From d8701259b219cf93212e4b7483727b33d177f87c Mon Sep 17 00:00:00 2001
From: ThomasBreuer <sam@math.rwth-aachen.de>
Date: Tue, 20 Dec 2022 17:43:09 +0100
Subject: [PATCH] adjust to the discussion

allow implications to representations that are already implied
---
 lib/filter.g               | 28 +++++++++++++++-------------
 tst/testinstall/method.tst |  7 ++++---
 2 files changed, 19 insertions(+), 16 deletions(-)

diff --git a/lib/filter.g b/lib/filter.g
index f318617f90..83b182a168 100644
--- a/lib/filter.g
+++ b/lib/filter.g
@@ -285,29 +285,32 @@ end);
 ##  Adding logical implications can change the rank of filters (see
 ##  <Ref Func="RankFilter"/>) and consequently the rank, and so choice of
 ##  methods for operations (see <Ref Sect="Applicable Methods and Method Selection"/>).
-##  By default <C>InstallTrueMethod</C> adjusts the method selection data
-##  structures to take care of this, but this process can be time-consuming,
+##  By default <Ref Func="InstallTrueMethod"/> adjusts the method selection
+##  data structures to take care of this,
+##  but this process can be time-consuming,
 ##  so functions <Ref Func="SuspendMethodReordering"/> and
 ##  <Ref Func="ResumeMethodReordering"/> are provided to allow control of this process.
 ##  </Description>
 ##  </ManSection>
 ##  <#/GAPDoc>
 ##
-
 BIND_GLOBAL( "InstallTrueMethod", function ( tofilt, from )
-    local i;
+    local fromflags, i, j;
 
     # Check whether 'tofilt' involves or implies representations.
+    fromflags:= TRUES_FLAGS( WITH_IMPS_FLAGS( FLAGS_FILTER( from ) ) );
     for i in TRUES_FLAGS( WITH_IMPS_FLAGS( FLAGS_FILTER( tofilt ) ) ) do
       if INFO_FILTERS[i] = FNUM_REP_KERN or INFO_FILTERS[i] = FNUM_REP then
-        # This is allowed only if 'from' consists of representations.
-        for i in TRUES_FLAGS( WITH_IMPS_FLAGS( FLAGS_FILTER( from ) ) ) do
-          if not ( INFO_FILTERS[i] = FNUM_REP_KERN or
-                   INFO_FILTERS[i] = FNUM_REP ) then
-            Error( "<tofilt> must not involve representation filters" );
-          fi;
-        od;
-        break;
+        # This is allowed only if either 'from' already implies filter 'i'
+        # or if 'from' consists of representations.
+        if not i in fromflags then
+          for j in fromflags do
+            if not ( INFO_FILTERS[j] = FNUM_REP_KERN or
+                     INFO_FILTERS[j] = FNUM_REP ) then
+              Error( "<tofilt> must not involve new representation filters" );
+            fi;
+          od;
+        fi;
       fi;
     od;
 
@@ -321,7 +324,6 @@ BIND_GLOBAL( "InstallTrueMethod", function ( tofilt, from )
     if REORDER_METHODS_SUSPENSION_LEVEL = 0 then
         RECALCULATE_ALL_METHOD_RANKS();
     fi;
-
 end );
 
 
diff --git a/tst/testinstall/method.tst b/tst/testinstall/method.tst
index 4e0cf0d796..671eed301f 100644
--- a/tst/testinstall/method.tst
+++ b/tst/testinstall/method.tst
@@ -10,8 +10,9 @@ Error, required filters [ "IsInt", "IsRat", "IsCyc", "IsExtAElement",
   "IsAdditivelyCommutativeElement", "IsCommutativeElement", "IsCyclotomic" ]
 for 1st argument do not match a declaration of Size
 gap> InstallTrueMethod( IsInternalRep, IsList );
-Error, <tofilt> must not involve representation filters
+Error, <tofilt> must not involve new representation filters
 gap> InstallTrueMethod( IsDenseCoeffVectorRep, IsList );
-Error, <tofilt> must not involve representation filters
+Error, <tofilt> must not involve new representation filters
 gap> InstallTrueMethod( IsInternalRep, IsSmallIntRep );
-gap> STOP_TEST("method.tst", 1);
+gap> InstallTrueMethod( IsList and IsInternalRep, IsList and IsSmallIntRep );
+gap> STOP_TEST("method.tst");