Deprecate alwaysStrict: false in TypeScript 6.0

[Copilot]

Deprecates alwaysStrict: false in 6.0 (will be removed in 7.0). This enables unconditional strict mode parsing, eliminating lookahead/reparse overhead for ambiguous constructs and simplifying AST caching across projects with different flags.

Changes

Deprecation warning in src/compiler/program.ts:

if (options.alwaysStrict === false) {
    createDeprecatedDiagnostic("alwaysStrict", "false", /*useInstead*/ undefined, /*related*/ undefined);
}

User impact:

error TS5107: Option 'alwaysStrict=false' is deprecated and will stop functioning in TypeScript 7.0.
Specify compilerOption '"ignoreDeprecations": "6.0"' to silence this error.

Tests:

• alwaysStrictDeprecation.ts - verifies warning emission
• alwaysStrictDeprecationSilenced.ts - verifies ignoreDeprecations: "6.0" silences warning
• Updated programUpdates.ts test to add ignoreDeprecations: "6.0" to silence the deprecation warning in the updates-errors-when-strictNullChecks-changes test, allowing it to continue testing strictNullChecks changes without interference
• Updated baseline for updates-errors-when-strictNullChecks-changes.js to reflect the silenced deprecation

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Assume "use strict" everywhere by default</issue_title>
<issue_description>From #54500:

The alwaysStrict flag refers to inference of the "use strict"; directive. In TypeScript 6.0, all code will be assumed to be in "strict mode", which is a set of JS semantics that mostly affects syntactic corner cases around reserved words.

This lets us be faster because it's no longer necessary to look ahead or repase on constructs like

var x = await

which is legal "sloppy mode" referring to a variable named await. This also reduces the number of flags which possibly prevent sharing of source files between projects with different compiler settings.

See #61888 for RWC run; we found one codebase that used static as a parameter name.

This also lets us be a bit more efficient because our language service (and --build mode?) tries its best to reuse syntax trees for files belonging to multiple projects. When flags differ across projects, we have to sometimes cache ASTs by creating a composite key of both their paths and their projects' flags. If we can remove alwaysStrict, we can simplify our code and ensure syntax trees are shared more often. Over time, we may be able to remove all configuration flags that impact sharing.

We may need to clarify a few details here: specifically, are we removing --alwaysStrict, or just turning it on by default? If the former, that means that in 6.0 we are deprecating --alwaysStrict false.</issue_description>

Comments on the Issue (you are @copilot in this section)

@jakebailey > We may need to clarify a few details here: specifically, are we removing --alwaysStrict, or just turning it on by default? If the former, that means that in 6.0 we are deprecating --alwaysStrict false.

I believe the right answer here is that we deprecate false in 6.0, and delete it entirely in 7.0.</comment_new>
<comment_new>@jakebailey
Note that the implication here is that we unconditionally emit "use strict" in non-ESM files.</comment_new>

• Fixes Assume "use strict" everywhere by default #62213

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[Copilot]

Pull request overview

This PR deprecates the alwaysStrict: false compiler option in TypeScript 6.0, with full removal planned for 7.0. This change enables unconditional strict mode parsing, which eliminates lookahead/reparse overhead for ambiguous constructs and simplifies AST caching across projects with different compiler flags.

Changes:

• Added deprecation warning for alwaysStrict: false in the compiler
• Created test cases to verify deprecation warning emission and silencing behavior
• Updated baseline for existing test affected by the deprecation warning

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/compiler/program.ts	Adds deprecation check for alwaysStrict: false option
tests/cases/compiler/alwaysStrictDeprecation.ts	Test case verifying deprecation warning is emitted
tests/cases/compiler/alwaysStrictDeprecationSilenced.ts	Test case verifying warning can be silenced with ignoreDeprecations: "6.0"
tests/baselines/reference/alwaysStrictDeprecation.errors.txt	Expected error output for deprecation warning
tests/baselines/reference/alwaysStrictDeprecation.js	Expected JavaScript output for deprecation test
tests/baselines/reference/alwaysStrictDeprecation.symbols	Expected symbol output for deprecation test
tests/baselines/reference/alwaysStrictDeprecation.types	Expected type output for deprecation test
tests/baselines/reference/alwaysStrictDeprecationSilenced.js	Expected JavaScript output when deprecation is silenced
tests/baselines/reference/alwaysStrictDeprecationSilenced.symbols	Expected symbol output when deprecation is silenced
tests/baselines/reference/alwaysStrictDeprecationSilenced.types	Expected type output when deprecation is silenced
tests/baselines/reference/tscWatch/programUpdates/updates-errors-when-strictNullChecks-changes.js	Updated baseline now showing deprecation error instead of null check error

[RyanCavanaugh]

@copilot do as Jake says