guix-package: Gracefully report non-existing outputs.

* guix-package.in (guix-package)[find-package](ensure-output): New
  procedure. Use it to validate SUB-DRV.

guix-package.in

@@ -402,6 +402,14 @@ Install, remove, or upgrade PACKAGES in a single transaction.\n"))
     ;; sub-derivation name.
     (define request name)
 
+    (define (ensure-output p sub-drv)
+      (if (member sub-drv (package-outputs p))
+          p
+          (leave (_ "~a: error: package `~a' lacks output `~a'~%")
+                 (location->string (package-location p))
+                 (package-full-name p)
+                 sub-drv)))
+
     (let*-values (((name sub-drv)
                    (match (string-rindex name #\:)
                      (#f    (values name "out"))
@@ -411,7 +419,7 @@ Install, remove, or upgrade PACKAGES in a single transaction.\n"))
                    (package-name->name+version name)))
       (match (find-packages-by-name name version)
         ((p)
-         (list name (package-version p) sub-drv p))
+         (list name (package-version p) sub-drv (ensure-output p sub-drv)))
         ((p p* ...)
          (format (current-error-port)
                  (_ "warning: ambiguous package specification `~a'~%")
@@ -420,7 +428,7 @@ Install, remove, or upgrade PACKAGES in a single transaction.\n"))
                  (_ "warning: choosing ~a from ~a~%")
                  (package-full-name p)
                  (location->string (package-location p)))
-         (list name (package-version p) sub-drv p))
+         (list name (package-version p) sub-drv (ensure-output p sub-drv)))
         (()
          (leave (_ "~a: package not found~%") request)))))
 

tests/guix-package.sh

@@ -120,6 +120,13 @@ fi
 # Make sure the `:' syntax works.
 guix-package --bootstrap -i "binutils:lib" -p "$profile" -n
 
+# Make sure nonexistent outputs are reported.
+guix-package --bootstrap -i "guile-bootstrap:out" -p "$profile" -n
+if guix-package --bootstrap -i "guile-bootstrap:does-not-exist" -p "$profile" -n;
+then false; else true; fi
+if guix-package --bootstrap -i "guile-bootstrap:does-not-exist" -p "$profile";
+then false; else true; fi
+
 # Check whether `--list-available' returns something sensible.
 guix-package -A 'gui.*e' | grep guile
 
@@ -161,4 +168,5 @@ do
 done
 
 # Extraneous argument.
-! guix-package install foo-bar
+if guix-package install foo-bar;
+then false; else true; fi