ref #20846; give a deprecation message for overriding = hook (#21020)

give a deprecation message for overriding `=` hook

compiler/semstmts.nim

@@ -1881,6 +1881,8 @@ proc semOverride(c: PContext, s: PSym, n: PNode) =
     if s.magic == mAsgn: return
     incl(s.flags, sfUsed)
     incl(s.flags, sfOverriden)
+    if name == "=":
+      message(c.config, n.info, warnDeprecated, "Overriding `=` hook is deprecated; Override `=copy` hook instead")
     let t = s.typ
     if t.len == 3 and t[0] == nil and t[1].kind == tyVar:
       var obj = t[1][0]

tests/deprecated/tequalhook.nim

@@ -0,0 +1,11 @@
+discard """
+  errormsg: "Overriding `=` hook is deprecated; Override `=copy` hook instead"
+  matrix: "--warningAsError[Deprecated]:on"
+"""
+
+type
+  SharedString = object
+    data: string
+
+proc `=`(x: var SharedString, y: SharedString) =
+  discard