fix corner case in reduce_vars (#5062)

fixes #5061

lib/compress.js

@@ -889,6 +889,7 @@ merge(Compressor.prototype, {
                     } else {
                         right.parent_scope.resolve().fn_defs.push(right);
                         right.safe_ids = null;
+                        if (!node.write_only) mark_fn_def(tw, ld, right);
                     }
                     return true;
                 } else if (scan) {

test/compress/functions.js

@@ -6346,3 +6346,55 @@ issue_5046: {
     }
     expect_stdout: "PASS"
 }
+
+issue_5061_1: {
+    options = {
+        evaluate: true,
+        reduce_vars: true,
+        toplevel: true,
+    }
+    input: {
+        var f, a = 1;
+        (f = function() {
+            console.log(a ? "foo" : "bar");
+        })();
+        f(a = 0);
+    }
+    expect: {
+        var f, a = 1;
+        (f = function() {
+            console.log(a ? "foo" : "bar");
+        })();
+        f(a = 0);
+    }
+    expect_stdout: [
+        "foo",
+        "bar",
+    ]
+}
+
+issue_5061_2: {
+    options = {
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var f, a = 1;
+        (f = function() {
+            console.log(a ? "foo" : "bar");
+        })();
+        f(a = 0);
+    }
+    expect: {
+        var f, a = 1;
+        (f = function() {
+            console.log(a ? "foo" : "bar");
+        })();
+        f(a = 0);
+    }
+    expect_stdout: [
+        "foo",
+        "bar",
+    ]
+}