Is `x!=x` a portable way to test for NaN?

Question

In C you can test to see if a double if NaN using isnan(x). However many places online, including for example this SO answer say that you can simply use x!=x

Answer 1

Please refer to the normative section Annex F: IEC 60559 floating-point arithmetic of the C standard:

F.1 Introduction

An implementation that defines __STDC_IEC_559__ shall conform to the specifications in this annex.

Implementations that do not define __STDC_IEC_559__ are not required to conform to these specifications.

F.9.3 Relational operators

The expression x ≠ x is true if x is a NaN.

The expression x = x is false if X is a Nan.

F.3 Operators and functions

The isnan macro in provides the isnan function recommended in the Appendix to IEC 60559.