fix corner case collapse_vars (#4921)

fixes #4920

Author: alexlamsl

lib/compress.js

@@ -2446,6 +2446,25 @@ merge(Compressor.prototype, {
                 return node;
             }
 
+            function find_stop_expr(expr, cont, node, parent, level) {
+                var replace = can_replace;
+                can_replace = false;
+                var after = stop_after;
+                var if_hit = stop_if_hit;
+                var stack = scanner.stack;
+                scanner.stack = [ parent ];
+                expr.transform(scanner);
+                scanner.stack = stack;
+                stop_if_hit = if_hit;
+                stop_after = after;
+                can_replace = replace;
+                if (abort) {
+                    abort = false;
+                    return node;
+                }
+                return cont(parent, level + 1);
+            }
+
             function find_stop_value(node, level) {
                 var parent = scanner.parent(level);
                 if (parent instanceof AST_Array) return find_stop_value(parent, level + 1);
@@ -2497,7 +2516,11 @@ merge(Compressor.prototype, {
                         return prop instanceof AST_ObjectKeyVal;
                     }) ? find_stop_value(obj, level + 2) : obj;
                 }
-                if (parent instanceof AST_PropAccess) return find_stop_value(parent, level + 1);
+                if (parent instanceof AST_PropAccess) {
+                    var exp = parent.expression;
+                    if (exp === node) return find_stop_value(parent, level + 1);
+                    return find_stop_expr(exp, find_stop_value, node, parent, level);
+                }
                 if (parent instanceof AST_Sequence) {
                     return (parent.tail_node() === node ? find_stop_value : find_stop_unused)(parent, level + 1);
                 }
@@ -2546,7 +2569,7 @@ merge(Compressor.prototype, {
                 if (parent instanceof AST_PropAccess) {
                     var exp = parent.expression;
                     if (exp === node) return find_stop_unused(parent, level + 1);
-                    return check_expr(exp);
+                    return find_stop_expr(exp, find_stop_unused, node, parent, level);
                 }
                 if (parent instanceof AST_Sequence) return find_stop_unused(parent, level + 1);
                 if (parent instanceof AST_SimpleStatement) return find_stop_unused(parent, level + 1);
@@ -2557,28 +2580,11 @@ merge(Compressor.prototype, {
                 if (parent instanceof AST_Yield) return node;
                 return null;
 
-                function check_expr(expr) {
-                    var replace = can_replace;
-                    can_replace = false;
-                    var after = stop_after;
-                    var if_hit = stop_if_hit;
-                    var stack = scanner.stack;
-                    scanner.stack = [ parent ];
-                    expr.transform(scanner);
-                    scanner.stack = stack;
-                    stop_if_hit = if_hit;
-                    stop_after = after;
-                    can_replace = replace;
-                    if (abort) {
-                        abort = false;
-                        return node;
-                    }
-                    return find_stop_unused(parent, level + 1);
-                }
-
                 function check_assignment(lhs) {
                     if (may_throw(parent)) return node;
-                    if (lhs !== node && lhs instanceof AST_Destructured) return check_expr(lhs);
+                    if (lhs !== node && lhs instanceof AST_Destructured) {
+                        return find_stop_expr(lhs, find_stop_unused, node, parent, level);
+                    }
                     return find_stop_unused(parent, level + 1);
                 }
             }

test/compress/collapse_vars.js

@@ -9194,3 +9194,29 @@ issue_4918: {
     }
     expect_stdout: "PASS"
 }
+
+issue_4920: {
+    options = {
+        collapse_vars: true,
+        toplevel: true,
+    }
+    input: {
+        var a = "PASS", b;
+        ({
+            get PASS() {
+                a = "FAIL";
+            },
+        })[b = a];
+        console.log(b);
+    }
+    expect: {
+        var a = "PASS", b;
+        ({
+            get PASS() {
+                a = "FAIL";
+            },
+        })[b = a];
+        console.log(b);
+    }
+    expect_stdout: "PASS"
+}