Fonts: Assert that in each GlyphRanges[] pairs first is <= second.

docs/CHANGELOG.txt

@@ -84,6 +84,7 @@ All changes:
 - Menus: Fixed layout of MenuItem()/BeginMenu() when label contains a '\n'. (#6116) [@imkcy9]
 - PlotHistogram, PlotLines: Passing negative sizes honor alignment like other widgets.
 - Combo: Allow SetNextWindowSize() to alter combo popup size. (#6130)
+- Fonts: Assert that in each GlyphRanges[] pairs first is <= second.
 - ImDrawList: Added missing early-out in AddPolyline() and AddConvexPolyFilled() when
   color alpha is zero.
 - Misc: Most text functions treat "%s" as a shortcut to no-formatting. (#3466)

imgui.h

@@ -2664,7 +2664,7 @@ struct ImFontConfig
     bool            PixelSnapH;             // false    // Align every glyph to pixel boundary. Useful e.g. if you are merging a non-pixel aligned font with the default font. If enabled, you can set OversampleH/V to 1.
     ImVec2          GlyphExtraSpacing;      // 0, 0     // Extra spacing (in pixels) between glyphs. Only X axis is supported for now.
     ImVec2          GlyphOffset;            // 0, 0     // Offset all glyphs from this font input.
-    const ImWchar*  GlyphRanges;            // NULL     // Pointer to a user-provided list of Unicode range (2 value per range, values are inclusive, zero-terminated list). THE ARRAY DATA NEEDS TO PERSIST AS LONG AS THE FONT IS ALIVE.
+    const ImWchar*  GlyphRanges;            // NULL     // THE ARRAY DATA NEEDS TO PERSIST AS LONG AS THE FONT IS ALIVE. Pointer to a user-provided list of Unicode range (2 value per range, values are inclusive, zero-terminated list).
     float           GlyphMinAdvanceX;       // 0        // Minimum AdvanceX for glyphs, set Min to align font icons, set both Min/Max to enforce mono-space font
     float           GlyphMaxAdvanceX;       // FLT_MAX  // Maximum AdvanceX for glyphs
     bool            MergeMode;              // false    // Merge into previous ImFont, so you can combine multiple inputs font into one ImFont (e.g. ASCII font + icons + Japanese glyphs). You may want to use GlyphOffset.y when merge font of different heights.

imgui_draw.cpp

@@ -2384,7 +2384,12 @@ static bool ImFontAtlasBuildWithStbTruetype(ImFontAtlas* atlas)
         ImFontBuildDstData& dst_tmp = dst_tmp_array[src_tmp.DstIndex];
         src_tmp.SrcRanges = cfg.GlyphRanges ? cfg.GlyphRanges : atlas->GetGlyphRangesDefault();
         for (const ImWchar* src_range = src_tmp.SrcRanges; src_range[0] && src_range[1]; src_range += 2)
+        {
+            // Check for valid range. This may also help detect *some* dangling pointers, because a common
+            // user error is to setup ImFontConfig::GlyphRanges with a pointer to data that isn't persistent.
+            IM_ASSERT(src_range[0] <= src_range[1]);
             src_tmp.GlyphsHighest = ImMax(src_tmp.GlyphsHighest, (int)src_range[1]);
+        }
         dst_tmp.SrcCount++;
         dst_tmp.GlyphsHighest = ImMax(dst_tmp.GlyphsHighest, src_tmp.GlyphsHighest);
     }

misc/freetype/imgui_freetype.cpp

@@ -444,7 +444,12 @@ bool ImFontAtlasBuildWithFreeTypeEx(FT_Library ft_library, ImFontAtlas* atlas, u
         ImFontBuildDstDataFT& dst_tmp = dst_tmp_array[src_tmp.DstIndex];
         src_tmp.SrcRanges = cfg.GlyphRanges ? cfg.GlyphRanges : atlas->GetGlyphRangesDefault();
         for (const ImWchar* src_range = src_tmp.SrcRanges; src_range[0] && src_range[1]; src_range += 2)
+        {
+            // Check for valid range. This may also help detect *some* dangling pointers, because a common
+            // user error is to setup ImFontConfig::GlyphRanges with a pointer to data that isn't persistent.
+            IM_ASSERT(src_range[0] <= src_range[1]);
             src_tmp.GlyphsHighest = ImMax(src_tmp.GlyphsHighest, (int)src_range[1]);
+        }
         dst_tmp.SrcCount++;
         dst_tmp.GlyphsHighest = ImMax(dst_tmp.GlyphsHighest, src_tmp.GlyphsHighest);
     }