fixes #2257

compiler/ast.nim

@@ -1335,8 +1335,12 @@ proc propagateToOwner*(owner, elem: PType) =
   if elem.isMetaType:
     owner.flags.incl tfHasMeta
 
-  if owner.kind != tyProc:
-    if elem.isGCedMem or tfHasGCedMem in elem.flags:
+  if owner.kind notin {tyProc, tyGenericInst, tyGenericBody,
+                       tyGenericInvocation}:
+    let elemB = elem.skipTypes({tyGenericInst})
+    if elemB.isGCedMem or tfHasGCedMem in elemB.flags:
+      # for simplicity, we propagate this flag even to generics. We then
+      # ensure this doesn't bite us in sempass2.
       owner.flags.incl tfHasGCedMem
 
 proc rawAddSon*(father, son: PType) =

compiler/sempass2.nim

@@ -209,8 +209,7 @@ proc useVar(a: PEffects, n: PNode) =
       a.init.add s.id
   if {sfGlobal, sfThread} * s.flags == {sfGlobal} and s.kind in {skVar, skLet}:
     if s.guard != nil: guardGlobal(a, n, s.guard)
-    if (tfHasGCedMem in s.typ.flags or s.typ.isGCedMem) and 
-        tfGcSafe notin s.typ.flags:
+    if (tfHasGCedMem in s.typ.flags or s.typ.isGCedMem):
       if warnGcUnsafe in gNotes: warnAboutGcUnsafe(n)
       markGcUnsafe(a)
 

tests/parallel/tarray_of_channels.nim

@@ -0,0 +1,26 @@
+# bug #2257
+import threadpool
+
+type StringChannel = TChannel[string]
+var channels: array[1..3, StringChannel]
+
+type
+  MyObject[T] = object
+    x: T
+
+var global: MyObject[string]
+var globalB: MyObject[float]
+
+proc consumer(ix : int) {.thread.} =
+  echo channels[ix].recv() ###### not GC-safe: 'channels'
+  echo globalB
+
+proc main =
+  for ix in 1..3: channels[ix].open()
+  for ix in 1..3: spawn consumer(ix)
+  for ix in 1..3: channels[ix].send("test")
+  sync()
+  for ix in 1..3: channels[ix].close()
+
+when isMainModule:
+  main()

tests/parallel/tgc_unsafe.nim

@@ -0,0 +1,32 @@
+discard """
+  errormsg: "'consumer' is not GC-safe"
+  line: 19
+"""
+
+# bug #2257
+import threadpool
+
+type StringChannel = TChannel[string]
+var channels: array[1..3, StringChannel]
+
+type
+  MyObject[T] = object
+    x: T
+
+var global: MyObject[string]
+var globalB: MyObject[float]
+
+proc consumer(ix : int) {.thread.} =
+  echo channels[ix].recv() ###### not GC-safe: 'channels'
+  echo global
+  echo globalB
+
+proc main =
+  for ix in 1..3: channels[ix].open()
+  for ix in 1..3: spawn consumer(ix)
+  for ix in 1..3: channels[ix].send("test")
+  sync()
+  for ix in 1..3: channels[ix].close()
+
+when isMainModule:
+  main()