when preserveConstEnums = true should allow use enum[key]

TypeScript Version: 3.4.0-dev.201xxxxx

Search Terms:

Code

preserveConstEnums = true

const enum EnumCacheName
{
	'toc_contents' = '.toc_contents.cache'
}

let { target } = yargs
	.option('target', {
		type: 'string',
	})
	.argv
;


let cache_file = path.join(ProjectConfig.cache_root, EnumCacheName[target as any]);

Expected behavior:

no error, because this code exists in .js

var EnumCacheName;
(function (EnumCacheName) {
    EnumCacheName["toc_contents"] = ".toc_contents.cache";
})(EnumCacheName || (EnumCacheName = {}));

Actual behavior:

Error: TS2476: A const enum member can only be accessed using a string literal.

Playground Link:

Related Issues:

Start by reproducing the reported TS2476 error with the const enum and indexed access shown in the issue, using preserveConstEnums=true. Inspect the compiler path that checks const enum member access and compare it with the expected JavaScript output. Done means this code compiles without the error and preserves the expected runtime lookup behavior.