fix #25976: treat proc-type forbids as an empty tag set (#25980)

fix #25976

Initialize tagEffects for proc types that declare .forbids but omit
.tags,
so they behave like explicit tags: [] during indirect-call effect
tracking.
Add a regression for the nested callback assignment case.

compiler/sempass2.nim

@@ -1784,13 +1784,18 @@ proc setEffectsForProcType*(g: ModuleGraph; t: PType, n: PNode; s: PSym = nil) =
     elif s != nil and (s.magic != mNone or {sfImportc, sfExportc} * s.flags == {sfImportc}):
       effects[exceptionEffects] = newNodeI(nkArgList, effects.info)
 
+    let forbidsSpec = effectSpec(n, wForbids)
     let tagsSpec = effectSpec(n, wTags)
     if not isNil(tagsSpec):
       effects[tagEffects] = tagsSpec
+    elif not isNil(forbidsSpec):
+      # `.forbids` without `.tags` still declares a known empty tag set.
+      # Leaving this as nil would mean "unknown tags", which later widens
+      # indirect calls to `RootEffect`.
+      effects[tagEffects] = newNodeI(nkArgList, effects.info)
     elif s != nil and (s.magic != mNone or {sfImportc, sfExportc} * s.flags == {sfImportc}):
       effects[tagEffects] = newNodeI(nkArgList, effects.info)
 
-    let forbidsSpec = effectSpec(n, wForbids)
     if not isNil(forbidsSpec):
       effects[forbiddenEffects] = forbidsSpec
     elif s != nil and (s.magic != mNone or {sfImportc, sfExportc} * s.flags == {sfImportc}):

tests/effects/tissue25976.nim

@@ -0,0 +1,28 @@
+type
+  NestedPoll = object of RootEffect
+
+  CallbackFunc = proc(arg: pointer) {.gcsafe, raises: [], forbids: [NestedPoll].}
+  TaggedCallbackFunc = proc(arg: pointer) {.gcsafe, raises: [], tags: [], forbids: [NestedPoll].}
+
+  InternalAsyncCallback = object
+    fn: CallbackFunc
+
+  TaggedInternalAsyncCallback = object
+    fn: TaggedCallbackFunc
+
+proc closeSocket(aftercb: CallbackFunc = nil) =
+  proc continuation(udata: pointer) =
+    aftercb(nil)
+
+  let acb = InternalAsyncCallback(fn: continuation)
+  discard acb
+
+proc closeSocketTagged(aftercb: TaggedCallbackFunc = nil) =
+  proc continuation(udata: pointer) =
+    aftercb(nil)
+
+  let acb = TaggedInternalAsyncCallback(fn: continuation)
+  discard acb
+
+closeSocket()
+closeSocketTagged()