Repository: babel/babili (js)

Issue:
[Webpack 3?] Duplicate declaration error
I've been trying to isolate this error, but I think that it has something to do with Webpack 3 and I can't pull anything of value from Babili and Webpack.

However, you can reproduce this issue by:

```
git clone https://github.com/ckeditor/ckeditor5-build-classic.git
git co babili-webpack-error
npm i
npm run build-ckeditor
```

This is the output I get:

```
(babili-webpack-error b720deb) p@m /workspace/test/ckeditor5-build-classic> npm run build-ckeditor

> @ckeditor/ckeditor5-build-classic@0.2.0 build-ckeditor /workspace/test/ckeditor5-build-classic
> ./bin/build-ckeditor.sh

Building 'build/ckeditor.js'...

Hash: 50fb7c52c064ebfdf790
Version: webpack 3.0.0
Time: 3798ms
          Asset    Size  Chunks                    Chunk Names
    ckeditor.js  405 kB       0  [emitted]  [big]  main
ckeditor.js.map  484 kB       0  [emitted]         main
  [37] ./ckeditor.js 191 bytes {0} [built]
  [58] (webpack)/buildin/harmony-module.js 596 bytes {0} [built]
  [59] (webpack)/buildin/global.js 509 bytes {0} [built]
    + 88 hidden modules

ERROR in unknown: Duplicate declaration "count"

Done.
```

This is the Webpack config: https://github.com/ckeditor/ckeditor5-build-classic/blob/babili-webpack-error/webpack.config.js

And the crucial file is this one: https://github.com/ckeditor/ckeditor5-typing/blob/958eeb26c6d862de3cdfbf53efd0a554e3c8e13e/src/typing.js

It imports:

* https://github.com/ckeditor/ckeditor5-typing/blob/958eeb26c6d862de3cdfbf53efd0a554e3c8e13e/src/input.js
* https://github.com/ckeditor/ckeditor5-typing/blob/958eeb26c6d862de3cdfbf53efd0a554e3c8e13e/src/delete.js

Which both depends on:

* https://github.com/ckeditor/ckeditor5-typing/blob/958eeb26c6d862de3cdfbf53efd0a554e3c8e13e/src/changebuffer.js

Both of these modules import `count` function and I think that this is that duplicated `count` identifier.

If I disable Babili plugin in `webpack.config.js` everything works fine. I also checked that builds done without Babili run correctly so most likely the issue is not on Webpack's side.

Output without Babili:

```
(babili-webpack-error b720deb M3) p@m /workspace/test/ckeditor5-build-classic> npm run build-ckeditor

> @ckeditor/ckeditor5-build-classic@0.2.0 build-ckeditor /workspace/test/ckeditor5-build-classic
> ./bin/build-ckeditor.sh

Building 'build/ckeditor.js'...

Hash: 50fb7c52c064ebfdf790
Version: webpack 3.0.0
Time: 759ms
          Asset    Size  Chunks                    Chunk Names
    ckeditor.js  405 kB       0  [emitted]  [big]  main
ckeditor.js.map  484 kB       0  [emitted]         main
  [37] ./ckeditor.js 191 bytes {0} [built]
  [58] (webpack)/buildin/harmony-module.js 596 bytes {0} [built]
  [59] (webpack)/buildin/global.js 509 bytes {0} [built]
    + 88 hidden modules

Done.
```

I've checked the unminified output but I can't find anything there which would suspiscious.

I'm using webpack 3.0.0 and babili-webpack-plugin 0.1.2.

Pull request description:
Detect duplicate declaration during one-use replacement
Fix #611

Patch:
diff --git a/packages/babel-plugin-minify-dead-code-elimination/src/index.js b/packages/babel-plugin-minify-dead-code-elimination/src/index.js
index 9f3bdbf..796c087 100644
--- a/packages/babel-plugin-minify-dead-code-elimination/src/index.js
+++ b/packages/babel-plugin-minify-dead-code-elimination/src/index.js
@@ -400,7 +400,8 @@ module.exports = ({ types: t, traverse }) => {
               const replaced = replace(binding.referencePaths[0], {
                 binding,
                 scope,
-                replacement
+                replacement,
+                replacementPath
               });
 
               if (replaced) {
@@ -936,7 +937,7 @@ module.exports = ({ types: t, traverse }) => {
   }
 
   function replace(path, options) {
-    const { replacement, scope, binding } = options;
+    const { replacement, replacementPath, scope, binding } = options;
 
     // Same name, different binding.
     if (scope.getBinding(path.node.name) !== binding) {
@@ -976,6 +977,18 @@ module.exports = ({ types: t, traverse }) => {
       return;
     }
 
+    // https://github.com/babel/babili/issues/611
+    // this is valid only for FunctionDeclaration where we convert
+    // function declaration to expression in the next step
+    if (replacementPath.isFunctionDeclaration()) {
+      const fnName = replacementPath.get("id").node.name;
+      for (let name in replacementPath.scope.bindings) {
+        if (name === fnName) {
+          return;
+        }
+      }
+    }
+
     // https://github.com/babel/babili/issues/130
     if (!t.isExpression(replacement)) {
       t.toExpression(replacement);


Test patch:
diff --git a/packages/babel-plugin-minify-dead-code-elimination/__tests__/dead-code-elimination-test.js b/packages/babel-plugin-minify-dead-code-elimination/__tests__/dead-code-elimination-test.js
index 3e4476b..abecf1d 100644
--- a/packages/babel-plugin-minify-dead-code-elimination/__tests__/dead-code-elimination-test.js
+++ b/packages/babel-plugin-minify-dead-code-elimination/__tests__/dead-code-elimination-test.js
@@ -2448,4 +2448,18 @@ describe("dce-plugin", () => {
       }
     `
   );
+
+  thePlugin(
+    "should fix issue#611 - transforming fn decl to expr",
+    `
+      function foo() {
+        function count() {
+          let count = 1;
+          bar(count);
+          return count;
+        }
+        return count;
+      }
+    `
+  );
 });