From ca237910b3a1f8ffd24f0742e1758ce8186796d5 Mon Sep 17 00:00:00 2001
From: Matthew Flatt <mflatt@racket-lang.org>
Date: Tue, 15 Dec 2015 17:06:48 -0700
Subject: [PATCH] fix `make-syntax-delta-introducer` with a #f argument

Closes PR 15202
---
 pkgs/racket-doc/scribblings/reference/stx-trans.scrbl |  2 ++
 pkgs/racket-test-core/tests/racket/stx.rktl           | 11 +++++++++++
 racket/src/racket/src/syntax.c                        |  4 +---
 3 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/pkgs/racket-doc/scribblings/reference/stx-trans.scrbl b/pkgs/racket-doc/scribblings/reference/stx-trans.scrbl
index e8d6832c95..3eca603e50 100644
--- a/pkgs/racket-doc/scribblings/reference/stx-trans.scrbl
+++ b/pkgs/racket-doc/scribblings/reference/stx-trans.scrbl
@@ -947,6 +947,8 @@ and different result procedures use distinct scopes.
 Produces a procedure that behaves like the result of
 @racket[make-syntax-introducer], but using the @tech{scopes} of
 @racket[ext-stx] that are not shared with @racket[base-stx].
+A @racket[#f] value for @racket[base-stx] is equivalent to a syntax
+object with no @tech{scopes}.
 
 This procedure is potentially useful when some @racket[_m-id] has a
 transformer binding that records some @racket[_orig-id], and a use of
diff --git a/pkgs/racket-test-core/tests/racket/stx.rktl b/pkgs/racket-test-core/tests/racket/stx.rktl
index 4b1e343f6d..13da53dbbf 100644
--- a/pkgs/racket-test-core/tests/racket/stx.rktl
+++ b/pkgs/racket-test-core/tests/racket/stx.rktl
@@ -148,6 +148,17 @@
 (test #f syntax-original? ((make-syntax-introducer) #'here))
 (test #t syntax-original? ((make-syntax-introducer #t) #'here))
 
+(let* ([a (datum->syntax #f 'a)]
+       [a1 ((make-syntax-introducer) a)]
+       [a2 ((make-syntax-introducer) a)])
+  (test #f bound-identifier=? a1 a2)
+  (test #t bound-identifier=? a1 ((make-syntax-delta-introducer a1 a2) a))
+  (test #t bound-identifier=? a2 ((make-syntax-delta-introducer a2 a1) a))
+  (test #t bound-identifier=? a2 ((make-syntax-delta-introducer a2 #f) a))
+  (test #t bound-identifier=?
+        ((make-syntax-delta-introducer a1 a2) a2)
+        ((make-syntax-delta-introducer a2 a1) a1)))
+
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; Test basic expansion and property propagation
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
diff --git a/racket/src/racket/src/syntax.c b/racket/src/racket/src/syntax.c
index 0ef0a6d685..de2cc3ae1b 100644
--- a/racket/src/racket/src/syntax.c
+++ b/racket/src/racket/src/syntax.c
@@ -8006,12 +8006,10 @@ Scheme_Object *scheme_syntax_make_transfer_intro(int argc, Scheme_Object **argv)
   } else
     m2 = NULL;
 
-  if (!m2) {
+  if (!m2 && !SCHEME_FALSEP(src)) {
     src = scheme_stx_lookup_w_nominal(argv[1], phase, 1,
                                       NULL, NULL, &m2,
                                       NULL, NULL, NULL, NULL, NULL);
-    if (SCHEME_FALSEP(src))
-      m2 = NULL;
   }
 
   if (m2) {