fix corner case in unused (#5847)

fixes #5843

Author: alexlamsl

lib/compress.js

@@ -8284,6 +8284,7 @@ Compressor.prototype.compress = function(node) {
                             if (prop instanceof AST_Spread) return prop;
                             var key = prop_keys[index];
                             if (key instanceof AST_Node) return prop;
+                            if (key === "__proto__") return prop;
                             if (drop_keys.has(key)) {
                                 var mapped = drop_keys.get(key);
                                 if (!mapped) return prop;
@@ -8317,7 +8318,10 @@ Compressor.prototype.compress = function(node) {
                         if (value.has_side_effects(compressor) && prop.value.match_symbol(function(node) {
                             return node instanceof AST_PropAccess;
                         })) break;
-                        value = make_node(AST_Sub, node, {
+                        value = is_identifier_string(prop.key) ? make_node(AST_Dot, node, {
+                            expression: value,
+                            property: prop.key,
+                        }) : make_node(AST_Sub, node, {
                             expression: value,
                             property: make_node_from_constant(prop.key, prop),
                         });

test/compress/default-values.js

@@ -2293,7 +2293,7 @@ issue_5340_3: {
     }
     expect: {
         var a;
-        (function() {})(a = true["p"]);
+        (function() {})(a = true.p);
         console.log(a);
     }
     expect_stdout: "undefined"

test/compress/destructured.js

@@ -1382,7 +1382,7 @@ side_effects_object: {
         }
     }
     expect: {
-        var a = null, c = (console, 42["c"]);
+        var a = null, c = (console, 42..c);
         try {
             c[a = "PASS"];
         } catch (e) {
@@ -1684,7 +1684,7 @@ singleton_1: {
     expect: {
         var b, a = "P"[0], o = {};
         o.p = [ "FAIL"["1"] ][0];
-        o.q = { foo: "S"[0] }["foo"];
+        o.q = { foo: "S"[0] }.foo;
         [ b = "S" ] = [];
         console.log(a + o.p + o.q + b);
     }
@@ -3886,3 +3886,54 @@ issue_5651: {
     expect_stdout: true
     node_version: ">=6"
 }
+
+issue_5843_1: {
+    options = {
+        unused: true,
+    }
+    input: {
+        var { p: a } = {
+            __proto__: {
+                p: "PASS",
+            },
+        };
+        console.log(a);
+    }
+    expect: {
+        var a = {
+            __proto__: {
+                p: "PASS",
+            },
+        }.p;
+        console.log(a);
+    }
+    expect_stdout: "PASS"
+    node_version: ">=6"
+}
+
+issue_5843_2: {
+    options = {
+        side_effects: true,
+        unused: true,
+    }
+    input: {
+        var a;
+        ({ p: a } = {
+            __proto__: {
+                p: "PASS",
+            },
+        });
+        console.log(a);
+    }
+    expect: {
+        var a;
+        a = {
+            __proto__: {
+                p: "PASS",
+            },
+        }.p;
+        console.log(a);
+    }
+    expect_stdout: "PASS"
+    node_version: ">=6"
+}

test/compress/exports.js

@@ -260,7 +260,7 @@ hoist_exports_2: {
         }
     }
     expect: {
-        let e, a = 42["foo"];
+        let e, a = 42..foo;
         function f(t, { [e]: o }) {
             t(o, f);
         }

test/compress/yields.js

@@ -1386,7 +1386,7 @@ issue_5076_1: {
     expect: {
         var a;
         console.log("PASS"),
-        a = 42["a"];
+        a = 42..a;
     }
     expect_stdout: "PASS"
     node_version: ">=6"