Other observation: if I comment out these lines:

i4 : blah 12
../../m2/debugging.m2:6:17:(1): error: debug me
../../m2/debugging.m2:6:17:(1): --entering debugger (type help to see debugger commands)
../../m2/debugging.m2:6:12-6:28: --source code:
blah = x -> error "debug me"

So maybe a naive solution is to move that segment to the interpreter.

Playing around with setting loadDepth to something lower before that section and resetting it after doesn't seem to help either.

@pzinn my best guess is that the FilePosition of the error code is somehow involved, but I don't know where to look for more clues.

In #3431 I'm adding a new keyword breakpoint which can be used to start the debugger in the correct location. e.g. instead of adding error "debug me" you could add:

kernel Matrix := Module => opts -> (cacheValue symbol kernel) ((m) -> (
	  N := source m;
	  if m == 0 then return N;
	  P := target m;
	  if m.?RingMap then (
	       f := m.RingMap;
	       n := map(target m,f source m,raw m);
-	       p := pushNonLinear(options pushForward, f, coimage n);
+	       p := breakpoint pushNonLinear(options pushForward, f, coimage n);
	       image p)
	  else (
	       m = matrix m;
	       if P.?generators then m = P.generators * m;
	       h := modulo(m, if P.?relations then P.relations);
	       if N.?generators then h = N.generators * h;
	       subquotient( h, if N.?relations then N.relations))))

Other observation: if I comment out these lines:

M2/M2/Macaulay2/m2/debugging.m2

Lines 15 to 18 in ec9e9ac

	olderror := error
	error = args -> (
	-- this is the body of the "error" function, which prints out error messages
	olderror processArgs args)

then everything works fine:

i4 : blah 12
../../m2/debugging.m2:6:17:(1): error: debug me
../../m2/debugging.m2:6:17:(1): --entering debugger (type help to see debugger commands)
../../m2/debugging.m2:6:12-6:28: --source code:
blah = x -> error "debug me"

So maybe a naive solution is to move that segment to the interpreter.

Playing around with setting loadDepth to something lower before that section and resetting it after doesn't seem to help either.

@pzinn my best guess is that the FilePosition of the error code is somehow involved, but I don't know where to look for more clues.

definitely these lines should go at some point (and be moved to the interpreter).

Interestingly, the same test you propose at the top produces a quite different result on Macaulay2Web, which incorporates some major changes I made to the error workflow. I'll try to analyse where the difference is.
Also, the interaction between TCO and debugging is messy, cf possibly related #3371 (on which I'm stuck).

Interestingly, the same test you propose at the top produces a quite different result on Macaulay2Web, which incorporates some major changes I made to the error workflow.

On the other hand, it used to work as late as version 1.21:

Macaulay2, version 1.21

i1 : debug Core

i2 : errorDepth=1

o2 = 1

i3 : blah()
M2/Macaulay2/m2/debugging.m2:6:13:(1):[1]: error: debug me
M2/Macaulay2/m2/debugging.m2:6:13:(1):[1]: --entering debugger (type help to see debugger commands)
M2/Macaulay2/m2/debugging.m2:6:13-6:13: --source code:
blah = x -> error "debug me"

So analyzing the differences since then could also work.

Very exiting realization: the breakpoint keyword that I added in #3431 allows you to drop into a thread for debugging! This is currently not possible using error, at least as far as I can tell.

@pzinn this error was introduced after 1.24.05. Did you have any luck isolating when it started?

Okay, found the commit that broke this: 94ac762. Going to submit a pull request to revert this for now. Later we can figure out how to fix error location in tail calls without breaking the debugger.

All I can add (a bit late) is that I think I'd effectively solved the problem on my branch (without any need to revert #3544), see attached screenshot. But as I mentioned in the other thread, will need more time to investigate.