Generic record values cannot be used as generic interface type

[svermeulen]

Example below:

-- Not using generics works as expected:

-- local interface IFoo
--    get_value: function(self): integer
-- end

-- local record Foo is IFoo
--    _value: integer
-- end

-- function Foo:get_value():integer
--    return self._value
-- end

-- function Foo.new(value: integer):Foo
--    local fields = { _value = value }
--    return setmetatable(fields, { __index = Foo })
-- end

-- local function create_foo(value: integer):IFoo
--    local foo = Foo.new(value)
--    return foo
-- end

------------------------

-- But with generics we get error:
-- "in return value: got Foo<T>, expected interface IFoo"
-- in create_foo function

local interface IFoo<T>
   get_value: function(self): T
end

local record Foo<T> is IFoo<T>
   _value: T
end

function Foo:get_value():T
   return self._value
end

function Foo.new(value: T):Foo<T>
   local fields = { _value = value }
   return setmetatable(fields, { __index = Foo })
end

local function create_foo<T>(value: T):IFoo<T>
   local foo = Foo.new(value)
   return foo

   -- Have to do this instead for now:
   -- return foo as IFoo<T>
end

------------------------

local foo = create_foo(5)
print(foo:get_value())