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Fix several regressions introduced by #6494 - #6510

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fingolfin merged 4 commits into
gap-system:masterfrom
hulpke:isom_better
Aug 15, 2026
Merged

Fix several regressions introduced by #6494#6510
fingolfin merged 4 commits into
gap-system:masterfrom
hulpke:isom_better

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@hulpke

@hulpke hulpke commented Aug 14, 2026

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This fixes the error notes in #6494 post-merge

(Some of it is not strictly needed but useful caution about the domain of nice morphisms)

@fingolfin fingolfin added topic: library release notes: not needed PRs introducing changes that are wholly irrelevant to the release notes labels Aug 14, 2026
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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.39%. Comparing base (0ff3c4f) to head (6fde561).

Additional details and impacted files 
@@            Coverage Diff             @@
##           master    #6510      +/-   ##
==========================================
- Coverage   78.91%   76.39%   -2.52%     
==========================================
  Files         685      684       -1     
  Lines      294163   293922     -241     
  Branches     8669     8652      -17     
==========================================
- Hits       232147   224555    -7592     
- Misses      60215    67541    +7326     
- Partials     1801     1826      +25

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fingolfin
fingolfin reviewed Aug 14, 2026
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@fingolfin fingolfin left a comment

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So what exactly is the fix, and why does it work?

Comment thread lib/grppcext.gi
Comment thread lib/grppcext.gi
Comment thread lib/grppcext.gi Outdated
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@fingolfin

fingolfin commented Aug 14, 2026

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Also, this really needs a regression test.

@fingolfin

fingolfin commented Aug 14, 2026

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With this branch:

gap> AutomorphismGroupSolvableGroup(SmallGroup(864,4675));
Error, Variable: 'u' must have an assigned value
Stack trace:
*[1] if u <> fail then
    B := List( GeneratorsOfGroup( A ), function ( x )
            return x[2];
        end );
    B := Group( B );
    if not IsSubset( u, B ) then
        Info( InfoCompPairs, 1, " projection wrong, go normal" );
    else
        return A;
    fi;
else
    return A;
fi;
   @ /Users/mhorn/Projekte/GAP/gap/lib/grppcext.gi:617
 [2] CompatiblePairs( F, M, D )
   @ /Users/mhorn/Projekte/GAP/gap/lib/grppcaut.gi:1343
<function "AutomorphismGroupSolvableGroup">( <arguments> )
 called from read-eval loop at *stdin*:1
you can enter 'quit;' to quit to outer loop

@hulpke

hulpke commented Aug 14, 2026 •
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So what exactly is the fix, and why does it work?

Magic :-)

No, The "else" case (old line 552) needs to be run if not doing the new "direct" case.

@fingolfin

fingolfin commented Aug 14, 2026

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It does work with AutomorphismGroup, but there are examples where that fails, too:

gap> AutomorphismGroup(SmallGroup(864,4675));
<group of size 10368 with 11 generators>
gap> AutomorphismGroup(SmallGroup(96, 76));
Error, Variable: 'u' must have an assigned value
Stack trace:
*[1] if u <> fail then
    B := List( GeneratorsOfGroup( A ), function ( x )
            return x[2];
        end );
    B := Group( B );
    if not IsSubset( u, B ) then
        Info( InfoCompPairs, 1, " projection wrong, go normal" );
    else
        return A;
    fi;
else
    return A;
fi;
   @ /Users/mhorn/Projekte/GAP/gap/lib/grppcext.gi:617
 [2] CompatiblePairs( F, M, D )
   @ /Users/mhorn/Projekte/GAP/gap/lib/grppcaut.gi:1343
 [3] AutomorphismGroupSolvableGroup( G )
   @ /Users/mhorn/Projekte/GAP/gap/lib/morpheus.gi:2562
<function "AutomorphismGroup finite groups">( <arguments> )
 called from read-eval loop at *stdin*:1
you can enter 'quit;' to quit to outer loop

@hulpke
FIX remaining gap-system#6494 bug
33fc5ac 
This fixes the error notes in gap-system#6494 post-merge
@fingolfin
Fix another error
eba1c45 
Fix this:
```
gap> AutomorphismGroupSolvableGroup(SmallGroup(147, 4));;
Error, usage: Group(<gen>,...), Group(<gens>), Group(<gens>,<id>)
Stack trace:
*[1] Error( "usage: Group(<gen>,...), Group(<gens>), Group(<gens>,<id>)" );
   @ /Users/mhorn/Projekte/GAP/gap/lib/grp.gi:5114
 [2] Group( GeneratorsOfGroup( K ){B} )
   @ /Users/mhorn/Projekte/GAP/gap/lib/grppcext.gi:590
 [3] CompatiblePairs( F, M, D )
   @ /Users/mhorn/Projekte/GAP/gap/lib/grppcaut.gi:1343
 [4] AutomorphismGroupSolvableGroup( grps[i] );
   @ *stdin*:3
<function "unknown">( <arguments> )
 called from read-eval loop at *stdin*:3
you can enter 'quit;' to quit to outer loop, or
you can enter 'return;' to continue
```
@fingolfin

fingolfin commented Aug 15, 2026

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Run a test computing many AutomorphismGroups, found another error (and pushed a fix):

gap> AutomorphismGroupSolvableGroup(SmallGroup(147, 4));;
Error, usage: Group(<gen>,...), Group(<gens>), Group(<gens>,<id>)
Stack trace:
*[1] Error( "usage: Group(<gen>,...), Group(<gens>), Group(<gens>,<id>)" );
   @ /Users/mhorn/Projekte/GAP/gap/lib/grp.gi:5114
 [2] Group( GeneratorsOfGroup( K ){B} )
   @ /Users/mhorn/Projekte/GAP/gap/lib/grppcext.gi:590
 [3] CompatiblePairs( F, M, D )
   @ /Users/mhorn/Projekte/GAP/gap/lib/grppcaut.gi:1343
 [4] AutomorphismGroupSolvableGroup( grps[i] );
   @ *stdin*:3
<function "unknown">( <arguments> )
 called from read-eval loop at *stdin*:3
you can enter 'quit;' to quit to outer loop, or
you can enter 'return;' to continue

@fingolfin

fingolfin commented Aug 15, 2026

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Also added regression tests for the three failing groups, and cleaned up the code a bit.

@fingolfin fingolfin changed the title FIX remaining #6494 bug Fix several regressions introduced by #6494 Aug 15, 2026
@fingolfin
fingolfin enabled auto-merge (squash) August 15, 2026 08:27
@fingolfin
fingolfin merged commit 8636b5a into gap-system:master Aug 15, 2026
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@hulpke @fingolfin