fix corner case in collapse_vars (#4919)

fixes #4918

lib/compress.js

@@ -2546,12 +2546,7 @@ merge(Compressor.prototype, {
                 if (parent instanceof AST_PropAccess) {
                     var exp = parent.expression;
                     if (exp === node) return find_stop_unused(parent, level + 1);
-                    var sym = root_expr(exp);
+                    return check_expr(exp);
-                    if (!(sym instanceof AST_SymbolRef)) return find_stop_unused(parent, level + 1);
-                    var lvalue = lvalues.get(sym.name);
-                    return !lvalue || all(lvalue, function(lhs) {
-                        return !(lhs instanceof AST_PropAccess);
-                    }) ? find_stop_unused(parent, level + 1) : node;
                 }
                 if (parent instanceof AST_Sequence) return find_stop_unused(parent, level + 1);
                 if (parent instanceof AST_SimpleStatement) return find_stop_unused(parent, level + 1);
@@ -2562,16 +2557,14 @@ merge(Compressor.prototype, {
                 if (parent instanceof AST_Yield) return node;
                 return null;
 
-                function check_assignment(lhs) {
+                function check_expr(expr) {
-                    if (may_throw(parent)) return node;
-                    if (lhs !== node && lhs instanceof AST_Destructured) {
                     var replace = can_replace;
                     can_replace = false;
                     var after = stop_after;
                     var if_hit = stop_if_hit;
                     var stack = scanner.stack;
                     scanner.stack = [ parent ];
-                        lhs.transform(scanner);
+                    expr.transform(scanner);
                     scanner.stack = stack;
                     stop_if_hit = if_hit;
                     stop_after = after;
@@ -2580,7 +2573,12 @@ merge(Compressor.prototype, {
                         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);
                     return find_stop_unused(parent, level + 1);
                 }
             }

test/compress/collapse_vars.js

@@ -9171,3 +9171,26 @@ issue_4914: {
     }
     expect_stdout: "function"
 }
+
+issue_4918: {
+    options = {
+        collapse_vars: true,
+    }
+    input: {
+        var a = "FAIL";
+        ({
+            get 42() {
+                console.log(a);
+            }
+        }[a = "PASS", 42] += "PASS");
+    }
+    expect: {
+        var a = "FAIL";
+        ({
+            get 42() {
+                console.log(a);
+            }
+        }[a = "PASS", 42] += "PASS");
+    }
+    expect_stdout: "PASS"
+}