Detect future syntax errors in the Syntax helper

Repository parsing and linting use a slightly different parsing route
which wasn't checking for future syntax errors.

master_changes.md

@@ -92,6 +92,7 @@ users)
 
 ## Opamfile
   * Make all writes atomic [#5489 @kit-ty-kate]
+  * Ensure future syntax errors are only reported when the syntax version is greater than the client, not the format library [#6199 @dra27 - fix #6188]
 
 ## External dependencies
  * Always pass --no-version-check and --no-write-registry to Cygwin setup [#6046 @dra27]

src/format/opamFile.ml

@@ -874,9 +874,14 @@ module Syntax = struct
     let filename = OpamFilename.to_string filename in
     lexbuf.Lexing.lex_curr_p <- { lexbuf.Lexing.lex_curr_p with
                                   Lexing.pos_fname = filename };
-    try OpamParser.main OpamLexer.token lexbuf filename with
-    | OpamLexer.Error msg -> error msg
-    | Parsing.Parse_error -> error "Parse error"
+    match OpamParser.main OpamLexer.token lexbuf filename with
+    | {file_contents = [{pelem = Variable({pelem = "opam-version"; _}, {pelem = String ver; _}); _ };
+                        {pelem = Section {section_kind = {pelem = "#"; _}; _}; _}]; _}
+      when OpamVersion.(compare (nopatch (of_string ver)) (nopatch OpamVersion.current)) <= 0 ->
+      error "Parse error"
+    | opamfile -> opamfile
+    | exception OpamLexer.Error msg -> error msg
+    | exception Parsing.Parse_error -> error "Parse error"
 
 
   let pp_channel filename ic oc =

tests/reftests/lint.test

@@ -1262,11 +1262,11 @@ ${BASEDIR}/future/pin-at-two-one.opam: Errors.
 # Return code 1 #
 ### opam lint future/pin-at-two-two.opam
 ${BASEDIR}/future/pin-at-two-two.opam: Errors.
-             error  2: File format error: unsupported or missing file format version; should be 2.0 or older
+             error  2: File format error at line 11, column 0: Parse error
 # Return code 1 #
 ### opam lint future/pin-at-two-three.opam
 ${BASEDIR}/future/pin-at-two-three.opam: Errors.
-             error  2: File format error: unsupported or missing file format version; should be 2.0 or older
+             error  2: File format error at line 11, column 0: Parse error
 # Return code 1 #
 ### opam lint future/pin-at-future.opam
 ${BASEDIR}/future/pin-at-future.opam: Errors.