[J3] Question on overflow and run-time detection of infinity?

[Van Snyder]

On Tue, 2018-11-27 at 11:13 +0900, Malcolm Cohen via J3 wrote:
> You can see this by the wording which allows
> IEEE_VALUE(x,IEEE_SIGNALING_NAN) to raise INVALID and have the
> signalling NaN converted to a quiet NaN.

Since it's OK for a processor to supply a quiet NaN when a signaling NaN
is requested, 

  x = IEEE_VALUE(x,IEEE_SIGNALING_NAN)

might not assign a signaling NaN to X.  How does one reliably fill a
real variable (scalar or otherwise) with signaling NaN?