Coercion to string does not call metamethod (Was: Lua minus null (-0)

[Dirk Laurie <dpl@...>]

On Thu, Jun 09, 2011 at 12:15:37PM +0200, David Kastrup wrote:
> 
> lua
> Lua 5.1.4  Copyright (C) 1994-2008 Lua.org, PUC-Rio
> > debug.setmetatable(0, {__tostring = 
> >>     function(n) local s=string.format("%.14g",n) 
> >>     if s=='-0' then s='0' end return s 
> >>     end})
> > print(-0)
> 0
> > print("Oh"..-0)
> Oh-0

Very interesting!  We're back to that other old favourite, implicit
conversion for concatenation, which bypasses the metatable.  It's not 
the fault of debug.setmetatable, you also get it with the non-debug one:

> t={}
> setmetatable(t, {__tostring = function(self) return "table t" end})
> =t
table t
> ="show "..t
stdin:1: attempt to concatenate global 't' (a table value)
stack traceback:
    stdin:1: in main chunk
    [C]: in ?

Is that a bug?

Dirk