Fix bad bound in forcibly overflowing optimal paragraph layout (#4975)

crates/typst/src/layout/inline/line.rs

@@ -698,6 +698,12 @@ impl<'a> DerefMut for Items<'a> {
     }
 }
 
+impl Debug for Items<'_> {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        f.debug_list().entries(&self.0).finish()
+    }
+}
+
 /// A reference to or a boxed item.
 pub enum ItemEntry<'a> {
     Ref(&'a Item<'a>),

crates/typst/src/layout/inline/linebreak.rs

@@ -481,9 +481,19 @@ fn linebreak_optimized_approximate(
         let Entry { end, breakpoint, unbreakable, .. } = table[idx];
 
         let attempt = line(engine, p, start..end, breakpoint, Some(&pred));
-        let (_, line_cost) =
+        let (ratio, line_cost) =
             ratio_and_cost(p, metrics, width, &pred, &attempt, breakpoint, unbreakable);
 
+        // If approximation produces a valid layout without too much shrinking,
+        // exact layout is guaranteed to find the same layout. If, however, the
+        // line is overfull, we do not have this guarantee. Then, our bound
+        // becomes useless and actively harmful (it could be lower than what
+        // optimal layout produces). Thus, we immediately bail with an infinite
+        // bound in this case.
+        if ratio < metrics.min_ratio(false) {
+            return Cost::INFINITY;
+        }
+
         pred = attempt;
         start = end;
         exact += line_cost;

tests/suite/layout/inline/justify.typ

@@ -167,3 +167,8 @@ int main() {
 // an underfull first line.
 #set par(hanging-indent: 2.5cm, justify: true)
 #lorem(5)
+
+--- issue-4651-justify-bad-bound ---
+// Test that overflow does not lead to bad bounds in paragraph optimization.
+#set par(justify: true)
+#block(width: 0pt)[A B]