Call the C++ compiler with -std=c++11 when using OCaml >= 5.0

Signed-off-by: Kate <kit-ty-kate@outlook.com>

src/dune_rules/cxx_flags.ml

@@ -1,24 +1,25 @@
 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) -> []
+let base_cxx_compile_flags ocaml_config = function
+  | Gcc | Clang ->
+    "-x"
+    :: "c++"
+    :: (if Ocaml_config.version ocaml_config >= (5, 0, 0) then [ "-std=c++11" ] else [])
+  | Msvc -> [ "/TP" ]
+  | Other _ -> []
+;;
+
+let base_cxx_link_flags = function
+  | Gcc -> [ "-lstdc++"; "-shared-libgcc" ]
+  | Clang -> [ "-lc++" ]
+  | Msvc -> []
+  | Other _ -> []
 ;;
 
 let fdiagnostics_color = function
@@ -59,8 +60,14 @@ let ccomp_type (ctx : Build_context.t) =
   ccomp_type
 ;;
 
-let get_flags ~for_ ctx =
+let get_compile_flags ocaml_version ctx =
+  let open Action_builder.O in
+  let+ ccomp_type = ccomp_type ctx in
+  base_cxx_compile_flags ocaml_version ccomp_type
+;;
+
+let get_link_flags ctx =
   let open Action_builder.O in
   let+ ccomp_type = ccomp_type ctx in
-  base_cxx_flags ~for_ ccomp_type
+  base_cxx_link_flags ccomp_type
 ;;

src/dune_rules/cxx_flags.mli

@@ -3,10 +3,6 @@
 
 open Import
 
-type phase =
-  | Compile
-  | Link
-
 (** The detected compiler *)
 type ccomp_type =
   | Gcc
@@ -21,9 +17,13 @@ val preprocessed_filename : string
 (** [ccomp_type ctx] returns the C/C++ compiler type. *)
 val ccomp_type : Build_context.t -> ccomp_type Action_builder.t
 
-(** [get_flags for_:phase ctx] returns the necessary flags to turn this compiler
-    into a c++ compiler for some of the most common compilers *)
-val get_flags : for_:phase -> Build_context.t -> string list Action_builder.t
+(** [get_compile_flags ctx] returns the necessary compile-time flags to turn
+    this compiler into a c++ compiler for some of the most common compilers *)
+val get_compile_flags : Ocaml_config.t -> Build_context.t -> string list Action_builder.t
+
+(** [get_link_flags ctx] returns the necessary link-time flags to turn
+    this compiler into a c++ compiler for some of the most common compilers *)
+val get_link_flags : Build_context.t -> string list Action_builder.t
 
 (** [fdiagnostics_color cc] returns the flags activating color diagnostics for
     the C/C++ compiler, if supported. *)

src/dune_rules/foreign_rules.ml

@@ -28,7 +28,7 @@ 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_compile_flags ocaml_config ctx in
         List.concat [ db_flags; cxxflags; fdiagnostics_color ]
       in
       c, cxx

src/dune_rules/lib_rules.ml

@@ -67,7 +67,7 @@ let build_lib
       let+ project = Dune_load.find_project ~dir in
       match Dune_project.use_standard_c_and_cxx_flags project with
       | Some true when Buildable.has_foreign_cxx lib.buildable ->
-        Cxx_flags.get_flags ~for_:Link (Context.build_context ctx)
+        Cxx_flags.get_link_flags (Context.build_context ctx)
       | _ -> Action_builder.return []
     in
     let cclibs = Expander.expand_and_eval_set expander lib.c_library_flags ~standard in
@@ -263,7 +263,7 @@ let foreign_rules (library : Foreign.Library.t) ~sctx ~expander ~dir ~dir_conten
     match Dune_project.use_standard_c_and_cxx_flags project with
     | Some true when Foreign.Sources.has_cxx_sources foreign_sources ->
       let ctx = Super_context.context sctx in
-      Cxx_flags.get_flags ~for_:Link (Context.build_context ctx)
+      Cxx_flags.get_link_flags (Context.build_context ctx)
     | _ -> Action_builder.return []
   in
   let c_library_flags =
@@ -317,7 +317,7 @@ let build_stubs lib ~cctx ~dir ~expander ~requires ~dir_contents ~vlib_stubs_o_f
       let+ project = Dune_load.find_project ~dir in
       match Dune_project.use_standard_c_and_cxx_flags project with
       | Some true when Foreign.Sources.has_cxx_sources foreign_sources ->
-        Cxx_flags.get_flags ~for_:Link (Context.build_context ctx)
+        Cxx_flags.get_link_flags (Context.build_context ctx)
       | _ -> Action_builder.return []
     in
     let c_library_flags =

src/dune_rules/ocaml_flags_db.ml

@@ -47,7 +47,7 @@ let link_env =
       ~name:"link-env"
       ~root:(fun ctx _ ->
         let default_cxx_link_flags =
-          Cxx_flags.get_flags ~for_:Link (Build_context.create ~name:ctx)
+          Cxx_flags.get_link_flags (Build_context.create ~name:ctx)
         in
         Link_flags.default ~default_cxx_link_flags |> Memo.return)
       ~f:(fun ~parent expander (env : Dune_env.config) ->