[Bug middle-end/87247] intrinsic acosh violates 2008 Standard rule 13.7.5 line 5

[joseph at codesourcery dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87247

--- Comment #4 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
The standard branch cut for acosh (not just a C standard, but as at 
https://dlmf.nist.gov/4.37 for example) follows from the principles that 
(a) acosh(conj(x)) = conj(acosh(x)) and (b) complex acosh should take the 
same value as real acosh for those real arguments for which acosh has a 
real value.  Fixing the sign of the imaginary part of the result is 
inconsistent with (a) (which is a general principle for complex libm 
functions, not just acosh).