Union of generic record types as types of function argument inside a record produces an internal compiler error

[FractalU]

An internal compiler error is being produced when an union of generic record types is a type of a function argument whose function is inside a record definition. This can be illustrated with the following example code.

local record A<T>
	a: T
end

local record B<T>
	b: T
end

local record C<T>
	f: function<T>(A<T> | B<T>)
end

When trying to run or check this code the following error message gets produced.

lua: C:\Lua\src\tl-0.24.1\/tl.lua:7304: Internal Compiler Error: error creating fresh type vari
ables
stack traceback:
        [C]: in function 'assert'
        C:\Lua\src\tl-0.24.1\/tl.lua:7304: in function <C:\Lua\src\tl-0.24.1\/tl.lua:7296>
        (...tail calls...)
        C:\Lua\src\tl-0.24.1\/tl.lua:4681: in upvalue 'recurse_type'
        C:\Lua\src\tl-0.24.1\/tl.lua:4634: in upvalue 'recurse_type'
        C:\Lua\src\tl-0.24.1\/tl.lua:4675: in upvalue 'recurse_type'
        C:\Lua\src\tl-0.24.1\/tl.lua:4914: in local 'fn'
        C:\Lua\src\tl-0.24.1\/tl.lua:4954: in upvalue 'recurse'
        C:\Lua\src\tl-0.24.1\/tl.lua:4812: in local 'fn'
        C:\Lua\src\tl-0.24.1\/tl.lua:4954: in upvalue 'recurse'
        C:\Lua\src\tl-0.24.1\/tl.lua:4747: in local 'fn'
        C:\Lua\src\tl-0.24.1\/tl.lua:4954: in function <C:\Lua\src\tl-0.24.1\/tl.lua:4929>
        (...tail calls...)
        C:\Lua\src\tl-0.24.1\/tl.lua:13311: in function 'tl.check'
        C:\Lua\src\tl-0.24.1\/tl.lua:13447: in function 'tl.check_string'
        (...tail calls...)
        C:\Lua\src\tl-0.24.1\tl:239: in upvalue 'process_module'
        C:\Lua\src\tl-0.24.1\tl:739: in field '?'
        C:\Lua\src\tl-0.24.1\tl:977: in main chunk
        [C]: in ?

When I slightly modify this example so that the function isn't inside a record and instead a standalone function then after running or checking the modified example code a proper error message gets produced. Below the modified example.

local record A<T>
	a: T
end

local record B<T>
	b: T
end

local function f<T>(a: A<T> | B<T>)
	print(a)
end

f(nil)

And the associated error message.

1 error:
union.tl:9:24: cannot discriminate a union between multiple table types: A<T> | B<T>
----------------------------------------
1 error

Note. The f(nil) line in the modified example code is to suppress a warning.

Originally, I encountered this error with trying to modify functional.d.tl. See https://github.com/wqferr/functional/blob/master/functional.d.tl. I tried to prune the Iterator<T> record by merging duplicate function definitions into one with function argument types such as {T} | IIterator<T>. I later realized that it isn't possible because teal cannot discriminate union of multiple table types at runtime. But by loading the modified functional.d.tl file, I encountered this strange internal compiler error. I managed to reduce it to this simple example code.

[hishamhm]

Thank you for the report and especially for the minimized test case!!