[gcc r17-2748] libstdc++: Fix num_children of the map pretty-printers

[Jonathan Wakely]

https://gcc.gnu.org/g:b80a4347fc63b1a6ed6e2b3ea26c5948f08ba3cd

commit r17-2748-gb80a4347fc63b1a6ed6e2b3ea26c5948f08ba3cd
Author: David Faure <david.faure@kdab.com>
Date:   Mon Jul 27 20:02:29 2026 +0200

    libstdc++: Fix num_children of the map pretty-printers
    
    StdMapPrinter and Tr1UnorderedMapPrinter yield a separate child for the
    key and for the value of every element, as their 'map' display hint
    requires, but their num_children methods return the number of elements,
    i.e. half of what children() produces.
    
    GDB counts the children itself where it does not use num_children, so
    the two disagree about the same map. For a std::map with 2 elements:
    
        (gdb) interpreter-exec mi "-enable-pretty-printing"
        (gdb) interpreter-exec mi "-var-create v * m"
        (gdb) interpreter-exec mi "-var-list-children v"
        ^done,numchild="4",displayhint="map",children=[...]
    
    while num_children() returns 2.  The DAP code in gdb treats the two as
    interchangeable: gdb/python/lib/gdb/dap/varref.py calls num_children()
    when the printer has one, and falls back to len(list(children())) when
    it does not, so a DAP client sees half the children of a std::map,
    std::multimap or std::unordered_map.
    
    StdSetPrinter and the sequence container printers yield one child per
    element, so they are correct as they are.
    
    libstdc++-v3/ChangeLog
    
            * python/libstdcxx/v6/printers.py (StdMapPrinter.num_children):
            Count two children per element.
            (Tr1UnorderedMapPrinter.num_children): Likewise.
    
    Signed-off-by: David Faure <david.faure@kdab.com>
    Reviewed-by: Tom Tromey <tromey@adacore.com>

Diff:
---
 libstdc++-v3/python/libstdcxx/v6/printers.py | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/python/libstdcxx/v6/printers.py b/libstdc++-v3/python/libstdcxx/v6/printers.py
index eff4b72c1d8a..1bba377513ca 100644
--- a/libstdc++-v3/python/libstdcxx/v6/printers.py
+++ b/libstdc++-v3/python/libstdcxx/v6/printers.py
@@ -909,7 +909,8 @@ class StdMapPrinter(printer_base):
         return self._iter(RbtreeIterator(self._val), node)
 
     def num_children(self):
-        return len(RbtreeIterator(self._val))
+        # Each element produces two children, the key and the value.
+        return 2 * len(RbtreeIterator(self._val))
 
     def display_hint(self):
         return 'map'
@@ -1301,7 +1302,8 @@ class Tr1UnorderedMapPrinter(printer_base):
         return izip(counter, data)
 
     def num_children(self):
-        return int(self._hashtable()['_M_element_count'])
+        # Each element produces two children, the key and the value.
+        return 2 * int(self._hashtable()['_M_element_count'])
 
     def display_hint(self):
         return 'map'