False type mismatch error using union types

[timothee-haudebourg]

Consider the following TreeLDR file:

use <http://www.example.com/> as ex;

type A {
	ex:prop: A | &A
	//       ^^^^^^ this is _:0
}

type B {
	ex:prop: A | &A
	//       ^^^^^^ this is _:1
}

The compiler raises an error because the two occurences of the same union type are given different blank node identifiers, so there is a type mismatch here. There are two issues at play here:

• Semanticaly equivalent blank nodes are merged too late. They should be merged before the type checking phase.
• A | &A is considered to be a union type even though it is in fact the type A with a different layout. If the union was properly erased, the previous issue should not interfere.