Fix variable CC content + add test

src/dune/super_context.ml

@@ -2,19 +2,12 @@ open! Stdune
 open Import
 
 let default_context_flags (ctx : Context.t) =
-  (* TODO (v3) Current flag behavior is different when calling the C compiler or
-     the C++ compiler:
-
-     1.[ocamlc_cflags] and [ocamlc_cppflags] are always prenpended to the C
-     compiler arguments to reproduce [ocamlc]'s behavior,
-
-     2. [ocamlc_cflags] are present in [:standard] and prepended to the C++
-     compiler arguments only if the user didn't redefined them (or used
-     [:standard] to extend them) *)
-  let c = [] in
+  (* TODO DUNE3 To ensure full backward compatibility, ocaml_cflags are still
+     present in the :standard set of flags. However these should not as they are
+     already prepended when calling the compiler, causing flag duplication. *)
+  let c = Ocaml_config.ocamlc_cflags ctx.ocaml_config in
   let cxx =
-    Ocaml_config.ocamlc_cflags ctx.ocaml_config
-    |> List.filter ~f:(fun s -> not (String.is_prefix s ~prefix:"-std="))
+    List.filter c ~f:(fun s -> not (String.is_prefix s ~prefix:"-std="))
   in
   Foreign.Language.Dict.make ~c ~cxx
 

test/blackbox-tests/test-cases/variables/var-cc.t/dune-project

@@ -0,0 +1 @@
+(lang dune 2.0)

test/blackbox-tests/test-cases/variables/var-cc.t/run.t

@@ -0,0 +1,33 @@
+According to the doc: CC is the C compiler command line (list made of the
+compiler name followed by its flags) that was used to compile OCaml in the
+current build context.
+
+In practice it consists in the concatenation of OCaml's `c_compiler` and flags
+The flags are made of the :standard (= ocamlc_cflags) set of ocamlc_flags
+merged with (and sometimes replaced by) the flags in the env stanza.
+
+  $ O_CC=$(ocamlc -config-var c_compiler)
+  $ O_CCF=$(ocamlc -config-var ocamlc_cflags)
+
+No env
+  $ cat > dune <<'EOF'
+  > (rule
+  >  (alias cc)
+  >   (action (echo %{cc})))
+
+  $ dune build @cc | sed "s,${O_CC} ${O_CCF},OK,"
+  OK
+
+With added env flags
+  $ cat >> dune <<'EOF'
+  > (env (_ (c_flags :standard -fPIC)))
+  > EOF
+
+  $ dune build @cc | sed "s,${O_CC} ${O_CCF} -fPIC,OK,"
+  OK
+
+With redefining env flags
+  $ sed -i "s/:standard //g" dune
+
+  $ dune build @cc | sed "s,${O_CC} -fPIC,OK,"
+  OK