fix corner case in collapse_vars (#4892)

fixes #4891

lib/compress.js

@@ -1884,9 +1884,17 @@ merge(Compressor.prototype, {
                     && node.name == def.name) {
                     if (!--replaced) abort = true;
                     if (is_lhs(node, multi_replacer.parent())) return node;
-                    def.replaced++;
                     var ref = rvalue.clone();
                     value_def.references.push(ref);
+                    if (abort && candidate instanceof AST_Assign
+                        && def.references.length - def.replaced - (assignments[def.name] || 0) > 1) {
+                        return make_node(AST_Assign, candidate, {
+                            operator: "=",
+                            left: node,
+                            right: ref,
+                        });
+                    }
+                    def.replaced++;
                     return ref;
                 }
                 // Skip (non-executed) functions and (leading) default case in switch statements

test/compress/collapse_vars.js

@@ -9057,3 +9057,31 @@ issue_4874: {
     }
     expect_stdout: "PASS"
 }
+
+issue_4891: {
+    options = {
+        collapse_vars: true,
+        reduce_vars: true,
+        toplevel: true,
+    }
+    input: {
+        var a = 0, b;
+        a++;
+        console.log(b = a, b);
+        b--;
+        a.a += 0;
+        console.log(b);
+    }
+    expect: {
+        var a = 0, b;
+        a++;
+        console.log(a, b = a);
+        b--;
+        a.a += 0;
+        console.log(b);
+    }
+    expect_stdout: [
+        "1 1",
+        "0",
+    ]
+}