Fix nix why-depends --derivation

src/libcmd/installables.cc

@@ -931,10 +931,7 @@ std::vector<std::pair<std::shared_ptr<Installable>, BuiltPathWithResult>> Instal
                                 DrvOutput outputId { *outputHash, output };
                                 auto realisation = store->queryRealisation(outputId);
                                 if (!realisation)
-                                    throw Error(
-                                        "cannot operate on an output of the "
-                                        "unbuilt derivation '%s'",
-                                        outputId.to_string());
+                                    throw MissingRealisation(outputId);
                                 outputs.insert_or_assign(output, realisation->outPath);
                             } else {
                                 // If ca-derivations isn't enabled, assume that

src/libstore/realisation.hh

@@ -93,4 +93,14 @@ struct RealisedPath {
     GENERATE_CMP(RealisedPath, me->raw);
 };
 
+class MissingRealisation : public Error
+{
+public:
+    MissingRealisation(DrvOutput & outputId)
+        : Error( "cannot operate on an output of the "
+                "unbuilt derivation '%s'",
+                outputId.to_string())
+    {}
+};
+
 }

src/libstore/remote-store.cc

@@ -879,10 +879,7 @@ std::vector<BuildResult> RemoteStore::buildPathsWithResults(
                                 auto realisation =
                                     queryRealisation(outputId);
                                 if (!realisation)
-                                    throw Error(
-                                        "cannot operate on an output of unbuilt "
-                                        "content-addressed derivation '%s'",
-                                        outputId.to_string());
+                                    throw MissingRealisation(outputId);
                                 res.builtOutputs.emplace(realisation->id, *realisation);
                             } else {
                                 // If ca-derivations isn't enabled, assume that

src/nix/why-depends.cc

@@ -95,23 +95,13 @@ struct CmdWhyDepends : SourceExprCommand
          * to build.
          */
         auto dependency = parseInstallable(store, _dependency);
-        auto derivedDependency = dependency->toDerivedPath();
-        auto optDependencyPath = std::visit(overloaded {
-            [](const DerivedPath::Opaque & nodrv) -> std::optional<StorePath> {
-                return { nodrv.path };
-            },
-            [&](const DerivedPath::Built & hasdrv) -> std::optional<StorePath> {
-                if (hasdrv.outputs.size() != 1) {
-                    throw Error("argument '%s' should evaluate to one store path", dependency->what());
-                }
-                auto outputMap = store->queryPartialDerivationOutputMap(hasdrv.drvPath);
-                auto maybePath = outputMap.find(*hasdrv.outputs.begin());
-                if (maybePath == outputMap.end()) {
-                    throw Error("unexpected end of iterator");
-                }
-                return maybePath->second;
-            },
-        }, derivedDependency.raw());
+        auto optDependencyPath = [&]() -> std::optional<StorePath> {
+            try {
+                return {Installable::toStorePath(getEvalStore(), store, Realise::Derivation, operateOn, dependency)};
+            } catch (MissingRealisation &) {
+                return std::nullopt;
+            }
+        }();
 
         StorePathSet closure;
         store->computeFSClosure({packagePath}, closure, false, false);

tests/local.mk

@@ -92,6 +92,7 @@ nix_tests = \
   fmt.sh \
   eval-store.sh \
   why-depends.sh \
+  ca/why-depends.sh \
   import-derivation.sh \
   ca/import-derivation.sh \
   nix_path.sh \

tests/why-depends.sh

@@ -6,6 +6,9 @@ cp ./dependencies.nix ./dependencies.builder0.sh ./config.nix $TEST_HOME
 
 cd $TEST_HOME
 
+nix why-depends --derivation --file ./dependencies.nix input2_drv input1_drv
+nix why-depends --file ./dependencies.nix input2_drv input1_drv
+
 nix-build ./dependencies.nix -A input0_drv -o dep
 nix-build ./dependencies.nix -o toplevel