PangoFT2 font maps don't honor aliases defined in custom FcConfigs

Please have a look at the following test program:

#include <stdio.h>
#include <assert.h>

#include <fontconfig/fontconfig.h>
#include <pango/pangoft2.h>

int main() {
  FcConfig *conf = FcConfigCreate();
  FcChar8 *default_conf_file = FcConfigGetFilename(NULL, NULL);
  assert(default_conf_file);
  printf("Reading default conf file: %s\n", default_conf_file);
  assert(FcConfigParseAndLoad(conf, default_conf_file, FcTrue));
  printf("Reading custom config that defines a \"Custom Family\" alias for the C059 font\n");
  FcChar8 *my_conf = (FcChar8 *)
    "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n"
    "<!DOCTYPE fontconfig SYSTEM \"fonts.dtd\">\n"
    "<fontconfig>\n"
    "  <alias binding=\"strong\">\n"
    "    <family>Custom Family</family>\n"
    "      <prefer>\n"
    "        <family>C059</family>\n"
    "      </prefer>\n"
    "  </alias>\n"
    "</fontconfig>\n";
  assert(FcConfigParseAndLoadFromMemory(conf, my_conf, FcTrue));

  FcConfigBuildFonts(conf);

  printf("Finding \"Custom Family\" font via Fontconfig...\n");

  FcPattern *pat = FcPatternCreate();
  FcPatternAddString(pat, FC_FAMILY, (FcChar8 *) "Custom Family");
  FcConfigSubstitute(conf, pat, FcMatchPattern);
  FcDefaultSubstitute(pat);
  FcResult result;
  FcPattern *found_pat = FcFontMatch(conf, pat, &result);
  assert(result == FcResultMatch);
  FcChar8 *fc_family;
  FcResult fam_result = FcPatternGetString(found_pat, FC_FAMILY, 0, &fc_family);
  assert(fam_result == FcResultMatch);
  printf("Found: %s\n", fc_family);

  printf("Finding \"Custom Family\" font via PangoFT2...\n");

  PangoFontMap *map = pango_ft2_font_map_new();
  pango_fc_font_map_set_config(PANGO_FC_FONT_MAP(map), conf);
  PangoContext *ctx = pango_font_map_create_context(map);

  PangoFontDescription *desc = pango_font_description_new();
  pango_font_description_set_family(desc, "Custom Family");

  PangoFont *found_font = pango_font_map_load_font(map, ctx, desc);
  PangoFontDescription *found_desc = pango_font_describe(found_font);
  char *desc_str = pango_font_description_to_string(found_desc);
  printf("Found: %s\n", desc_str);

  return 0;
}

Here is the output on my system:

~/tmp $ gcc $(pkg-config --cflags pangoft2) $(pkg-config --libs pangoft2) -o pango-bug pango-bug.c && ./pango-bug
Reading default conf file: /etc/fonts/fonts.conf
Reading custom config that defines a "Custom Family" alias for the C059 font
Finding "Custom Family" font via Fontconfig...
Found: C059
Finding "Custom Family" font via PangoFT2...
Found: Noto Sans 0
~/tmp $ pkg-config --modversion pangoft2
1.50.14
~/tmp $ pkg-config --modversion fontconfig
2.14.1

In this program, I am using a custom FcConfig object which loads a bit of Fontconfig XML defining a font alias "Custom Family" that is the same as C059 (a font found on my system by fc-list).

• When I use the Fontconfig API directly to look up "Custom Family", I get C059.
• When I use a Pango FT2 font map, I get "Noto Sans", the default font on my system.

I'm pretty sure this happens because of the call to FcConfigSubstitute in pangoft2-fontmap.c:

$ rg "FcConfigSubstitute"
pango/pangocairo-fcfontmap.c
106:  FcConfigSubstitute (pango_fc_font_map_get_config (fcfontmap), pattern, FcMatchPattern);

pango/pangoxft-fontmap.c
393:  FcConfigSubstitute (NULL, pattern, FcMatchPattern);

pango/pangoft2-fontmap.c
353:  FcConfigSubstitute (NULL, pattern, FcMatchPattern);

Shouldn't the third one, and possibly the second, be using the font map's FcConfig rather than NULL, which means to use the global FcConfig?

CC @behdad as the author of http://mces.blogspot.com/2015/05/how-to-use-custom-application-fonts.html who also added pango_fc_font_map_set_config.

Thanks!