Improve error when parsing several licenses

Closes ocaml#6103

Signed-off-by: Etienne Millon <me@emillon.org>

CHANGES.md

@@ -42,6 +42,9 @@
 
 - Fix compilation of Dune under esy on Windows (#6109, fixes #6098, @nojb)
 
+- Improve error message when parsing several licenses in `(license)` (#6114,
+  fixes #6103, @emillon)
+
 3.4.1 (26-07-2022)
 ------------------
 

src/dune_engine/package.ml

@@ -498,10 +498,14 @@ module Info = struct
         (Dune_lang.Syntax.since Stanza.syntax (v (1, 9)) >>> repeat string)
     and+ license =
       field_o "license"
-        (Dune_lang.Syntax.since Stanza.syntax (v (3, 2))
-        >>> repeat1 string
-        <|> ( Dune_lang.Syntax.since Stanza.syntax (v (1, 9)) >>> string
-            >>| fun s -> [ s ] ))
+        (Dune_lang.Syntax.since Stanza.syntax (v (1, 9))
+        >>> let* l = repeat1 string in
+            (if List.length l > 1 then
+             Dune_lang.Syntax.since ~what:"Parsing several licenses"
+               Stanza.syntax
+               (v (3, 2))
+            else return ())
+            >>> return l)
     and+ homepage =
       field_o "homepage"
         (Dune_lang.Syntax.since Stanza.syntax (v (1, 10)) >>> string)

test/blackbox-tests/test-cases/dune-project-meta/main.t/run.t

@@ -606,10 +606,11 @@ Reject multiple licences in version [1.9, 3.2)
   > EOF
 
   $ dune build
-  File "dune-project", line 4, characters 13-16:
+  File "dune-project", line 4, characters 0-17:
   4 | (license MIT ISC)
-                   ^^^
-  Error: Too many argument for license
+      ^^^^^^^^^^^^^^^^^
+  Error: Parsing several licenses is only available since version 3.2 of the
+  dune language. Please update your dune-project file to have (lang dune 3.2).
   [1]
 
 Allow multiple licences in version >= 3.2