expander: repair local-expand with definition context

When `local-expand` receives one or more internal definition contexts,
it would forget about any current post-expansion scopes. That's
particularly a problem in a 'module-begin expansion context, where the
post-expansion scope ensures that any bindings are suitably
phase-specific.

Closes #2115

pkgs/racket-test-core/tests/racket/macro.rktl

@@ -2173,6 +2173,38 @@
 
 (test 1 dynamic-require ''local-expand-result-depends-on-use-site-scope 'result)
 
+;; ----------------------------------------
+;; Make sure `local-expand` doesn't get confused about the
+;; post-expansion scope needed to make a binding phase-specific
+
+(module test-for-scope-specific-binding racket/base
+
+  (module l racket/base
+    (require (for-syntax racket/base racket/syntax)
+             syntax/parse/define)
+
+    (provide #%module-begin m
+             (all-from-out racket/base))
+
+    (define-syntax-parser #%module-begin
+      [(_ form ...)
+       (let ([intdef-ctx (syntax-local-make-definition-context #f #f)])
+         (local-expand #'(#%plain-module-begin form ...)
+                       'module-begin
+                       '()
+                       intdef-ctx))])
+
+    (define-syntax-parser m
+      [(_)
+       #:with x (generate-temporary)
+       #'(begin
+           (define x #f)
+           ;; The #' here triggers a `syntax-local-value` call:
+           (begin-for-syntax #'x))]))
+
+  (module c (submod ".." l)
+    (#%module-begin
+     (m))))
 
 ;; ----------------------------------------
 

racket/src/expander/expand/definition-context.rkt

@@ -232,6 +232,9 @@
   (define def-ctx-scopes (if (expand-context-def-ctx-scopes ctx)
                              (unbox (expand-context-def-ctx-scopes ctx))
                              null))
+  (define placeholder-sc (and intdefs
+                              (not (null? intdefs))
+                              (new-scope 'macro)))
   (struct*-copy expand-context ctx
                 [context context]
                 [env (add-intdef-bindings (expand-context-env ctx)
@@ -256,11 +259,12 @@
                              [else (or frame-id i-frame-id)]))]
                 [post-expansion-scope
                  #:parent root-expand-context
-                 (if (and intdefs (not (null? intdefs)))
-                     (new-scope 'macro) ; placeholder; action uses `indefs`
-                     (and same-kind?
+                 (or (and same-kind?
                           (memq context '(module module-begin top-level))
-                          (root-expand-context-post-expansion-scope ctx)))]
+                          (root-expand-context-post-expansion-scope ctx))
+                     ;; Placeholder to make sure `post-expansion-scope-action`
+                     ;; is used
+                     placeholder-sc)]
                 [post-expansion-shifts
                  #:parent root-expand-context
                  (if (and same-kind?
@@ -268,10 +272,14 @@
                      (root-expand-context-post-expansion-shifts ctx)
                      null)]
                 [post-expansion-scope-action
-                 (if (and intdefs (not (null? intdefs)))
-                     (lambda (s placeholder-sc)
-                       (add-intdef-scopes s intdefs))
-                     (expand-context-post-expansion-scope-action ctx))]
+                 (let ([act (expand-context-post-expansion-scope-action ctx)])
+                   (if (and intdefs (not (null? intdefs)))
+                       (lambda (s sc)
+                         (define s2 (if (eq? sc placeholder-sc)
+                                        s
+                                        (act s sc)))
+                         (add-intdef-scopes s2 intdefs))
+                       act))]
                 [scopes
                  (append def-ctx-scopes
                          (expand-context-scopes ctx))]