why := ImplementsExplain(n.Type(), t)
		if why != "" {
			base.Fatalf("impossible type assertion:\n\t%s", why)
			n.SetType(nil)
			return n
		}
	}
	return n
}

// tcITab typechecks an OITAB node.
func tcITab(n *ir.UnaryExpr) ir.Node {
	n.X = Expr(n.X)
	t := n.X.Type()
	if t == nil {
		n.SetType(nil)
		return n
	}
	if !t.IsInterface() {
		base.Fatalf("OITAB of %v", t)
	}

		n := n.(*ir.BinaryExpr)
		return tcUnsafeString(n)

	case ir.OUNSAFESTRINGDATA:
		n := n.(*ir.UnaryExpr)
		return tcUnsafeData(n)

	case ir.OITAB:
		n := n.(*ir.UnaryExpr)
		return tcITab(n)

	case ir.OIDATA:
		// Whoever creates the OIDATA node must know a priori the concrete type at that moment,
		// usually by just having checked the OITAB.
		n := n.(*ir.UnaryExpr)