fixes #1551

compiler/semexprs.nim

@@ -1614,6 +1614,11 @@ proc instantiateCreateFlowVarCall(c: PContext; t: PType;
   initIdTable(bindings)
   bindings.idTablePut(sym.ast[genericParamsPos].sons[0].typ, t)
   result = c.semGenerateInstance(c, sym, bindings, info)
+  # since it's an instantiation, we unmark it as a compilerproc. Otherwise
+  # codegen would fail:
+  if sfCompilerProc in result.flags:
+    result.flags = result.flags - {sfCompilerProc, sfExportC, sfImportC}
+    result.loc.r = nil
 
 proc setMs(n: PNode, s: PSym): PNode = 
   result = n

lib/pure/concurrency/threadpool.nim

@@ -95,7 +95,7 @@ type
 
   FlowVarBase* = ref FlowVarBaseObj ## untyped base class for 'FlowVar[T]'
   FlowVarBaseObj = object of TObject
-    ready, usesCondVar: bool
+    ready, usesCondVar, awaited: bool
     cv: CondVar #\
     # for 'awaitAny' support
     ai: ptr AwaitInfo
@@ -129,8 +129,8 @@ type
 proc await*(fv: FlowVarBase) =
   ## waits until the value for the flowVar arrives. Usually it is not necessary
   ## to call this explicitly.
-  if fv.usesCondVar:
-    fv.usesCondVar = false
+  if fv.usesCondVar and not fv.awaited:
+    fv.awaited = true
     await(fv.cv)
     destroyCondVar(fv.cv)