fix(dce): bail on non-constant replacement path (#755)

* fix(dce): bail on non-constant replacement path

In one use variable replacement, if the right hand side contains
constant violations, the left-hand side is supposed to be preserved, so
we detect this and bail out for this case

+ Fix #685

* Fix binding check and checks for not Identifier

* Add testcase for non-identifier replacementPaths

Author: boopathi

packages/babel-plugin-minify-dead-code-elimination/__tests__/fixtures/issue-685/actual.js

@@ -0,0 +1,16 @@
+function loop() {
+  var end = 0;
+  var start = end;
+  while (end < 10) {
+    console.log(start, end);
+    var end = end + 1;
+  }
+}
+loop();
+
+function bar() {
+  var x = 1;
+  var y = x + 2;
+  var x = 3;
+  return x + y;
+}

packages/babel-plugin-minify-dead-code-elimination/__tests__/fixtures/issue-685/expected.js

@@ -0,0 +1,16 @@
+function loop() {
+  var end = 0;
+  var start = end;
+  while (end < 10) {
+    console.log(start, end);
+    var end = end + 1;
+  }
+}
+loop();
+
+function bar() {
+  var x = 1;
+  var y = x + 2;
+  var x = 3;
+  return x + y;
+}

packages/babel-plugin-minify-dead-code-elimination/src/index.js

@@ -349,9 +349,16 @@ module.exports = ({ types: t, traverse }) => {
               let bail = false;
 
               if (replacementPath.isIdentifier()) {
-                bail =
-                  refPath.scope.getBinding(replacement.name) !==
-                  scope.getBinding(replacement.name);
+                const binding = scope.getBinding(replacement.name);
+                // the reference should be in the same scope
+                // and the replacement should be a constant - this is to
+                // ensure that the duplication of replacement is not affected
+                // https://github.com/babel/minify/issues/685
+                bail = !(
+                  binding &&
+                  refPath.scope.getBinding(replacement.name) === binding &&
+                  binding.constantViolations.length === 0
+                );
               } else {
                 replacementPath.traverse({
                   Function(path) {
@@ -362,9 +369,13 @@ module.exports = ({ types: t, traverse }) => {
                     if (bail) {
                       return;
                     }
-                    bail =
-                      refPath.scope.getBinding(node.name) !==
-                      scope.getBinding(node.name);
+                    const binding = scope.getBinding(node.name);
+                    if (
+                      binding &&
+                      refPath.scope.getBinding(node.name) === binding
+                    ) {
+                      bail = binding.constantViolations.length > 0;
+                    }
                   }
                 });
               }