Issue #4004 AssertionError from Range.gap.

[Koooooo-7]

Fix #4004, Got IAE instead of AssertionError.

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[cpovirk]

Thanks. I suspect that what we'd prefer is to avoid calling describeAsLowerBound at all in this case. But I'd have to look at gap more to confirm (and to figure out what to do instead).

[Koooooo-7]

I think the problem is the ranges contain each other(intersection), it is illegal for gap.
and I made an opposite case.

Range.atMost(3).gap(Range.atMost(2));

java.lang.AssertionError
	at com.google.common.collect.Cut$BelowAll.describeAsUpperBound(Cut.java:165)
	at com.google.common.collect.Range.toString(Range.java:677)
	at com.google.common.collect.Range.<init>(Range.java:358)
	at com.google.common.collect.Range.create(Range.java:156)
	at com.google.common.collect.Range.gap(Range.java:583)

so, I added the ranges check for the gap.
PTAL @cpovirk .

[cpovirk]

Sounds reasonable, thanks. Would you also:

• Add your tests for the AssertionError cases

• include the 2 input ranges in the exception message

[Koooooo-7]

@cpovirk I added the test and the invalid ranges in the IAE message.

[cpovirk]

Thanks. I got another set of eyes on this, and hopefully we'll get it submitted internally and merged out soon.