fix corner case in reduce_vars (#4585)

fixes #4584

lib/compress.js

@@ -786,7 +786,10 @@ merge(Compressor.prototype, {
                     }
                     var d = sym.definition();
                     d.assignments++;
-                    if (fixed && !is_modified(compressor, tw, node, node.right, 0) && safe_to_assign(tw, d)) {
+                    if (fixed
+                        && !is_modified(compressor, tw, node, node.right, 0)
+                        && !sym.in_arg
+                        && safe_to_assign(tw, d)) {
                         push_ref(d, sym);
                         mark(tw, d);
                         if (d.single_use && left instanceof AST_Destructured) d.single_use = false;
@@ -809,7 +812,7 @@ merge(Compressor.prototype, {
                 }
                 var safe = safe_to_read(tw, d);
                 node.right.walk(tw);
-                if (safe && safe_to_assign(tw, d)) {
+                if (safe && !left.in_arg && safe_to_assign(tw, d)) {
                     push_ref(d, left);
                     mark(tw, d);
                     if (d.single_use) d.single_use = false;
@@ -1117,7 +1120,7 @@ merge(Compressor.prototype, {
             var d = exp.definition();
             d.assignments++;
             var fixed = d.fixed;
-            if (safe_to_read(tw, d) && safe_to_assign(tw, d)) {
+            if (safe_to_read(tw, d) && !exp.in_arg && safe_to_assign(tw, d)) {
                 push_ref(d, exp);
                 mark(tw, d);
                 if (d.single_use) d.single_use = false;

test/compress/destructured.js

@@ -2487,3 +2487,30 @@ issue_4554: {
     expect_stdout: "PASS"
     node_version: ">=6"
 }
+
+issue_4584: {
+    options = {
+        evaluate: true,
+        reduce_vars: true,
+    }
+    input: {
+        try {
+            (function f({
+                [console.log(a = "FAIL")]: a,
+            }) {})(0);
+        } catch (e) {
+            console.log("PASS");
+        }
+    }
+    expect: {
+        try {
+            (function f({
+                [console.log(a = "FAIL")]: a,
+            }) {})(0);
+        } catch (e) {
+            console.log("PASS");
+        }
+    }
+    expect_stdout: "PASS"
+    node_version: ">=6"
+}

test/ufuzz/index.js

@@ -431,12 +431,18 @@ function createAssignmentPairs(recurmax, stmtDepth, canThrow, nameLenBefore, was
     unique_vars.length = len;
     return pairs;
 
-    function mapShuffled(array, fn) {
-        var result = [];
-        for (var i = array.length; --i >= 0;) {
-            result.splice(rng(result.length + 1), 0, fn(array[i], i));
-        }
-        return result;
+    function mapShuffled(values, fn) {
+        var ordered = [];
+        var shuffled = [];
+        values.forEach(function(value, index) {
+            value = fn(value, index);
+            if (/]:/.test(value) ? canThrow && rng(10) == 0 : rng(5)) {
+                shuffled.splice(rng(shuffled.length + 1), 0, value);
+            } else {
+                ordered.push(value);
+            }
+        });
+        return shuffled.concat(ordered);
     }
 
     function convertToRest(names) {