A dlopened macro binds to its own copy, not the merged host's

flan dev's one-process build links the program and the compiler into one
-rdynamic executable, so it exports flan.rl/with-drawing -- the package's
defmacro compiled as an ordinary function, whose body was qualified at the
Ast level, after the quasiquote had already become a string literal. ELF
gives the executable precedence over a dlopened object, so the macro module
called the host's copy and (rl/with-drawing ...) expanded to an unqualified
begin-drawing the checker then refused. The module is self-contained, so
-Wl,-Bsymbolic is the whole fix.

merged_setup reports a Loc.Error as a diagnostic now, which is how the
failure had a location at all instead of an exception constructor after an
apparently successful build.

lib/build.ml

@@ -1053,7 +1053,19 @@ let macro_module ?(opts = default) ?(csrcs = []) ?(lflags = []) ~macros
   let cmd =
     String.concat " "
       ([ Filename.quote (compiler opts); opts.opt; "-Wno-override-module";
-         "-shared"; "-fPIC" ]
+         "-shared"; "-fPIC";
+         (* Bind this module's own references to its own definitions. ELF
+            gives the executable's symbols precedence over a shared object's,
+            and [flan dev]'s merged build is an executable that carries the
+            whole program -- including the package's [defmacro]s, compiled
+            from a body whose names were qualified after the quasiquote became
+            string literals and so are not the ones this module holds. Without
+            this, dlopening the macro module into that host makes
+            [flan.rl/with-drawing] resolve to the host's copy, and
+            [(rl/with-drawing ...)] expands to an unqualified [begin-drawing]
+            that the checker then refuses. The module is self-contained by
+            design, so there is nothing it wants from the host. *)
+         "-Wl,-Bsymbolic" ]
        @ tflags
        @ [ Filename.quote ll ]
        @ List.map Filename.quote objs

lib/dev.ml

@@ -3038,9 +3038,25 @@ let merged_setup () =
     merged_state := Some (t, ls, sock);
     Printf.eprintf "flan dev: %s ready on %s (%.0fms, one process)\n%!" file
       sock ((Unix.gettimeofday () -. t0) *. 1000.)
-  with e ->
-    Printf.eprintf "flan dev: %s\n%!"
-      (match e with Failure m -> m | e -> Printexc.to_string e);
+  with
+  (* The executable has just replaced the launcher, and recreates the session
+     above in order to own it for the rest of the dev run.  That is still a
+     frontend boundary: rendering [Loc.Error] as an exception constructor here
+     loses the source location, the reason, its span, and any notes.  It also
+     made a source error look like a compiler crash after an apparently
+     successful build.  Keep this identical to the command driver's reporting
+     for both the one-error and whole-file-error channels. *)
+  | Loc.Error d ->
+    Printf.eprintf "%s\n%!" (Loc.report d);
+    exit 1
+  | Loc.Errors ds ->
+    Printf.eprintf "%s\n%!" (Loc.report_all ds);
+    exit 1
+  | Failure m ->
+    Printf.eprintf "flan dev: %s\n%!" m;
+    exit 1
+  | e ->
+    Printf.eprintf "flan dev: %s\n%!" (Printexc.to_string e);
     exit 1
 
 (* Called from the compiler thread after the program has started. Never