Added runtime check for odd condition where user may target vthreads but then run on a non-vthreads jvm.

Author: fogus

src/main/clojure/clojure/core/async.clj

@@ -515,7 +515,11 @@ use vthreads in io-thread when available
   completed"
   [& body]
   (if (or (dispatch/aot-vthreads?) (dispatch/runtime-vthreads?))
-    `(thread-call (^:once fn* [] ~@body) :io)
+    `(if (and (= dispatch/compiled-vthreads-flag "target") (not (dispatch/runtime-vthreads?)))
+       (throw (ex-info "Code compiled to target virtual threads, but is running on a JVM without vthread support."
+                       {:compiled-vthreads-flag dispatch/compiled-vthreads-flag
+                        :runtime-jvm-version (System/getProperty "java.version")}))
+       (thread-call (^:once fn* [] ~@body) :io))
     ((find-var 'clojure.core.async.impl.go/go-impl) &env body)))
 
 (defonce ^:private thread-macro-executor nil)

src/main/clojure/clojure/core/async/impl/dispatch.clj

@@ -85,14 +85,18 @@
   []
   (System/getProperty "clojure.core.async.vthreads"))
 
+(def compiled-vthreads-flag (vthreads-directive))
+
 (defn aot-vthreads? []
   (and clojure.core/*compile-files*
        (= (vthreads-directive) "target")))
 
-(defn runtime-vthreads? []
-  (and (not clojure.core/*compile-files*)
-       (not= (vthreads-directive) "avoid")
-       @virtual-threads-available?))
+(def runtime-vthreads?
+  (memoize
+   (fn []
+     (and (not clojure.core/*compile-files*)
+          (not= (vthreads-directive) "avoid")
+          @virtual-threads-available?))))
 
 (defn- make-io-executor
   []