _unit) {
+  std::cout << strdup("Hi from C++11") << std::endl;
+  return Val_unit;
+}

test/blackbox-tests/test-cases/cpp-std-ocaml5.t/cpp11.ml

@@ -0,0 +1,3 @@
+external cpp11 : unit -> unit = "cpp11"
+
+let () = cpp11 ()

test/blackbox-tests/test-cases/cpp-std-ocaml5.t/run.t

@@ -0,0 +1,19 @@
+-std=c++11 is given on OCaml 5.0
+================================
+
+  $ cat >dune-project <<EOF
+  > (lang dune 3.14)
+  > EOF
+
+  $ cat >dune <<EOF
+  > ;; will fail if :standard doesn't have -std=c++11 on OCaml >= 5.0
+  > (executable
+  >  (name cpp11)
+  >  (foreign_stubs
+  >   (language cxx)
+  >   (names cpp11)
+  >   (flags -std=c++98 :standard)))
+  > EOF
+
+  $ (dune build ./cpp11.exe 2>/dev/null) && _build/default/cpp11.exe
+  Hi from C++11