Use "best choice type" for || and ?: operators#10069
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This PR introduces a best choice type for the
||and?:operators. Given two types T1 and T2, the best choice type of T1 and T2 is determined as follows:The first two steps are the ones introduced by this PR. They make the operators slightly less likely to produce a union type than they were before. In particular, if T2 is a subtype of T1 except for optional properties in T1 that are missing in T2, we will now pick T1 instead of producing the union type T1 | T2.
Fixes #10041.