Jest: expect('null').toContain(null) should fail

[jeysal]

In https://github.com/facebook/jest/pull/8008#discussion_r261748762, we discovered that expect('asdf null asdf').toContain(null) passes and agreed that it shouldn't. This breaking change should be included in the next major. @SimenB

[pedrottimark]

Let’s also remember https://github.com/facebook/jest/pull/8008#issue-257184687

Breaking change to require .exec method in addition to .test method for RegExp

EDIT also for .toThrow and .toThrowError see https://github.com/facebook/jest/pull/8033#issue-257769410

Or to get to the root of the problem, see https://github.com/facebook/jest/pull/8033#discussion_r

[pedrottimark]

Please critique 3 possible matcher error messages. Or suggest your own Goldilocks message :)

Baby bear is direct parallel with other error messages:

Mama bear makes the conditional requirement explicit:

Papa bear also displays received value because the message refers to it:

[jeysal]

I vote 3 ("Papa bear").
This is the only really helpful message in case received turns out to be a string when the user actually thought it was an array and intended to use the array version of toContain.

Also, TIL where the term Goldilocks zone originates 😅