fixes #20515; base method requires explicit {.gcsafe.} to be GC-safe (#20574)

* fixes #20515; base requires explicit `{.gcsafe.}` to be GC-safe

* add tests

compiler/sempass2.nim

@@ -259,6 +259,11 @@ proc listGcUnsafety(s: PSym; onlyWarning: bool; cycleCheck: var IntSet; conf: Co
     of routineKinds:
       # recursive call *always* produces only a warning so the full error
       # message is printed:
+      if u.kind == skMethod and {sfBase, sfThread} * u.flags == {sfBase}:
+        message(conf, u.info, msgKind,
+          "Base method '$#' requires explicit '{.gcsafe.}' to be GC-safe" %
+          [u.name.s])
+      else:
         listGcUnsafety(u, true, cycleCheck, conf)
         message(conf, s.info, msgKind,
           "'$#' is not GC-safe as it calls '$#'" %
@@ -836,6 +841,9 @@ proc trackCall(tracked: PEffects; n: PNode) =
         discard
       var effectList = op.n[0]
       if a.kind == nkSym and a.sym.kind == skMethod:
+        if {sfBase, sfThread} * a.sym.flags == {sfBase}:
+          if tracked.config.hasWarn(warnGcUnsafe): warnAboutGcUnsafe(n, tracked.config)
+          markGcUnsafe(tracked, a)
         propagateEffects(tracked, n, a.sym)
       elif isNoEffectList(effectList):
         if isForwardedProc(a):

tests/method/t20515.nim

@@ -0,0 +1,20 @@
+discard """
+  errormsg: "Base method 'zzz' requires explicit '{.gcsafe.}' to be GC-safe"
+  line: 10
+"""
+
+type
+  A = ref object of RootObj
+  B = ref object of A
+
+method zzz(a: A) {.base.} =
+  discard
+
+var s: seq[int]
+method zzz(a: B) =
+  echo s
+
+proc xxx(someObj: A) {.gcsafe.} =
+  someObj.zzz()
+
+xxx(B())