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>).

[svermeulen]

I keep getting a similar problem:

local record Foo
   metamethod __add: function(Foo, Foo): Foo
end

local record Bar
end

local foo:Foo
local bar:Bar

-- Teal allows this
local result:any = foo + bar

print(result)

This only results in runtime errors but would be great if it was a compiler error instead. I would expect the add method to not be allowed since Foo explicitly requires Foo type for it

[hishamhm]

@svermeulen This looks like a different issue. Could you open a separate ticket? Thanks!!

Edit: also, I tried your example above in v0.15.3, master and next, and all three branches reported

========================================
1 error:
633.tl:12:26: argument 2: Bar is not a Foo

...so I assume you typed this from memory. But if you could come up with a minimal repro that triggers the problem you're facing at operator evaluation, that would help a lot! But let's do it in a separate issue to avoid burying the original topic of this one. Thanks!!

[svermeulen]

@hishamhm I upgraded from 0.15.2+dev to 0.15.3+dev and it is now being detected properly. Thank you!