fixes #20846; warn on overloaded =copy with refc

compiler/condsyms.nim

@@ -154,3 +154,4 @@ proc initDefines*(symbols: StringTableRef) =
   defineSymbol("nimHasGenericDefine")
   defineSymbol("nimHasDefineAliases")
   defineSymbol("nimHasWarnBareExcept")
+  defineSymbol("nimHasWarnCopyHookForRefc")

compiler/lineinfos.nim

@@ -88,6 +88,7 @@ type
     warnUnnamedBreak = "UnnamedBreak",
     warnStmtListLambda = "StmtListLambda",
     warnBareExcept = "BareExcept",
+    warnCopyHookForRefc = "CopyHookForRefc",
     warnUser = "User",
     # hints
     hintSuccess = "Success", hintSuccessX = "SuccessX",
@@ -189,6 +190,7 @@ const
     warnUnnamedBreak: "Using an unnamed break in a block is deprecated; Use a named block with a named break instead",
     warnStmtListLambda: "statement list expression assumed to be anonymous proc; this is deprecated, use `do (): ...` or `proc () = ...` instead",
     warnBareExcept: "$1",
+    warnCopyHookForRefc: "Overriding `=copy` hook is not reliable for refc",
     warnUser: "$1",
     hintSuccess: "operation successful: $#",
     # keep in sync with `testament.isSuccess`

compiler/nim.cfg

@@ -46,3 +46,7 @@ define:useStdoutAsStdmsg
 @if nimHasWarnBareExcept:
   warningAserror[BareExcept]:on
 @end
+
+@if nimHasWarnCopyHookForRefc:
+  warningAserror[CopyHookForRefc]:on
+@end

compiler/semstmts.nim

@@ -1857,6 +1857,8 @@ proc semOverride(c: PContext, s: PSym, n: PNode) =
     incl(s.flags, sfOverriden)
     if name == "=":
       message(c.config, n.info, warnDeprecated, "Overriding `=` hook is deprecated; Override `=copy` hook instead")
+    elif name == "=copy" and c.config.selectedGC == gcRefc:
+      message(c.config, n.info, warnCopyHookForRefc)
     let t = s.typ
     if t.len == 3 and t[0] == nil and t[1].kind == tyVar:
       var obj = t[1][0]