feat(shapers): Warn when asked to measure a character not shaped in a…

… font

packages/math/base-elements.lua

@@ -1625,7 +1625,10 @@ function elements.sqrt:shape ()
    -- Note: In TeX, the radical sign extends a lot below the baseline,
    -- and MathML Core also has a lot of layout text about it.
    -- Not only it doesn't look good, but it's not very clear vs. OpenType.
-   local radicalGlyph = SILE.shaper:measureChar("√")
+   local radicalGlyph, found = SILE.shaper:measureChar("√")
+   if not found then
+      SU.error("Math font does not contain a square root glyph")
+   end
    local ratio = (self.radicand.height:tonumber() + self.radicand.depth:tonumber())
       / (radicalGlyph.height + radicalGlyph.depth)
    local vertAdHocOffset = (ratio > 1 and math.log(ratio) or 0) * self.radicalVerticalGap

shapers/base.lua

@@ -67,8 +67,9 @@ function shaper:measureChar (char)
    local options = SILE.font.loadDefaults({})
    options.tracking = SILE.settings:get("shaper.tracking")
    local items = self:shapeToken(char, options)
-   if #items > 0 then
-      return { height = items[1].height, width = items[1].width, depth = items[1].depth }
+   if items and items[1] then
+      local item = items[1]
+      return item, item.gid ~= 0
    else
       SU.error("Unable to measure character", char)
    end

types/unit.lua

@@ -223,16 +223,16 @@ unittypes["zw"] = {
    relative = true,
    definition = function (v)
       local zenkakuchar = SILE.settings:get("document.zenkakuchar")
-      local measurable, zenkaku = pcall(SILE.shaper.measureChar, SILE.shaper, zenkakuchar)
-      if not measurable then
+      local measurable, zenkaku, found = pcall(SILE.shaper.measureChar, SILE.shaper, zenkakuchar)
+      if not found or not measurable then
          SU.warn(([[
             Zenkaku width (全角幅) unit zw is falling back to 1em == 1zw as we cannot measure %s
 
             Either change this char to one suitable for your language, or load a font that
             has it.
          ]]):format(zenkakuchar))
       end
-      local width = measurable and zenkaku.width or SILE.settings:get("font.size")
+      local width = found and measurable and zenkaku.width or SILE.settings:get("font.size")
       return v * width
    end,
 }