diff --git a/src/dune_project.ml b/src/dune_project.ml
index 0de6b34a688..077549d7a1a 100644
--- a/src/dune_project.ml
+++ b/src/dune_project.ml
@@ -716,8 +716,9 @@ let parse ~dir ~lang ~opam_packages ~file =
      let wrapped_executables =
        Option.value wrapped_executables
          ~default:(wrapped_executables_default ~lang) in
+     let dune_version = lang.version in
      let allow_approx_merlin =
-       Option.value ~default:false allow_approx_merlin in
+       Option.value ~default:(dune_version < (1, 9)) allow_approx_merlin in
      let generate_opam_files =
        Option.value ~default:false generate_opam_files in
      let root = dir in
@@ -745,7 +746,7 @@ let parse ~dir ~lang ~opam_packages ~file =
      ; parsing_context
      ; implicit_transitive_deps
      ; wrapped_executables
-     ; dune_version = lang.version
+     ; dune_version
      ; allow_approx_merlin
      ; generate_opam_files
      ; dialects
diff --git a/test/blackbox-tests/dune.inc b/test/blackbox-tests/dune.inc
index 8ed2245437e..d1f221421e0 100644
--- a/test/blackbox-tests/dune.inc
+++ b/test/blackbox-tests/dune.inc
@@ -22,6 +22,14 @@
    test-cases/all-alias
    (progn (run %{exe:cram.exe} -test run.t) (diff? run.t run.t.corrected)))))
 
+(alias
+ (name allow_approximate_merlin)
+ (deps (package dune) (source_tree test-cases/allow_approximate_merlin))
+ (action
+  (chdir
+   test-cases/allow_approximate_merlin
+   (progn (run %{exe:cram.exe} -test run.t) (diff? run.t run.t.corrected)))))
+
 (alias
  (name bad-alias-error)
  (deps (package dune) (source_tree test-cases/bad-alias-error))
@@ -1638,6 +1646,7 @@
   (alias action-modifying-a-dependency)
   (alias aliases)
   (alias all-alias)
+  (alias allow_approximate_merlin)
   (alias bad-alias-error)
   (alias block-strings)
   (alias byte-code-only)
@@ -1837,6 +1846,7 @@
   (alias action-modifying-a-dependency)
   (alias aliases)
   (alias all-alias)
+  (alias allow_approximate_merlin)
   (alias bad-alias-error)
   (alias block-strings)
   (alias byte-code-only)
diff --git a/test/blackbox-tests/test-cases/allow_approximate_merlin/a.ml b/test/blackbox-tests/test-cases/allow_approximate_merlin/a.ml
new file mode 100644
index 00000000000..e69de29bb2d
diff --git a/test/blackbox-tests/test-cases/allow_approximate_merlin/b.ml b/test/blackbox-tests/test-cases/allow_approximate_merlin/b.ml
new file mode 100644
index 00000000000..e69de29bb2d
diff --git a/test/blackbox-tests/test-cases/allow_approximate_merlin/dune b/test/blackbox-tests/test-cases/allow_approximate_merlin/dune
new file mode 100644
index 00000000000..2ea4a7195b9
--- /dev/null
+++ b/test/blackbox-tests/test-cases/allow_approximate_merlin/dune
@@ -0,0 +1,8 @@
+(executable
+ (name a)
+ (modules a)
+ (preprocess future_syntax))
+
+(executable
+ (name b)
+ (modules b))
diff --git a/test/blackbox-tests/test-cases/allow_approximate_merlin/run.t b/test/blackbox-tests/test-cases/allow_approximate_merlin/run.t
new file mode 100644
index 00000000000..e60b01772c9
--- /dev/null
+++ b/test/blackbox-tests/test-cases/allow_approximate_merlin/run.t
@@ -0,0 +1,26 @@
+If different options apply to two stanzas in the same directory, the .merlin
+file is the union of the two and a warning is emitted.
+
+The output depends on dune-project.
+
+For lang >= 1.9, a warning is printed:
+
+  $ echo '(lang dune 1.9)' > dune-project
+  $ dune build @check
+  File "dune", line 4, characters 13-26:
+  4 |  (preprocess future_syntax))
+                   ^^^^^^^^^^^^^
+  Warning: .merlin generated is inaccurate. Cannot mix preprocessed and non
+  preprocessed specificiations.
+  Split the stanzas into different directories or silence this warning by
+  adding (allow_approximate_merlin) to your dune-project.
+
+Indeed, adding this will suppress the warning:
+
+  $ echo '(lang dune 1.9)\n(allow_approximate_merlin)' > dune-project
+  $ dune build @check
+
+However, the warning is not emitted if it is not fixable (#2399).
+
+  $ echo '(lang dune 1.8)' > dune-project
+  $ dune build @check
diff --git a/test/blackbox-tests/test-cases/github1946/run.t b/test/blackbox-tests/test-cases/github1946/run.t
index e931c1a40a6..6a18e6deefc 100644
--- a/test/blackbox-tests/test-cases/github1946/run.t
+++ b/test/blackbox-tests/test-cases/github1946/run.t
@@ -1,9 +1,2 @@
   $ dune build --display short --profile release | grep "FLG -ppx"
-  File "dune", line 4, characters 13-23:
-  4 |  (preprocess (pps ppx1)))
-                   ^^^^^^^^^^
-  Warning: .merlin generated is inaccurate. pps specification isn't identical
-  in all stanzas.
-  Split the stanzas into different directories or silence this warning by
-  adding (allow_approximate_merlin) to your dune-project.
   [1]