Fix #1856

CHANGES.md

@@ -18,6 +18,9 @@ unreleased
   is used. Add `Pkg_config.query_expr` for cases when the full power of
   pkg-config's querying is needed (#1842, fix #1833, @rgrinberg)
 
+- Fix unavailable, optional implementations eagerly breaking the build (#1857,
+  fix #1856, @rgrinberg)
+
 1.7.1 (13/02/2019)
 ------------------
 

src/dir_contents.ml

@@ -52,11 +52,18 @@ module Modules = struct
             | From _, From _ ->
               let name = (fst lib.name, Library.best_name lib) in
               Result.ok_exn (
-                let open Result.O in
-                Lib.DB.resolve (Scope.libs scope) name >>= fun lib ->
-                Lib.main_module_name lib >>= fun main_module_name ->
-                Lib.wrapped lib >>| fun wrapped ->
-                (main_module_name, Option.value_exn wrapped)
+                match
+                  Lib.DB.find_even_when_hidden (Scope.libs scope) (snd name)
+                with
+                | None ->
+                  (* can't happen because this library is defined using the
+                     current stanza *)
+                  assert false
+                | Some lib ->
+                  let open Result.O in
+                  Lib.main_module_name lib >>= fun main_module_name ->
+                  Lib.wrapped lib >>| fun wrapped ->
+                  (main_module_name, Option.value_exn wrapped)
               )
           in
           Left ( lib

test/blackbox-tests/dune.inc

@@ -527,6 +527,14 @@
    test-cases/github1811
    (progn (run %{exe:cram.exe} -test run.t) (diff? run.t run.t.corrected)))))
 
+(alias
+ (name github1856)
+ (deps (package dune) (source_tree test-cases/github1856))
+ (action
+  (chdir
+   test-cases/github1856
+   (progn (run %{exe:cram.exe} -test run.t) (diff? run.t run.t.corrected)))))
+
 (alias
  (name github20)
  (deps (package dune) (source_tree test-cases/github20))
@@ -1352,6 +1360,7 @@
   (alias github1560)
   (alias github1616)
   (alias github1811)
+  (alias github1856)
   (alias github20)
   (alias github24)
   (alias github25)
@@ -1509,6 +1518,7 @@
   (alias github1560)
   (alias github1616)
   (alias github1811)
+  (alias github1856)
   (alias github20)
   (alias github24)
   (alias github25)

test/blackbox-tests/test-cases/github1856/dune-project

@@ -0,0 +1 @@
+(lang dune 1.7)

test/blackbox-tests/test-cases/github1856/impl/dune

@@ -0,0 +1,6 @@
+(library 
+ (name foo_impl)
+ (public_name foo.impl)
+ (implements foo)
+ (libraries dontexist)
+ (optional))

test/blackbox-tests/test-cases/github1856/run.t

@@ -0,0 +1,2 @@
+Optional implementation of virtual library
+  $ dune build @install

test/blackbox-tests/test-cases/github1856/vlib/dune

@@ -0,0 +1,4 @@
+(library
+ (name foo)
+ (public_name foo)
+ (virtual_modules foo))