Backport PR matplotlib#32066: Fix crash on prime after single-character superscript

Author: tacaswell

lib/matplotlib/_mathtext.py

@@ -2607,6 +2607,10 @@ def subsuper(self, s: str, loc: int, toks: ParseResults) -> T.Any:
         if napostrophes:
             if super is None:
                 super = Hlist([])
+            elif not isinstance(super, Hlist):
+                # A single-char superscript is a bare Char; wrap it so the
+                # prime glyphs can be appended.
+                super = Hlist([super])
             for i in range(napostrophes):
                 super.children.extend(self.symbol(s, loc, {"sym": "\\prime"}))
             # kern() and hpack() needed to get the metrics right after

lib/matplotlib/tests/test_mathtext.py

@@ -569,6 +569,13 @@ def test_mathtext_operators():
     fig.draw_without_rendering()
 
 
+@pytest.mark.parametrize("expr", [r"$x^2'$", r"$x^a'$", r"$x^2''$", r"$x^\alpha'$"])
+def test_mathtext_single_char_super_with_prime(expr):
+    # Regression test for a crash: prime after a single-char superscript.
+    parser = mathtext.MathTextParser('agg')
+    parser.parse(expr)
+
+
 @check_figures_equal()
 def test_boldsymbol(fig_test, fig_ref):
     fig_test.text(0.1, 0.2, r"$\boldsymbol{\mathrm{abc0123\alpha}}$")