Downlevel emit for template strings should use 'void 0' not 'undefined' #38430

TypeScript Version: 3.9.x, 4.0.x-dev

Search Terms: template string emit

Code

From ~/tests/cases/conformance/es2018/invalidTaggedTemplateEscapeSequences.ts:

const x = tag`\u{hello} ${ 100 } \xtraordinary ${ 200 } wonderful ${ 300 } \uworld`;

Expected behavior:

ES5/ES3 emit:

const x = tag(__makeTemplateObject([void 0, void 0, " wonderful ", void 0], ["\\u{hello} ", " \\xtraordinary ", " wonderful ", " \\uworld"]), 100, 200, 300);

Actual behavior:

ES5/ES3 emit:

const x = tag(__makeTemplateObject([undefined, undefined, " wonderful ", undefined], ["\\u{hello} ", " \\xtraordinary ", " wonderful ", " \\uworld"]), 100, 200, 300);

In ES5, undefined is an identifier and not a keyword and can be redeclared. In ES3, undefined isn't declared. In general we emit void 0 in these cases.

This can be easily addressed by changing createTemplateCooked in ~/src/compiler/transformers/taggedTemplate.ts to return createVoidZero() instead of createIdentifier("undefined") when a template contains an invalid escape sequence..

Playground Link: link

Related Issues: #23801, #12700