fix corner case in reduce_vars

fixes #5949

Author: alexlamsl

lib/compress.js

@@ -1400,22 +1400,23 @@ Compressor.prototype.compress = function(node) {
             pop_scope(tw, fn);
             return true;
         });
-        def(AST_Sub, function(tw, descend) {
+        def(AST_Sub, function(tw) {
             var node = this;
             var expr = node.expression;
+            var prop = node.property;
+            expr.walk(tw);
             if (node.optional) {
-                expr.walk(tw);
                 push(tw, true, true);
-                node.property.walk(tw);
+                prop.walk(tw);
                 pop(tw);
             } else {
-                descend();
                 while (expr instanceof AST_Assign && expr.operator == "=") {
                     var lhs = expr.left;
                     if (lhs instanceof AST_SymbolRef) access(tw, lhs.definition());
                     expr = expr.right;
                 }
                 if (expr instanceof AST_SymbolRef) access(tw, expr.definition());
+                prop.walk(tw);
             }
             return true;
         });

test/compress/properties.js

@@ -1992,3 +1992,57 @@ issue_5927: {
     }
     expect_stdout: "PASS"
 }
+
+issue_5949_1: {
+    options = {
+        pure_getters: "strict",
+        reduce_vars: true,
+        side_effects: true,
+    }
+    input: {
+        var a = 42;
+        a[a = null];
+        try {
+            a.p;
+            console.log("FAIL");
+        } catch (e) {
+            console.log("PASS");
+        }
+    }
+    expect: {
+        var a = 42;
+        a[a = null];
+        try {
+            a.p;
+            console.log("FAIL");
+        } catch (e) {
+            console.log("PASS");
+        }
+    }
+    expect_stdout: "PASS"
+}
+
+issue_5949_2: {
+    options = {
+        pure_getters: "strict",
+        reduce_vars: true,
+        side_effects: true,
+    }
+    input: {
+        try {
+            a[42];
+            console.log("FAIL");
+        } catch (e) {
+            console.log("PASS");
+        }
+    }
+    expect: {
+        try {
+            a[42];
+            console.log("FAIL");
+        } catch (e) {
+            console.log("PASS");
+        }
+    }
+    expect_stdout: "PASS"
+}