From 250f0b10fbd0758c86abe1c0517a63e8babdeaac Mon Sep 17 00:00:00 2001
From: Kate <kit.ty.kate@disroot.org>
Date: Sun, 6 Oct 2024 17:44:40 +0100
Subject: [PATCH] Call the C++ compiler with -std=c++11 when using OCaml >= 5.0
 (#10962)

* Call the C++ compiler with -std=c++11 when using OCaml >= 5.0
---
 doc/changes/10962.md                          |  2 ++
 src/dune_rules/cxx_flags.ml                   | 36 +++++++++++--------
 src/dune_rules/cxx_flags.mli                  |  2 +-
 src/dune_rules/foreign_rules.ml               |  6 +++-
 src/ocaml/version.ml                          |  1 +
 src/ocaml/version.mli                         |  2 ++
 .../test-cases/cpp-std-ocaml5.t/run.t         |  2 +-
 test/blackbox-tests/test-cases/dune           |  2 +-
 8 files changed, 35 insertions(+), 18 deletions(-)
 create mode 100644 doc/changes/10962.md

diff --git a/doc/changes/10962.md b/doc/changes/10962.md
new file mode 100644
index 00000000000..b5073d70591
--- /dev/null
+++ b/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)
diff --git a/src/dune_rules/cxx_flags.ml b/src/dune_rules/cxx_flags.ml
index d4daffd73ea..d2fcad154fb 100644
--- a/src/dune_rules/cxx_flags.ml
+++ b/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=c++11" ] else [])
+  | Msvc -> [ "/TP" ]
+  | Other _ -> []
+;;
+
+let base_cxx_link_flags = function
+  | Gcc -> [ "-lstdc++"; "-shared-libgcc" ]
+  | Clang -> [ "-lc++" ]
+  | Msvc -> []
+  | Other _ -> []
 ;;
 
 let fdiagnostics_color = function
@@ -62,5 +67,8 @@ 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
 ;;
diff --git a/src/dune_rules/cxx_flags.mli b/src/dune_rules/cxx_flags.mli
index eed02da3c9f..0c01ae86bb8 100644
--- a/src/dune_rules/cxx_flags.mli
+++ b/src/dune_rules/cxx_flags.mli
@@ -4,7 +4,7 @@
 open Import
 
 type phase =
-  | Compile
+  | Compile of Ocaml.Version.t
   | Link
 
 (** The detected compiler *)
diff --git a/src/dune_rules/foreign_rules.ml b/src/dune_rules/foreign_rules.ml
index 86e9929558e..de7f82c6ec9 100644
--- a/src/dune_rules/foreign_rules.ml
+++ b/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
diff --git a/src/ocaml/version.ml b/src/ocaml/version.ml
index 3caf3c7c444..51f71e75187 100644
--- a/src/ocaml/version.ml
+++ b/src/ocaml/version.ml
@@ -30,3 +30,4 @@ 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 supports_hidden_includes version = version >= (5, 2, 0)
+let add_std_cxx_flag version = version >= (5, 0, 0)
diff --git a/src/ocaml/version.mli b/src/ocaml/version.mli
index dc79429144e..c8d1fdb15ca 100644
--- a/src/ocaml/version.mli
+++ b/src/ocaml/version.mli
@@ -79,3 +79,5 @@ val supports_bin_annot_occurrences : t -> bool
 
 (** Whether the compiler supports the -H flag *)
 val supports_hidden_includes : t -> bool
+
+val add_std_cxx_flag : t -> bool
diff --git a/test/blackbox-tests/test-cases/cpp-std-ocaml5.t/run.t b/test/blackbox-tests/test-cases/cpp-std-ocaml5.t/run.t
index ea569532442..29cc0ac0431 100644
--- a/test/blackbox-tests/test-cases/cpp-std-ocaml5.t/run.t
+++ b/test/blackbox-tests/test-cases/cpp-std-ocaml5.t/run.t
@@ -16,4 +16,4 @@
   > EOF
 
   $ (dune build ./cpp11.exe 2>/dev/null) && _build/default/cpp11.exe
-  [1]
+  Hi from C++11
diff --git a/test/blackbox-tests/test-cases/dune b/test/blackbox-tests/test-cases/dune
index 7cefc4d35db..59e394349d5 100644
--- a/test/blackbox-tests/test-cases/dune
+++ b/test/blackbox-tests/test-cases/dune
@@ -119,7 +119,7 @@
   (enabled_if %{bin-available:hg})))
 
 (cram
- (applies_to patch-back-source-tree cpp-std-ocaml5)
+ (applies_to patch-back-source-tree)
  (enabled_if
   (<> %{system} macosx)))