Five static errors in /lang/rascal/tests/basic/Maps.rsc, mostly, related to empty maps

[PaulKlint]

error("Product not defined on void and void",|project://rascal/src/org/rascalmpl/library/lang/rascal/tests/basic/Maps.rsc|(915,3,<26,32>,<26,35>))
error("map[int, int] and map[str, str] incomparable",|project://rascal/src/org/rascalmpl/library/lang/rascal/tests/basic/Maps.rsc|(1728,10,<49,8>,<49,18>))
error("map[int, int] and map[str, str] incomparable",|project://rascal/src/org/rascalmpl/library/lang/rascal/tests/basic/Maps.rsc|(2819,10,<85,10>,<85,20>))
error("value must be a subtype of void",|project://rascal/src/org/rascalmpl/library/lang/rascal/tests/basic/Maps.rsc|(626,8,<20,41>,<20,49>))
error("map[int, int] and map[real, int] incomparable",|project://rascal/src/org/rascalmpl/library/lang/rascal/tests/basic/Maps.rsc|(3020,18,<94,24>,<94,42>))

[jurgenvinju]

• the first message is very interesting. The type checker is right because truly the whole expression does not have any possible meaning. It sees the list is empty and because emptiness is represented first class in the type system by void it can compute that the multiplication does not make sense. I believe this is great and we should make the test more complex to make the type checker not able to see this.
• the second message is a correct static error afaik; this is where the type checker can help if types are incomparable a comparison between values does not make sense (i.e. is either tautological or contradictory).
• same for the third
• the fourth message is the same story as the first message
• the fifth is the same story as the second

I propose we:

• rewrite the tests such that the type checker can't find the tautologies
• invest in the future to explain these tautologies better in the error message

[jurgenvinju]

Either that or we define that void is defined on all operators, which is also a valid static interpretation and will hide the first and the fourth message but not the second, third and fifth.