closure-compiler

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Input:

// ==ClosureCompiler==
// @compilation_level ADVANCED_OPTIMIZATIONS
// @output_file_name default.js
// @language_out ECMASCRIPT_2015
// ==/ClosureCompiler==

const A = class {
  constructor(opts) {
  }
};

new A(1);

Expected output:

'use strict';

Actual output:

'use strict';new class{constructor(){}}(1);

Works as expected with language_out ECMASCRIPT5, but any language_out ECMASCRIPT_2015 or above does not DCE the unused instance.

https://closure-compiler.appspot.com/home#code%3D%252F%252F%2520%253D%253DClosureCompiler%253D%253D%250A%252F%252F%2520%2540compilation_level%2520ADVANCED_OPTIMIZATIONS%250A%252F%252F%2520%2540output_file_name%2520default.js%250A%252F%252F%2520%2540language_out%2520ECMASCRIPT_2015%250A%252F%252F%2520%253D%253D%252FClosureCompiler%253D%253D%250A%250Aconst%2520A%2520%253D%2520class%2520%257B%250A%2520%2520constructor(opts)%2520%257B%250A%2520%2520%257D%250A%257D%253B%250A%250Anew%2520A(1)%253B%250A%250Aclass%2520B%2520%257B%250A%2520%2520constructor(opts)%2520%257B%250A%2520%2520%257D%250A%257D%250A%250Anew%2520B(2)%253B%250A%250Aconsole.log(1)%253B%250A

By using --print_source_after_each_pass I found that after transpilation and inlining the compiler converts the code above to new function(){}(1); for ES5 output or new class {constructor(){}}(1);.

Then the peepholeOptimizations pass knows how to remove new function(){}(1);, but apparently does not know it is safe to remove new class {constructor(){}}(1);.