lib: clear error when a dependency's library was not in the install plan

lib/default.nix

@@ -356,18 +356,25 @@ in {
   # In the case of sublibs the `depends` value should already be the derivation.
   dependToLib = d:
     # Do not simplify this to `d.components.library or d`, as that
-    # will not give a good error message if the `.library`
-    # is missing (happens if the package is unplanned,
-    # but has overrides).
-    # It would be nice to put an `assert` here, but there is
-    # currently no good way to get the name of the dependency
-    # when it is not in the plan.  The attribute path of
-    # `d` in the `nix` error should include the name
-    # eg. `packages.Cabal.components.library`.
+    # would silently drop the dependency when `.library` is missing
+    # (which happens when the package is unplanned but has overrides).
+    # A package value carries its `identifier` (see hspkg-builder.nix), so
+    # when the library component is absent we can throw a message naming the
+    # dependency instead of the opaque `attribute 'library' missing` error
+    # (see #1379).  Checking `d.components ? library` adds no strictness: the
+    # non-error path already forces `d.components` to read `.library`.
     if d ? components
-    then if d ? instantiations
-         then d.components.library.override { inherit (d) instantiations; }
-         else d.components.library
+    then if d.components ? library
+         then if d ? instantiations
+              then d.components.library.override { inherit (d) instantiations; }
+              else d.components.library
+         else throw ("haskell.nix: the library of package '${d.identifier.name or "<unknown>"}'"
+           + " is not available because it was not in the install plan."
+           + " This usually means the package was given overrides (e.g. via"
+           + " `modules` / `packages.${d.identifier.name or "<name>"}`) but is not part"
+           + " of the cabal plan, so none of its components were planned."
+           + " Add it to the project's dependencies (so it appears in the plan),"
+           + " or remove the override.")
     else d;
 
   projectOverlays = import ./project-overlays.nix {

test/unit.nix

@@ -57,6 +57,29 @@ lib.runTests {
     expected = 1;
   };
 
+  # `dependToLib` returns the library component of a dependency package.
+  testDependToLibResolvesLibrary = {
+    expr = haskellLib.dependToLib { identifier.name = "foo"; components.library = "the-lib"; };
+    expected = "the-lib";
+  };
+
+  # A sublib `depends` value is already the derivation (no `components`), so it
+  # is passed through unchanged.
+  testDependToLibPassesSublibThrough = {
+    expr = haskellLib.dependToLib "already-a-derivation";
+    expected = "already-a-derivation";
+  };
+
+  # An unplanned package (has `components`, but no planned `library`) must throw
+  # rather than silently vanish or give an opaque `attribute 'library' missing`
+  # (see #1379).  `runTests` can only assert that evaluation fails; the message
+  # wording is checked out of band.
+  testDependToLibThrowsForUnplannedPackage = {
+    expr = (builtins.tryEval
+      (haskellLib.dependToLib { identifier.name = "foo"; components = { }; })).success;
+    expected = false;
+  };
+
   testParseBlock1 = {
     expr = __toJSON (haskellLib.parseSourceRepositoryPackageBlock "cabal.project" {} {} "" ''
         type: git