fix corner case in collapse_vars (#4911)

fixes #4910

lib/compress.js

@@ -1777,6 +1777,7 @@ merge(Compressor.prototype, {
                         }
                         return node;
                     } else if (value_def) {
+                        if (stop_if_hit && assign_pos == 0) assign_pos = remaining - replaced;
                         if (!hit_rhs) replaced++;
                         return node;
                     } else {
@@ -1904,14 +1905,14 @@ merge(Compressor.prototype, {
                     if (is_lhs(node, multi_replacer.parent())) return node;
                     var ref = rvalue.clone();
                     value_def.references.push(ref);
-                    if (abort && candidate instanceof AST_Assign && remaining_refs(def) > 1) {
+                    if (replaced == assign_pos) {
+                        abort = true;
                         return make_node(AST_Assign, candidate, {
                             operator: "=",
                             left: node,
                             right: ref,
                         });
                     }
-                    ref.fixed = false;
                     def.replaced++;
                     return ref;
                 }
@@ -1931,6 +1932,8 @@ merge(Compressor.prototype, {
                     hit_stack = candidates.pop();
                     var hit_index = 0;
                     var candidate = hit_stack[hit_stack.length - 1];
+                    var assign_pos = -1;
+                    var assign_used = false;
                     var remaining;
                     var value_def = null;
                     var stop_after = null;
@@ -1962,7 +1965,6 @@ merge(Compressor.prototype, {
                     var hit = funarg;
                     var abort = false;
                     var replaced = 0;
-                    var assign_used = false;
                     var can_replace = !args || !hit;
                     if (!can_replace) {
                         for (var j = candidate.arg_index + 1; !abort && j < args.length; j++) {
@@ -2621,9 +2623,15 @@ merge(Compressor.prototype, {
                     if (scope.uses_arguments && is_funarg(def)) return lhs;
                     if (compressor.exposed(def)) return lhs;
                     remaining = remaining_refs(def);
-                    if (def.fixed && lhs.fixed) remaining = Math.min(remaining, def.references.filter(function(ref) {
-                        return ref.fixed === lhs.fixed;
-                    }).length - 1);
+                    if (def.fixed && lhs.fixed) {
+                        var matches = def.references.filter(function(ref) {
+                            return ref.fixed === lhs.fixed;
+                        }).length - 1;
+                        if (matches < remaining) {
+                            remaining = matches;
+                            assign_pos = 0;
+                        }
+                    }
                     mangleable_var(expr.right);
                     return lhs;
                 }

test/compress/collapse_vars.js

@@ -9049,7 +9049,7 @@ issue_4874: {
     }
     expect: {
         var a;
-        a = null;
+        null;
         (function(b) {
             for (var c in a && a[console.log("PASS")])
                 console;
@@ -9125,8 +9125,29 @@ issue_4908: {
     expect: {
         var a = 0, b;
         console || a++;
-        var c = d = a, d = [ d && d, d += 42 ];
+        var c = a, d = [ (d = a) && d, d += 42 ];
         console.log(d[1]);
     }
     expect_stdout: "42"
 }
+
+issue_4910: {
+    options = {
+        collapse_vars: true,
+        reduce_vars: true,
+        toplevel: true,
+    }
+    input: {
+        var a = "foo", b;
+        var c = b = a;
+        1 && c[a = "bar"];
+        console.log(a, b);
+    }
+    expect: {
+        var a = "foo", b;
+        var c = b = a;
+        1 && b[a = "bar"];
+        console.log(a, b);
+    }
+    expect_stdout: "bar foo"
+}

test/compress/functions.js

@@ -6107,7 +6107,7 @@ reduce_cross_reference_2: {
 reduce_cross_reference_2_toplevel: {
     options = {
         collapse_vars: true,
-        passes: 3,
+        passes: 2,
         pure_getters: "strict",
         reduce_vars: true,
         sequences: true,
@@ -6150,7 +6150,7 @@ reduce_cross_reference_3: {
 reduce_cross_reference_3_toplevel: {
     options = {
         collapse_vars: true,
-        passes: 3,
+        passes: 2,
         pure_getters: "strict",
         reduce_vars: true,
         sequences: true,