allow (-128 : Int8)

[ggreif]

PR is being prepared: #638.

https://dfinity.atlassian.net/browse/AST-89

#487 (as delivered) has a small deficiency, namely the newly introduced Int* types fail to typecheck the lowest legal (negative) value. I.e. -128 : Int8 is rejected. The reason is, that - is an operator - : a -> a, and thus there is an intermediate 128 : Int8 being type-checked, which fails with range violation.

So, for checking mode, the operator - : Int8 -> Int8 in a compositional context causes the described problem, which might confuse AS users.

Other Nat* behave similarly.

An analogous problem arises at the pattern side.

Referencing the Slack discussion thread here for the record:

https://dfinity.slack.com/archives/CCK4B0XE3/p1560273268058800

[nomeata]

Let me ask the obvious question: can we make the parser parse signed literals?

[ggreif]

When fixing this, also revert the workaround in the fuzzer! grep for actorscript/issues/505.

[ggreif]

Here is an idea. When type checking a unary operator (these always have fixed semantics), always try to abstractly interpret the result first. When it evaluates to a constant, then check that. If not, use the current algorithm to check the original AST compositionally.

Pros: no need to change the parser
Cons: introduces a slight non-compositionality into the typechecker, and needs a helper Ast -> Ast option for simplification. This guy then can be used in IR-lowering too.

Of course this won't work. (I have coded up something in https://github.com/dfinity-lab/actorscript/tree/gabor/issue-505-take1.) Consider (---128) : Int8. This would abstractly evaluate to Some (LitE (PreLit (-128) Int)) and thus pass the type checker. But at runtime the unary expression --128 should trap. Eating only the innermost negation operator as a sign might be a remedy.

[nomeata]

Pinging @rossberg as our front-end type-checker guru

[ggreif]

The other approach I came up with (https://github.com/dfinity-lab/actorscript/tree/gabor/issue-505-take2) fixes the problem by treating the innermost NegOp around a literal as a sign.