guix build: -S returns the replacement's source.

Reported by Mark H Weaver.

* guix/scripts/build.scm (options->derivations): When SRC and GRAFT? are
true, use the source of P's replacement.
* tests/guix-build.sh: Add test.

guix/scripts/build.scm

@@ -592,6 +592,7 @@ build."
   (parameterize ((%graft? graft?))
     (append-map (match-lambda
                   ((? package? p)
+                   (let ((p (or (and graft? (package-replacement p)) p)))
                      (match src
                        (#f
                         (list (package->derivation store p system)))
@@ -600,7 +601,7 @@ build."
                           (list (package-source-derivation store s))))
                        (proc
                         (map (cut package-source-derivation store <>)
-                           (proc p)))))
+                             (proc p))))))
                   ((? derivation? drv)
                    (list drv))
                   ((? procedure? proc)

tests/guix-build.sh

@@ -43,6 +43,7 @@ trap "rm -rf $module_dir" EXIT
 
 cat > "$module_dir/foo.scm"<<EOF
 (define-module (foo)
+  #:use-module (guix tests)
   #:use-module (guix packages)
   #:use-module (guix download)
   #:use-module (guix build-system trivial))
@@ -88,6 +89,10 @@ cat > "$module_dir/foo.scm"<<EOF
     (synopsis "Dummy package")
     (description "bar is a dummy package for testing.")
     (license #f)))
+
+(define-public baz
+  (dummy-package "baz" (replacement foo)))
+
 EOF
 
 GUIX_PACKAGE_PATH="$module_dir"
@@ -97,6 +102,10 @@ export GUIX_PACKAGE_PATH
 guix build -d -S foo
 guix build -d -S foo | grep -e 'foo\.tar\.gz'
 
+# 'baz' has a replacement so we should be getting the replacement's source.
+(unset GUIX_BUILD_OPTIONS;
+ test "`guix build -d -S baz`" = "`guix build -d -S foo`")
+
 guix build -d --sources=package foo
 guix build -d --sources=package foo | grep -e 'foo\.tar\.gz'