Metamethod type not checked in metatable definition

[bjornbm]

Hi, is the below code supposed to pass the typechecker? (The type of __add in rec_mt does not have the correct type for metamethod __add in Rec.)

local type Rec = record
   metamethod __add: function(Rec, Rec): Rec
end

local rec_mt: metatable<Rec>
rec_mt = {
   __add = function(_a: Rec, _b: number): number  -- Type error??
      return 1
   end,
}

local r: Rec = setmetatable({ }, rec_mt)
print(r + r)      -- prints 1

What about this one? (__add is not defined in rec_mt)

local type Rec = record
   metamethod __add: function(Rec, Rec): Rec
end

local rec_mt: metatable<Rec> = {}  -- __add is not defined

local r: Rec = setmetatable({ }, rec_mt)

[hishamhm]

The second case (__add is not defined in rec_mt) is unlikely to be supported soon (the compiler currently does not do nil checks in general), but the first one sounds like it should have been checked — (note to self: this may require some adding custom code for checking setmetatable arguments, similarly to how we handle pcall, etc.).

[bjornbm]

OK, so my understanding for the second case: the record definition defines what keys/values are valid, but not that they are defined (or more generally perhaps the values may be nil, since nil is a valid value of every type). What is checked is that values for the defined keys have the right type, and that no other keys are added to the record.

[bjornbm]

For the first case, I guess the safest place to check is at setmetatable.

local r: Rec = setmetatable({ }, rec_mt)

I assume the inferred type of { } here is Rec, thanks to the assignment to r: Rec? And thus rec_mt must be metatable<Rec>, which in turn should satisfy the metamethods of Rec?

You could (should?) also check at the assignment to rec_mt that it satisfies the metamethods of Rec, since rec_mt is declared as a metatable<Rec>.

Currently it seems the metatable<Rec> type does nothing at all:

1. The contents of rec_mt are not checked versus Recs metamethods.
2. setmetatable does not check that rec_mt is a metatable<Rec> (it will accept any table, including of type metatable<SomeOtherType>).