Make ?-expander? allow any value, so that (~var exp (static foo-expander? "a foo expander")) can safely be used with syntax-parse.

private/define-expanders.rkt

@@ -27,7 +27,8 @@
                            (define-for-syntax (make-?-expander transformer)
                              (expander ?-expander-type transformer))
                            (define-for-syntax (?-expander? v)
-                             (expander-of-type? ?-expander-type v))
+                             (and (expander? v)
+                                  (expander-of-type? ?-expander-type v)))
                            (define-syntax define-?-expander
                              (syntax-parser
                                [(_ expander-name:id transformer:expr)

test/test-foo-mixin-expander-predicate.rkt

@@ -0,0 +1,61 @@
+#lang racket
+
+(require generic-syntax-expanders
+         (for-syntax syntax/parse
+                     rackunit))
+(require (for-syntax generic-syntax-expanders))
+(define-expander-type foo)
+(define-expander-type other)
+(define-foo-expander foo-exp (λ (stx) #''foo-exp-is-a-foo-expander))
+(define-other-expander other-exp (λ (stx) #''other-exp-is-not-a-foo-expander))
+(define-syntax not-an-expander 'syntax-local-value-is-not-an-expander)
+(begin-for-syntax
+  (test-not-exn
+   "Check that foo-expander? can be passed any value, not just an expander?"
+   (λ ()
+     (foo-expander? 123)
+     (void)))
+  
+  (test-false
+   "Check that (static foo-expander?) rejects syntax that is not an identifier?"
+   (syntax-parse #'(definitely not-a-foo-expander)
+     [(~var exp (static foo-expander? "a foo expander")) #t]
+     [_ #f]))
+  
+  (test-false
+   "Check that (static foo-expander?) rejects an id without syntax-local-value"
+   (syntax-parse #'no-syntax-local-value
+     [(~var exp (static foo-expander? "a foo expander")) #t]
+     [_ #f]))
+
+  (test-begin
+   (test-false
+    "Check that foo-expander? rejects an id which is not an expander?"
+    (foo-expander? (syntax-local-value #'not-an-expander)))
+   (test-false
+    "Check that foo-expander? rejects an id which is not an expander?"
+    (syntax-parse #'not-an-expander
+      [(~var exp (static foo-expander? "a foo expander")) #t]
+      [_ #f])))
+
+  (test-begin
+   (test-false
+    (string-append "Check that foo-expander? rejects an id which is an"
+                   " expander? but not a foo-expander?")
+    (foo-expander? (syntax-local-value #'other-exp)))
+   (test-false
+    (string-append "Check that foo-expander? rejects an id which is an"
+                   " expander? but not a foo-expander?")
+    (syntax-parse #'other-exp
+      [(~var exp (static foo-expander? "a foo expander")) #t]
+      [_ #f])))
+
+  (test-begin
+   (test-true
+    "Check that foo-expander? accepts an id which is a foo-expander?"
+    (foo-expander? (syntax-local-value #'foo-exp)))
+   (test-true
+    "Check that foo-expander? accepts an id which is a foo-expander?"
+    (syntax-parse #'foo-exp
+      [(~var exp (static foo-expander? "a foo expander")) #t]
+      [_ #f]))))