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

[jakebailey]

@typescript-bot test it

[typescript-bot]

Starting jobs; this comment will be updated as builds start and complete.

Command	Status	Results
test top400	✅ Started	✅ Results
user test this	✅ Started	✅ Results
run dt	✅ Started	✅ Results
perf test this faster	✅ Started	👀 Results

[typescript-bot]

Hey @jakebailey, the results of running the DT tests are ready.

Everything looks the same!

You can check the log here.

[typescript-bot]

@jakebailey Here are the results of running the user tests with tsc comparing main and refs/pull/63030/merge:

There were infrastructure failures potentially unrelated to your change:

• 1 instance of "Package install failed"
• 1 instance of "Git clone failed"

Otherwise...

Everything looks good!

[typescript-bot]

@jakebailey
The results of the perf run you requested are in!

Here they are:

tsc

Comparison Report - baseline..pr

Metric	baseline	pr	Delta	Best	Worst	p-value
Compiler-Unions - node (v18.15.0, x64)
Errors	2	2	~	~	~	p=1.000 n=6
Symbols	62,370	62,370	~	~	~	p=1.000 n=6
Types	50,387	50,387	~	~	~	p=1.000 n=6
Memory used	193,754k (± 0.94%)	194,389k (± 1.01%)	~	192,568k	196,217k	p=0.378 n=6
Parse Time	1.30s (± 0.42%)	1.30s (± 0.49%)	~	1.29s	1.31s	p=0.201 n=6
Bind Time	0.76s	0.76s	~	~	~	p=1.000 n=6
Check Time	9.86s (± 0.28%)	9.89s (± 0.21%)	~	9.86s	9.91s	p=0.073 n=6
Emit Time	2.74s (± 0.91%)	2.74s (± 0.60%)	~	2.71s	2.76s	p=0.935 n=6
Total Time	14.66s (± 0.30%)	14.69s (± 0.21%)	~	14.65s	14.74s	p=0.146 n=6
angular-1 - node (v18.15.0, x64)
Errors	2	2	~	~	~	p=1.000 n=6
Symbols	955,823	955,823	~	~	~	p=1.000 n=6
Types	415,853	415,853	~	~	~	p=1.000 n=6
Memory used	1,253,904k (± 0.00%)	1,253,899k (± 0.00%)	~	1,253,847k	1,253,941k	p=0.810 n=6
Parse Time	6.54s (± 0.46%)	6.53s (± 0.43%)	~	6.51s	6.58s	p=0.366 n=6
Bind Time	1.96s	1.96s (± 0.68%)	~	1.94s	1.98s	p=0.599 n=6
Check Time	32.41s (± 0.32%)	32.47s (± 0.27%)	~	32.32s	32.55s	p=0.297 n=6
Emit Time	15.00s (± 0.33%)	14.98s (± 0.33%)	~	14.91s	15.04s	p=0.469 n=6
Total Time	55.91s (± 0.25%)	55.94s (± 0.13%)	~	55.84s	56.05s	p=0.688 n=6
mui-docs - node (v18.15.0, x64)
Errors	11,439	11,439	~	~	~	p=1.000 n=6
Symbols	2,700,258	2,700,258	~	~	~	p=1.000 n=6
Types	930,444	930,444	~	~	~	p=1.000 n=6
Memory used	3,036,168k (± 0.00%)	3,036,130k (± 0.00%)	~	3,035,985k	3,036,245k	p=0.575 n=6
Parse Time	8.59s (± 0.23%)	8.59s (± 0.11%)	~	8.58s	8.61s	p=1.000 n=6
Bind Time	2.30s (± 0.45%)	2.30s (± 0.22%)	~	2.30s	2.31s	p=0.928 n=6
Check Time	92.72s (± 0.74%)	93.13s (± 0.32%)	~	92.77s	93.44s	p=0.229 n=6
Emit Time	0.31s (± 2.41%)	0.31s (± 3.19%)	~	0.30s	0.32s	p=0.554 n=6
Total Time	103.93s (± 0.68%)	104.34s (± 0.29%)	~	103.96s	104.66s	p=0.296 n=6
self-build-src - node (v18.15.0, x64)
Errors	0	0	~	~	~	p=1.000 n=6
Symbols	1,252,447	1,252,449	+2 (+ 0.00%)	~	~	p=0.001 n=6
Types	259,946	259,946	~	~	~	p=1.000 n=6
Memory used	2,387,987k (± 0.03%)	2,508,754k (±11.83%)	~	2,387,113k	3,115,333k	p=0.689 n=6
Parse Time	5.13s (± 0.43%)	5.16s (± 1.25%)	~	5.10s	5.28s	p=0.294 n=6
Bind Time	1.87s (± 0.63%)	1.85s (± 0.93%)	~	1.82s	1.87s	p=0.050 n=6
Check Time	35.47s (± 0.22%)	35.60s (± 0.39%)	~	35.39s	35.81s	p=0.149 n=6
Emit Time	2.97s (± 2.94%)	3.01s (± 1.01%)	~	2.96s	3.04s	p=0.336 n=6
Total Time	45.45s (± 0.24%)	45.66s (± 0.34%)	+0.21s (+ 0.46%)	45.47s	45.84s	p=0.045 n=6
self-build-src-public-api - node (v18.15.0, x64)
Errors	0	0	~	~	~	p=1.000 n=6
Symbols	1,252,447	1,252,449	+2 (+ 0.00%)	~	~	p=0.001 n=6
Types	259,946	259,946	~	~	~	p=1.000 n=6
Memory used	2,941,345k (±12.73%)	3,183,461k (± 0.02%)	~	3,182,697k	3,184,290k	p=0.378 n=6
Parse Time	6.79s (± 1.75%)	6.78s (± 0.68%)	~	6.71s	6.84s	p=0.575 n=6
Bind Time	2.27s (± 0.71%)	2.26s (± 1.13%)	~	2.23s	2.30s	p=0.575 n=6
Check Time	42.93s (± 0.31%)	43.06s (± 0.22%)	~	42.96s	43.19s	p=0.093 n=6
Emit Time	3.52s (± 2.07%)	3.48s (± 1.60%)	~	3.41s	3.54s	p=0.575 n=6
Total Time	55.51s (± 0.42%)	55.58s (± 0.20%)	~	55.43s	55.70s	p=0.688 n=6
self-compiler - node (v18.15.0, x64)
Errors	0	0	~	~	~	p=1.000 n=6
Symbols	264,690	264,690	~	~	~	p=1.000 n=6
Types	104,079	104,079	~	~	~	p=1.000 n=6
Memory used	443,079k (± 0.01%)	443,091k (± 0.02%)	~	442,982k	443,212k	p=0.810 n=6
Parse Time	3.51s (± 0.50%)	3.52s (± 1.05%)	~	3.49s	3.59s	p=0.743 n=6
Bind Time	1.39s (± 1.66%)	1.37s (± 0.59%)	~	1.36s	1.38s	p=0.183 n=6
Check Time	19.14s (± 0.35%)	19.16s (± 0.43%)	~	19.08s	19.30s	p=0.573 n=6
Emit Time	1.55s (± 0.68%)	1.55s (± 0.67%)	~	1.53s	1.56s	p=0.801 n=6
Total Time	25.59s (± 0.21%)	25.59s (± 0.39%)	~	25.48s	25.77s	p=1.000 n=6
ts-pre-modules - node (v18.15.0, x64)
Errors	73	73	~	~	~	p=1.000 n=6
Symbols	225,493	225,493	~	~	~	p=1.000 n=6
Types	94,373	94,373	~	~	~	p=1.000 n=6
Memory used	369,941k (± 0.05%)	370,007k (± 0.08%)	~	369,735k	370,374k	p=0.689 n=6
Parse Time	2.81s (± 0.74%)	2.83s (± 0.79%)	~	2.79s	2.85s	p=0.190 n=6
Bind Time	1.66s (± 1.17%)	1.65s (± 0.49%)	~	1.64s	1.66s	p=0.869 n=6
Check Time	16.64s (± 0.27%)	16.63s (± 0.45%)	~	16.52s	16.71s	p=0.936 n=6
Emit Time	0.00s	0.00s (±244.70%)	~	0.00s	0.01s	p=0.405 n=6
Total Time	21.11s (± 0.16%)	21.11s (± 0.38%)	~	21.02s	21.23s	p=1.000 n=6
vscode - node (v18.15.0, x64)
Errors	11	11	~	~	~	p=1.000 n=6
Symbols	4,157,470	4,157,470	~	~	~	p=1.000 n=6
Types	1,316,480	1,316,480	~	~	~	p=1.000 n=6
Memory used	3,950,721k (± 0.00%)	3,950,611k (± 0.00%)	~	3,950,429k	3,950,806k	p=0.298 n=6
Parse Time	16.04s (± 0.60%)	16.10s (± 0.73%)	~	15.96s	16.26s	p=0.335 n=6
Bind Time	5.52s (± 2.44%)	5.57s (± 3.11%)	~	5.33s	5.72s	p=1.000 n=6
Check Time	115.72s (± 2.72%)	116.09s (± 3.16%)	~	112.62s	119.93s	p=0.575 n=6
Emit Time	48.59s (± 4.85%)	53.62s (±11.51%)	~	47.00s	63.72s	p=0.173 n=6
Total Time	185.87s (± 2.95%)	191.38s (± 3.94%)	~	181.56s	197.90s	p=0.230 n=6
webpack - node (v18.15.0, x64)
Errors	41	41	~	~	~	p=1.000 n=6
Symbols	383,986	383,986	~	~	~	p=1.000 n=6
Types	167,978	167,978	~	~	~	p=1.000 n=6
Memory used	543,826k (± 0.03%)	543,688k (± 0.02%)	~	543,511k	543,898k	p=0.173 n=6
Parse Time	4.63s (± 0.75%)	4.63s (± 0.42%)	~	4.60s	4.65s	p=1.000 n=6
Bind Time	2.04s (± 0.91%)	2.03s (± 1.06%)	~	2.00s	2.06s	p=0.871 n=6
Check Time	23.68s (± 1.68%)	23.82s (± 1.30%)	~	23.24s	24.11s	p=0.689 n=6
Emit Time	0.00s	0.00s	~	~	~	p=1.000 n=6
Total Time	30.34s (± 1.31%)	30.48s (± 1.02%)	~	29.89s	30.75s	p=0.688 n=6
xstate-main - node (v18.15.0, x64)
Errors	30	30	~	~	~	p=1.000 n=6
Symbols	729,756	729,756	~	~	~	p=1.000 n=6
Types	218,510	218,510	~	~	~	p=1.000 n=6
Memory used	633,761k (± 0.01%)	633,713k (± 0.00%)	~	633,685k	633,742k	p=0.066 n=6
Parse Time	4.56s (± 0.39%)	4.58s (± 0.78%)	~	4.52s	4.62s	p=0.226 n=6
Bind Time	1.48s (± 0.82%)	1.48s (± 0.35%)	~	1.48s	1.49s	p=1.000 n=6
Check Time	22.02s (± 0.15%)	22.10s (± 0.30%)	+0.08s (+ 0.35%)	22.03s	22.19s	p=0.037 n=6
Emit Time	0.00s	0.00s	~	~	~	p=1.000 n=6
Total Time	28.07s (± 0.08%)	28.17s (± 0.20%)	+0.10s (+ 0.34%)	28.12s	28.26s	p=0.005 n=6

System info unknown

Hosts

• node (v18.15.0, x64)

Scenarios

• Compiler-Unions - node (v18.15.0, x64)
• angular-1 - node (v18.15.0, x64)
• mui-docs - node (v18.15.0, x64)
• self-build-src - node (v18.15.0, x64)
• self-build-src-public-api - node (v18.15.0, x64)
• self-compiler - node (v18.15.0, x64)
• ts-pre-modules - node (v18.15.0, x64)
• vscode - node (v18.15.0, x64)
• webpack - node (v18.15.0, x64)
• xstate-main - node (v18.15.0, x64)

Benchmark	Name	Iterations
Current	pr	6
Baseline	baseline	6

Developer Information:

Download Benchmarks

[typescript-bot]

@jakebailey Here are the results of running the top 400 repos with tsc comparing main and refs/pull/63030/merge:

Everything looks good!

[jakebailey]

This PR seems correct enough, but alwaysStrict is a "strict option", so you can actually get false by setting strict: false in a config, so this won't actually detect the problem.

I think we might be forced to check getStrictOptionValue(compilerOptions, "alwaysStrict") and then somehow force everyone setting { "strict": false } to write { "strict": false, "alwaysStrict": true }, which sucks.