Fixes #5738 (#6059)

tests/async/tasync_gcunsafe.nim

@@ -0,0 +1,30 @@
+discard """
+  cmd: "nim c --threads:on $file"
+  file: "asyncmacro.nim"
+  errormsg: "'anotherGCSafeAsyncProcIter' is not GC-safe as it calls 'asyncGCUnsafeProc'"
+"""
+
+assert compileOption("threads"), "this test will not do anything useful without --threads:on"
+
+import asyncdispatch
+
+var globalDummy: ref int
+proc gcUnsafeProc() =
+    if not globalDummy.isNil:
+        echo globalDummy[]
+
+proc asyncExplicitlyGCSafeProc() {.gcsafe, async.} =
+    echo "hi"
+
+proc asyncImplicitlyGCSafeProc() {.async.} =
+    echo "hi"
+
+proc asyncGCUnsafeProc() {.async.} =
+    gcUnsafeProc()
+
+proc anotherGCSafeAsyncProc() {.async, gcsafe.} =
+    # We should be able to call other gcsafe procs
+    await asyncExplicitlyGCSafeProc()
+    await asyncImplicitlyGCSafeProc()
+    # But we can't call gcunsafe procs
+    await asyncGCUnsafeProc()