mjhjmtu

I see this with TeXLive 2018, which includes version 3.26 of babel.sty (2018/10/16 3.26"). I think this works with some earlier versions of babel.sty, but I'm not sure where the cutoff is.

documentclass[english]article
usepackagebabel
usepackagetextcomp
renewcommand*LaTeXhello
begindocument

Testing: LaTeX

enddocument

When I run PDFLaTeX on this, I do not see "Testing: hello" but instead see "Testing: LaTeX". Why can't I redefine the macro LaTeX?

Note that I only see the problem if I use both packages babel and textcomp; omit either and things work as they should.

This happens because textcomp loads a non-ASCII encoding (ts1enc.def), and babel explicitly checks for that and, if that's the case, it changes the definition of the LaTeX macro. The relevant part of the file:

When babel is being loaded it defines the list of non-ASCII encodings:

newcommandBabelNonASCII % vvv Here's the one loaded by textcomp
 LGR,X2,OT2,OT3,OT6,LHE,LWN,LMA,LMC,LMS,LMU,TS1,T3,TS3

then it saves the original definitions of TeX and LaTeX:

letorg@TeXTeX
letorg@LaTeXLaTeX
letensureascii@firstofone

Now, when the begindocument is executed, babel checks if any of the encodings defined above were loaded:

AtBeginDocument%
 in@false
 bbl@foreachBabelNonASCII% is there a non-ascii enc?
 ifin@else
 lowercasebbl@xin@,#1enc.def,,@filelist,%
 fi%

if any, then it redefines LaTeX to be ensureasciiorg@LaTeX (and a couple more things):

 ifin@ % if a non-ascii has been loaded
 defensureascii#1fontencodingOT1selectfont#1%
 DeclareTextCommandDefaultTeXorg@TeX%
 DeclareTextCommandDefaultLaTeXorg@LaTeX%
 defbbl@tempb#1@@uppercasebbl@tempc#1ENC.DEF@empty@@%
 defbbl@tempc#1ENC.DEF#2@@%
 ifx@empty#2else
 bbl@ifunsetT@#1%
 %
 bbl@xin@,#1,,BabelNonASCII,%
 ifin@
 DeclareTextCommandTeX#1ensureasciiorg@TeX%
 DeclareTextCommandLaTeX#1ensureasciiorg@LaTeX%
 else
 defensureascii##1fontencoding#1selectfont##1%
 fi%
 fi%
 bbl@foreach@filelistbbl@tempb#1@@% TODO - @@ de mas??
 bbl@xin@,cf@encoding,,BabelNonASCII,%
 ifin@else
 edefensureascii#1%
 noexpandfontencodingcf@encodingnoexpandselectfont#1%
 fi
 fi

The problem is that org@LaTeX is defined when babel is loaded, and LaTeX is _re_defined AtBeginDocument, so your renewcommand is overwritten.

To overcome this you'll need to redefine LaTeX either before babel is loaded, after the begindocument, or AtBeginDocument after babel is loaded:

documentclassarticle
% renewcommand*LaTeXhello % Option 1
usepackagebabel
usepackagetextcomp
% AtBeginDocumentrenewcommand*LaTeXhello % Option 2
begindocument
% renewcommand*LaTeXhello % Option 3

Testing: LaTeX

enddocument

1. Option 1 works because when babel is loaded, LaTeX is hello, so org@LaTeX becomes hello and your definition is used.




2. Option 2 works because the AtBeginDocument command adds the argument to the end of the token list, so your redefinition is executed after babel's. It is essentially the same as doing it after the BeginDocument.




3. Option 3 trivially works because nothing happens between your redefinition and the usage of the command.

This change happened in babel version 3.23, when the encodings TS1, T3, TS3 were added to the non-ASCII list:

% Line 8996 of an up-to-date (26-10-2018) `babel.dtx`:
% changesbabel~3.232018/08/28Added TS1, T3, TS3