fix corner case in reduce_vars (#5099)

fixes #5098

lib/compress.js

@@ -981,7 +981,10 @@ merge(Compressor.prototype, {
                     if (fixed && !modified && !sym.in_arg && safe_to_assign(tw, d)) {
                         push_ref(d, sym);
                         mark(tw, d);
-                        if (d.single_use && left instanceof AST_Destructured) d.single_use = false;
+                        if (left instanceof AST_Destructured
+                            || d.orig.length == 1 && d.orig[0] instanceof AST_SymbolDefun) {
+                            d.single_use = false;
+                        }
                         tw.loop_ids[d.id] = tw.in_loop;
                         mark_escaped(tw, d, sym.scope, node, right, 0, 1);
                         sym.fixed = d.fixed = fixed;

test/compress/functions.js

@@ -6535,3 +6535,33 @@ issue_5096_4: {
     }
     expect_stdout: "PASS"
 }
+
+issue_5098: {
+    options = {
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        (function(o) {
+            function f() {
+                f = console.log;
+                if (o.p++)
+                    throw "FAIL";
+                f("PASS");
+            }
+            return f;
+        })({ p: 0 })();
+    }
+    expect: {
+        (function(o) {
+            function f() {
+                f = console.log;
+                if (o.p++)
+                    throw "FAIL";
+                f("PASS");
+            }
+            return f;
+        })({ p: 0 })();
+    }
+    expect_stdout: "PASS"
+}