Generic parameters constrained to unions of tuples are not inferred correctly in rest parameter positions

[jcalz]

TypeScript Version: 3.1.0-dev.20180813

Search Terms:
Tuple rest generic union

Code

declare function hmm<A extends [] | [number, string]>(...args: A): void;
hmm(); // okay, A = []
hmm(1, "s"); // okay, A = [1, "s"]
hmm("what"); // no error? A = [] | [number, string] ?

Expected behavior:
Error on hmm("what"). A should be inferred as [string], which does not extend [] | [number, string].

Actual behavior:
No error on hmm("what"). Apparently A is inferred as the constraint type [] | [number, string] despite that not matching the actual argument type.

Playground Link:
Link

Related Issues:
#24897: introduction of tuples into rest/spread expressions
#26110: similar issue with bad or weird inference behavior on rest arguments

[RyanCavanaugh]

Anders Hejlsberg (@ahejlsberg) I'm probably missing something here but this seems bad