Preserve ThisBinding in conditionals & collapse_vars

Fixes #973

lib/compress.js

@@ -176,6 +176,21 @@ merge(Compressor.prototype, {
         }
     };
 
+    // we shouldn't compress (1,func)(something) to
+    // func(something) because that changes the meaning of
+    // the func (becomes lexical instead of global).
+    function maintain_this_binding(parent, orig, val) {
+        if (parent instanceof AST_Call && parent.expression === orig && val instanceof AST_PropAccess) {
+            return make_node(AST_Seq, orig, {
+                car: make_node(AST_Number, orig, {
+                    value: 0
+                }),
+                cdr: val
+            });
+        }
+        return val;
+    }
+
     function as_statement_array(thing) {
         if (thing === null) return [];
         if (thing instanceof AST_BlockStatement) return thing.body;
@@ -366,7 +381,7 @@ merge(Compressor.prototype, {
                 if (is_lvalue(node, parent)) return node;
 
                 // Remove var definition and return its value to the TreeTransformer to replace.
-                var value = var_decl.value;
+                var value = maintain_this_binding(parent, node, var_decl.value);
                 var_decl.value = null;
 
                 var_defs.splice(var_defs_index, 1);
@@ -2108,13 +2123,7 @@ merge(Compressor.prototype, {
         if (!compressor.option("side_effects"))
             return self;
         if (!self.car.has_side_effects(compressor)) {
-            // we shouldn't compress (1,func)(something) to
+            return maintain_this_binding(compressor.parent(), self, self.cdr);
-            // func(something) because that changes the meaning of
-            // the func (becomes lexical instead of global).
-            var p = compressor.parent();
-            if (!(p instanceof AST_Call && p.expression === self)) {
-                return self.cdr;
-            }
         }
         if (compressor.option("cascade")) {
             if (self.car instanceof AST_Assign
@@ -2304,11 +2313,10 @@ merge(Compressor.prototype, {
                 if (ll.length > 1) {
                     if (ll[1]) {
                         compressor.warn("Condition left of && always true [{file}:{line},{col}]", self.start);
-                        var rr = self.right.evaluate(compressor);
+                        return maintain_this_binding(compressor.parent(), self, self.right.evaluate(compressor)[0]);
-                        return rr[0];
                     } else {
                         compressor.warn("Condition left of && always false [{file}:{line},{col}]", self.start);
-                        return ll[0];
+                        return maintain_this_binding(compressor.parent(), self, ll[0]);
                     }
                 }
             }
@@ -2317,11 +2325,10 @@ merge(Compressor.prototype, {
                 if (ll.length > 1) {
                     if (ll[1]) {
                         compressor.warn("Condition left of || always true [{file}:{line},{col}]", self.start);
-                        return ll[0];
+                        return maintain_this_binding(compressor.parent(), self, ll[0]);
                     } else {
                         compressor.warn("Condition left of || always false [{file}:{line},{col}]", self.start);
-                        var rr = self.right.evaluate(compressor);
+                        return maintain_this_binding(compressor.parent(), self, self.right.evaluate(compressor)[0]);
-                        return rr[0];
                     }
                 }
             }
@@ -2546,10 +2553,10 @@ merge(Compressor.prototype, {
         if (cond.length > 1) {
             if (cond[1]) {
                 compressor.warn("Condition always true [{file}:{line},{col}]", self.start);
-                return self.consequent;
+                return maintain_this_binding(compressor.parent(), self, self.consequent);
             } else {
                 compressor.warn("Condition always false [{file}:{line},{col}]", self.start);
-                return self.alternative;
+                return maintain_this_binding(compressor.parent(), self, self.alternative);
             }
         }
         var negated = cond[0].negate(compressor);

test/compress/issue-782.js

@@ -5,19 +5,19 @@ remove_redundant_sequence_items: {
         (0, 1, _decorators.logThis)();
     }
     expect: {
-        (0, logThis)();
+        logThis();
         (0, _decorators.logThis)();
     }
 }
 
-dont_remove_lexical_binding_sequence: {
+dont_remove_this_binding_sequence: {
     options = { side_effects: true };
     input: {
         (0, logThis)();
         (0, _decorators.logThis)();
     }
     expect: {
-        (0, logThis)();
+        logThis();
         (0, _decorators.logThis)();
     }
 }

test/compress/issue-973.js

@@ -0,0 +1,52 @@
+this_binding_conditionals: {
+    options = {
+        conditionals: true,
+        evaluate    : true
+    };
+    input: {
+        (1 && a)();
+        (0 || a)();
+        (0 || 1 && a)();
+        (1 ? a : 0)();
+
+        (1 && a.b)();
+        (0 || a.b)();
+        (0 || 1 && a.b)();
+        (1 ? a.b : 0)();
+
+        (1 && a[b])();
+        (0 || a[b])();
+        (0 || 1 && a[b])();
+        (1 ? a[b] : 0)();
+    }
+    expect: {
+        a();
+        a();
+        a();
+        a();
+
+        (0, a.b)();
+        (0, a.b)();
+        (0, a.b)();
+        (0, a.b)();
+
+        (0, a[b])();
+        (0, a[b])();
+        (0, a[b])();
+        (0, a[b])();
+    }
+}
+
+this_binding_collapse_vars: {
+    options = {
+        collapse_vars: true,
+    };
+    input: {
+        var c = a; c();
+        var d = a.b; d();
+    }
+    expect: {
+        a();
+        (0, a.b)();
+    }
+}