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Fixes #39715.
Also gives a partial fix to #396.

Example

Code

String.raw`\x \012`;

`\x \012`;

Current Behavior

src/backslashx.ts:1:14 - error TS1125: Hexadecimal digit expected.

1 String.raw`\x \012`;
               

src/backslashx.ts:3:4 - error TS1125: Hexadecimal digit expected.

3 `\x \012`;
     


Found 2 errors.

New Behavior

src/backslashx.ts:3:4 - error TS1125: Hexadecimal digit expected.

3 `\x \012`;
     

src/backslashx.ts:3:7 - error TS1392: Octal escape sequences are not allowed in template strings.

3 `\x \012`;
        


Found 2 errors.

(Error disappeared from tagged template literal, and new error for octal escape sequence in untagged template literal)

Implementation Detail

With this PR, invalid escape sequences do not emit errors when they are first scanned. Untagged template literals with invalid escape sequence in it are re-scanned to emit syntax errors.