optimize conditional when condition symbol matches consequent (#1684)

lib/compress.js

@@ -3565,8 +3565,19 @@ merge(Compressor.prototype, {
                 alternative: self.consequent
             });
         }
+        var condition = self.condition;
         var consequent = self.consequent;
         var alternative = self.alternative;
+        // x?x:y --> x||y
+        if (condition instanceof AST_SymbolRef
+            && consequent instanceof AST_SymbolRef
+            && condition.definition() === consequent.definition()) {
+            return make_node(AST_Binary, self, {
+                operator: "||",
+                left: condition,
+                right: alternative
+            });
+        }
         // if (foo) exp = something; else exp = something_else;
         //                   |
         //                   v

test/compress/conditionals.js

@@ -933,3 +933,32 @@ issue_1645_2: {
     }
     expect_stdout: true
 }
+
+condition_symbol_matches_consequent: {
+    options = {
+        conditionals: true,
+    }
+    input: {
+        function foo(x, y) {
+            return x ? x : y;
+        }
+        function bar() {
+            return g ? g : h;
+        }
+        var g = 4;
+        var h = 5;
+        console.log(foo(3, null), foo(0, 7), foo(true, false), bar());
+    }
+    expect: {
+        function foo(x, y) {
+            return x || y;
+        }
+        function bar() {
+            return g || h;
+        }
+        var g = 4;
+        var h = 5;
+        console.log(foo(3, null), foo(0, 7), foo(true, false), bar());
+    }
+    expect_stdout: "3 7 true 4"
+}