Otdfbt

I want to append some text to a command but expand a counter while doing so. Here is an example of what the problem is:

MWE:

documentclassarticle

newcommandtopics
newcountertopic
newcommandtopic[1]stepcountertopicexpandafterdefexpandaftertopicsexpandaftertopicshangindent=0.5cm Topic #thetopic: #1par

topictest A
topictest B
topictest C

begindocument
 topics
enddocument

This produces:

Topic #3: test A
Topic #3: test B
Topic #3: test C

But it should produce:

Topic #1: test A
Topic #2: test B
Topic #3: test C

I've also tried this, to no avail:

newcommandtopic[1]stepcountertopicg@addto@macrotopicshangindent=0.5cm Topic #thetopic: #1par

You can use edef here:

newcommandtopic[1]%
 stepcountertopic%
 edeftopicsunexpandedexpandaftertopicshangindent=0.5cm
 noexpandtextbfTopic #thetopic: unexpanded#1par%

The unexpandedexpandafter... expands the current topics list once but prevents it from being fully expanded. This is important on later calls of topic to make the expansion not break other commands that shouldn't be fully expanded (see the use of textbf in my example).

Using this macros gives the desired output

documentclassarticle

newcommandtopics
newcountertopic
newcommandtopic[1]%
 stepcountertopic%
 edeftopicstopicshangindent=0.5cm Topic #thetopic: #1par

topictest A
topictest B
topictest C

begindocument
 topics
enddocument

Your macro works fine if you move the stepcounter after the expansion of topics:

documentclassarticle

deftopics
newcountertopic

deftopics
deftopic#1%
 expandafterdefexpandaftertopicsexpandaftertopics 
 stepcountertopic 
 hangindent=0.5cm
 Topic #thetopic: #1par

topictest A
topictest B
topictest C

begindocument
 topics
enddocument

Since topics is only expanded once, you can have al kind of macros in it and in the argument of topic.

In case you both don't wish to use edef (which would also totally expand the token list contained in the argument #1 of the topic-macro) and don't wish to integrate the stepcounter-directives into the topics-macro, you can easily use a combination of romannumeral0-expansion and exchange— the gist is:

romannumeral is used for "gobbling" tokens that form a number and in return for them delivering character-tokens which form the representation of that number in lowercase roman numerals.
romannumeral in any case triggers "gobbling" those tokens that form the number. But in case that number does form an integer which is not positive, silently, i.e., without error-message or the like, no tokens at all will be delivered in return.

Besides this when searching for more digits or the terminating space-token of a digit-sequence, romannumeral will trigger expansion of expandable tokens. With a digit-sequence, the terminating space-token will be removed silently.

Thus you can (ab?)use romannumeral for having (La)TeX doing a lot of expansion-work as long as it is ensured that in the end romannumeral will find a non-positive integral number.

In the example below romannumeral is used for triggering expansion of thetopic three times (because expansion needs to be triggered three times until you get the value of the topic-counter) before exchanging things and triggering expansion of the topics-macro once.

In the example below each instance of romannumeral will in the end find the digit(-sequence) "0", which is formed by the character-token(-sequence) 0, trailed by a space-token (which terminates that digit-sequence(character-token-sequence) and thus will take these things for the non-positive number 0 and therefore will "gobble" both the character-token "0" which forms the digit(-sequence)/number "0" and the space-token while silently in return for them not delivering any token at all.

documentclassarticle

newcommandexchange[2]#2#1%

newcommand*topics%
newcountertopic
newcommandtopic[1]%
 stepcountertopic%
 expandafterdefexpandaftertopicsexpandafter%
 romannumeral0%
 expandafterexchangeexpandafter%
 romannumeral0exchange %
 expandafterexpandafterexpandafter
 expandafterexpandafterexpandafter
 expandafter
 %
 thetopic: #1par%
 ifxtopicsempty
 exchange expandafterexpandafterexpandafter%
 expandafterexpandafterexpandafterpar
 else
 exchange expandafterexpandafterexpandafter%
 fi
 topicshangindent=0.5cm Topic #%
 %
 %
%

topictest A
topictest B
topictest C

%showtopics

begindocument
frenchspacingtextttstringtopics: meaningtopics%

bigskip
noindenthrulefillnull

bigskip
topics
enddocument

Nonetheless the solution provided by Raoul Kessels, where stepcounter-directives get integrated into the topics-macro is preferable because when you do it in Raoul Kessel's way you can instead also use refstepcounter-directives and place referencing-labels into the argument of the topic-macro for referencing single topics by means of the label-(page)ref-mechanism.

You need to expand thetopic before doing the append operation:

documentclassarticle

newcommandtopics
newcountertopic
newcommandtopic[1]%
 stepcountertopic%
 % newer versions of the engines (TeX Live 2019)
 expandafterappendtotopicsexpandafterexpandedthetopic#1%
 % for older versions of the TeX engines use the following
 %begingroupedefxendgroupnoexpandappendtotopicsthetopicx#1%

newcommandappendtotopics[2]%
 expandafterdefexpandaftertopicsexpandafter%
 topicshangindent=0.5cm Topic ##1: #2par
 %


topictest A
topictest B
topictest C

begindocument

topics

enddocument

An expl3 version:

documentclassarticle
usepackagexparse

ExplSyntaxOn
NewDocumentCommandtopics
 
 tl_use:N g_kessels_topics_tl
 
NewDocumentCommandtopicm
 
 kessels_topic:n #1 
 

int_new:N g_kessels_topic_int
tl_new:N g_kessels_topics_tl

cs_new_protected:Nn kessels_topic:n
 
 int_gincr:N g_kessels_topic_int
 __kessels_topic_add:fn int_to_arabic:n g_kessels_topic_int #1 
 

cs_new_protected:Nn __kessels_topic_add:nn
 
 tl_gput_right:Nn g_kessels_topics_tl hangindent=0.5cm~Topic~##1:~#2par 
 
cs_generate_variant:Nn __kessels_topic_add:nn f 

ExplSyntaxOff

topictest A
topictest B
topictest C

begindocument

topics

enddocument