expander: repair phase >= 1 undefined-variable checking

Closes #1996

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

@@ -2443,4 +2443,19 @@ case of module-leve bindings; it doesn't cover local bindings.
 
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
+(let ([check
+       (lambda (later rx)
+         (err/rt-test (expand `(module m racket/base
+                                 (require (for-syntax racket/base))
+                                 (begin-for-syntax
+                                   (define (foo) (bar)))
+                                 ,later))
+                      (lambda (exn)
+                        (regexp-match? rx (exn-message exn)))))])
+  (check '(void) "unbound identifier")
+  (check '(begin-for-syntax (struct bar ())) #rx"later defined as syntax")
+  (check '(require (for-syntax (only-in racket/base [car bar]))) #rx"later bound differently"))
+
+;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
 (report-errs)

racket/src/expander/expand/module.rkt

@@ -369,7 +369,7 @@
                                            #:mpis-to-reset mpis-to-reset)))
 
      ;; Check that any tentatively allowed reference at phase >= 1 is ok
-     (check-defined-by-now need-eventually-defined self ctx)
+     (check-defined-by-now need-eventually-defined self ctx requires+provides)
      
      ;; - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
      ;; Pass 3: resolve provides at all phases
@@ -790,7 +790,7 @@
                                                       #:requires+provides requires+provides
                                                       #:in exp-body
                                                       #:as-transformer? #t))
-          (add-defined-syms! requires+provides syms phase)
+          (add-defined-syms! requires+provides syms phase #:as-transformer? #t)
           ;; Expand and evaluate RHS:
           (define-values (exp-rhs parsed-rhs vals)
             (expand+eval-for-syntaxes-binding (m 'rhs) ids
@@ -1019,17 +1019,27 @@
        (cons exp-body
              (loop tail? rest-bodys)))])))
 
-(define (check-defined-by-now need-eventually-defined self ctx)
+(define (check-defined-by-now need-eventually-defined self ctx requires+provides)
   ;; If `need-eventually-defined` is not empty, report an error
   (for ([(phase l) (in-hash need-eventually-defined)])
     (for ([id (in-list l)])
       (define b (resolve+shift id phase))
-      ;; FIXME: check that the binding is for a variable
-      (unless (and b
-                   (module-binding? b)
-                   (eq? (module-binding-sym b) (syntax-e id))
-                   (eq? (module-binding-module b) self))
-        (raise-syntax-error #f "reference to an unbound identifier"
+      (define bound-here? (and b
+                               (module-binding? b)
+                               (eq? (module-binding-sym b) (syntax-e id))
+                               (eq? (module-binding-module b) self)))
+      (define bound-kind (and bound-here?
+                              (defined-sym-kind requires+provides (module-binding-sym b) phase)))
+      (unless (eq? bound-kind 'variable)
+        (raise-syntax-error #f
+                            (string-append
+                             (cond
+                               [(not b) "reference to an unbound identifier"]
+                               [(eq? bound-kind 'transformer) "identifier treated as a variable, but later defined as syntax"]
+                               [else "identifier treated as a variable, but later bound differently"])
+                             (format "\n  at phase: ~a" (case phase
+                                                          [(1) "1; the transformer environment"]
+                                                          [else phase])))
                             id #f null
                             (syntax-debug-info-string id ctx))))))
 

racket/src/expander/expand/require+provide.rkt

@@ -30,6 +30,7 @@
          remove-required-id!
          check-not-defined
          add-defined-syms!
+         defined-sym-kind
          extract-module-requires
          extract-module-definitions
          extract-all-module-requires
@@ -48,7 +49,7 @@
                            require-mpis-in-order ; require-phase -> list of module-path-index
                            requires   ; mpi [interned] -> require-phase -> sym -> list-ish of [bulk-]required
                            provides   ; phase -> sym -> binding or protected
-                           phase-to-defined-syms ; phase -> sym -> boolean
+                           phase-to-defined-syms ; phase -> sym -> (or/c 'variable 'transformer)
                            also-required ; sym -> binding
                            [can-cross-phase-persistent? #:mutable]
                            [all-bindings-simple? #:mutable]) ; tracks whether bindings are easily reconstructed
@@ -397,14 +398,20 @@
                           (remove-non-matching-requireds reqds id phase mpi nominal-phase (syntax-e id))))])
           #f])])]))
 
-(define (add-defined-syms! r+p syms phase)
+(define (add-defined-syms! r+p syms phase #:as-transformer? [as-transformer? #f])
   (define phase-to-defined-syms (requires+provides-phase-to-defined-syms r+p))
   (define defined-syms (hash-ref phase-to-defined-syms phase #hasheq()))
   (define new-defined-syms
     (for/fold ([defined-syms defined-syms]) ([sym (in-list syms)])
-      (hash-set defined-syms sym #t)))
+      (hash-set defined-syms sym (if as-transformer? 'transformer 'variable))))
   (hash-set! phase-to-defined-syms phase new-defined-syms))
 
+;; Returns 'variable, 'transformer, or #f
+(define (defined-sym-kind r+p sym phase)
+  (define phase-to-defined-syms (requires+provides-phase-to-defined-syms r+p))
+  (define defined-syms (hash-ref phase-to-defined-syms phase #hasheq()))
+  (hash-ref defined-syms sym #f))
+
 ;; Get all the bindings imported from a given module
 (define (extract-module-requires r+p mod-name phase)
   (define mpi (intern-mpi r+p mod-name))

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

@@ -25,7 +25,7 @@
           all-scopes-stx  ; scopes like the initial import, which correspond to original forms
           * use-site-scopes ; #f or boxed list: scopes that should be pruned from binders
           defined-syms    ; phase -> sym -> id; symbols picked for bindings
-          * frame-id        ; #f or a gensym to identify a binding frame; 'all matches any for use-site scopes
+          * frame-id      ; #f or a gensym to identify a binding frame; 'all matches any for use-site scopes
           counter         ; box of an integer; used for generating names deterministically
           lift-key        ; identifies (via `syntax-local-lift-context`) a target for lifts
           )) ; after adding a field, update `copy-module-context` in "context.rkt"