Fix #10124 by adding check for undefined behaviour

[simonbyrne]

Fixes #10124, also adds explanation of logic. Checked on ARM machine.

cc: @eschnett, @nkottary, @ViralBShah

[ViralBShah]

Looks like this is failing the whitespace test.

[simonbyrne]

Updated. Is there an easy way to add the whitespace check as a pre-commit hook?

[simonbyrne]

Ah, figured it out:

cp .git/hooks/pre-commit.sample .git/hooks/pre-commit

[vtjnash]

is this overly conservative?

+# - typemax(Ti) can't be exactly represented by Tf:

for example, typemax(Int32) can be exactly represented in Float64

[simonbyrne]

Yes, these methods would not be valid in that case (note that Int32 vs Float64 are defined elsewhere).

The extra condition that I've added (fy != $(Tf(typemax(Ti)))) assumes that Tf(typemax(Ti))) involves some sort of rounding (either to 2^n or +Inf).

[yuyichao]

Hmmm, I suspect this PR has turned a similar test failure on AArch64 into an abort. Not exactly sure what happens yet trying to get a backtrace.

[yuyichao]

Actually the failing test passes and it is probably something else going wrong.