mjhjmtu

I have a problem with the following code under TexLive 2018:

documentclassreport

usepackagecalc
usepackage[inline]enumitem
usepackageparskip

newdimenlblwidthlblwidth=widthof8.88. 

newlengthtestb
setlengthtestbwidthof8.88. 


begindocument

chapterExercises

beginenumerate[labelindent=0pt,labelwidth=widthof8.88. ,label=textbfthechapter.arabic*.,leftmargin=!,ref=thechapter.arabic*]
item Hallo
endenumerate

enddocument

The newdimen produces an error on my system while the setlength doesn't:

Part of logfile:

lblwidth=dimen106

! Missing number, treated as zero.
<to be read again> 
 widthof 
l.7 newdimenlblwidthlblwidth=widthof
 8.88. 
A number should have been here; I inserted `0'.
(If you can't figure out why I needed to see a number,
look up `weird error' in the index to The TeXbook.)

! Illegal unit of measure (pt inserted).
<to be read again> 
 widthof 
l.7 newdimenlblwidthlblwidth=widthof
 8.88. 
Dimensions can be in units of em, ex, in, pt, pc,
cm, mm, dd, cc, nd, nc, bp, or sp; but yours is a new one!
I'll assume that you meant to say pt, for printer's points.
To recover gracefully from this error, it's best to
delete the erroneous units; e.g., type `2' to delete
two letters. (See Chapter 27 of The TeXbook.)


! LaTeX Error: Missing begindocument.

See the LaTeX manual or LaTeX Companion for explanation.
Type H <return> for immediate help.
 ... 

l.7 newdimenlblwidthlblwidth=widthof8
 .88. 
You're in trouble here. Try typing <return> to proceed.
If that doesn't work, type X <return> to quit.

testb=skip47
(./widthof.aux)

Changing newdimen to newskip doesn't solve the problem.

settowidthlblwidth8.88. has the correct LaTeX syntax and doesn't even require calc.

If you insist in using calc, you must use setlength:

setlengthlblwidthwidthof8.88. 

Why is this? The primitive assignment lblwidth= requires a legal dimension after the =, which widthof isn't.

By the way, the meaning of widthof is ignorespaces and only in legal calc expressions it becomes really useful. The calc package modifies setlength so that it can understand calc expressions. The primitive assignment cannot be handled this way.