Fix generic nullable checks by ahejlsberg · Pull Request #59437 · microsoft/TypeScript · GitHub

This PR builds on #59415 to implement the following in aggregate:

In an indexed access expression obj[key], when obj and key both have generic types we no longer require the constraint of obj to be a non-nullable type. The rationale for this change is that when key is a valid property name for obj, it must be because obj is of a non-nullable type, as discussed here.
We now consider unconstrained generic types and generic types constrained to unknown to possibly be null or undefined in --strictNullChecks mode. Previously, we would only consider generic types with constraints including null or undefined to possibly be nullable.

TypeScript Bot (@typescript-bot) test it

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

Anders Hejlsberg (@ahejlsberg) Here are the results of running the user tests with tsc comparing main and refs/pull/59437/merge:

Everything looks good!

Hey Anders Hejlsberg (@ahejlsberg), the results of running the DT tests are ready.

Everything looks the same!

You can check the log here.

Anders Hejlsberg (@ahejlsberg)
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	30	30	~	~	~	p=1.000 n=6
Symbols	62,153	62,153	~	~	~	p=1.000 n=6
Types	50,242	50,242	~	~	~	p=1.000 n=6
Memory used	193,065k (± 0.72%)	194,729k (± 0.96%)	~	192,305k	196,060k	p=0.378 n=6
Parse Time	1.30s (± 1.57%)	1.31s (± 0.89%)	~	1.29s	1.32s	p=0.391 n=6
Bind Time	0.71s	0.71s	~	~	~	p=1.000 n=6
Check Time	9.59s (± 0.41%)	9.53s (± 0.32%)	-0.06s (- 0.63%)	9.47s	9.56s	p=0.024 n=6
Emit Time	2.74s (± 0.78%)	2.72s (± 1.38%)	~	2.65s	2.76s	p=0.293 n=6
Total Time	14.33s (± 0.23%)	14.26s (± 0.48%)	-0.07s (- 0.52%)	14.13s	14.32s	p=0.016 n=6
angular-1 - node (v18.15.0, x64)
Errors	7	17	🔻+10 (+142.86%)	~	~	p=0.001 n=6
Symbols	945,532	945,532	~	~	~	p=1.000 n=6
Types	409,507	409,512	+5 (+ 0.00%)	~	~	p=0.001 n=6
Memory used	1,221,152k (± 0.00%)	1,221,178k (± 0.00%)	~	1,221,099k	1,221,213k	p=0.630 n=6
Parse Time	6.68s (± 0.83%)	6.68s (± 0.57%)	~	6.64s	6.75s	p=1.000 n=6
Bind Time	1.90s (± 0.54%)	1.90s (± 0.58%)	~	1.89s	1.92s	p=0.558 n=6
Check Time	31.26s (± 0.27%)	31.25s (± 0.57%)	~	31.08s	31.51s	p=0.377 n=6
Emit Time	15.18s (± 0.40%)	15.19s (± 0.33%)	~	15.14s	15.27s	p=0.809 n=6
Total Time	55.02s (± 0.19%)	55.02s (± 0.37%)	~	54.87s	55.33s	p=0.687 n=6
mui-docs - node (v18.15.0, x64)
Errors	0	16	🔻+16 (+ ∞%)	~	~	p=0.001 n=6
Symbols	2,437,253	2,437,253	~	~	~	p=1.000 n=6
Types	1,004,097	1,004,141	+44 (+ 0.00%)	~	~	p=0.001 n=6
Memory used	2,426,339k (± 0.00%)	2,426,265k (± 0.00%)	~	2,426,226k	2,426,333k	p=0.066 n=6
Parse Time	8.43s (± 0.31%)	8.44s (± 0.21%)	~	8.42s	8.47s	p=0.686 n=6
Bind Time	2.08s (± 0.56%)	2.08s (± 0.30%)	~	2.07s	2.09s	p=1.000 n=6
Check Time	75.18s (± 0.66%)	74.77s (± 0.39%)	-0.40s (- 0.54%)	74.23s	75.00s	p=0.045 n=6
Emit Time	0.28s (± 1.47%)	0.28s (± 1.47%)	~	0.27s	0.28s	p=1.000 n=6
Total Time	85.96s (± 0.59%)	85.57s (± 0.32%)	~	85.06s	85.77s	p=0.066 n=6
self-build-src - node (v18.15.0, x64)
Errors	0	0	~	~	~	p=1.000 n=6
Symbols	1,227,735	1,227,781	+46 (+ 0.00%)	~	~	p=0.001 n=6
Types	265,136	265,143	+7 (+ 0.00%)	~	~	p=0.001 n=6
Memory used	2,346,556k (± 0.01%)	2,346,515k (± 0.03%)	~	2,345,869k	2,347,878k	p=0.378 n=6
Parse Time	5.03s (± 0.57%)	5.01s (± 1.07%)	~	4.94s	5.08s	p=0.471 n=6
Bind Time	1.92s (± 0.43%)	1.92s (± 0.39%)	~	1.91s	1.93s	p=0.729 n=6
Check Time	34.74s (± 0.57%)	34.78s (± 0.22%)	~	34.69s	34.89s	p=0.936 n=6
Emit Time	3.33s (± 0.94%)	3.30s (± 1.61%)	~	3.21s	3.35s	p=0.229 n=6
Total Time	45.03s (± 0.46%)	45.03s (± 0.27%)	~	44.90s	45.20s	p=0.936 n=6
self-build-src-public-api - node (v18.15.0, x64)
Errors	0	0	~	~	~	p=1.000 n=6
Symbols	1,227,735	1,227,781	+46 (+ 0.00%)	~	~	p=0.001 n=6
Types	265,136	265,143	+7 (+ 0.00%)	~	~	p=0.001 n=6
Memory used	2,420,283k (± 0.03%)	2,420,571k (± 0.03%)	~	2,419,926k	2,421,919k	p=1.000 n=6
Parse Time	6.19s (± 0.70%)	6.19s (± 0.69%)	~	6.14s	6.25s	p=1.000 n=6
Bind Time	2.02s (± 1.15%)	2.03s (± 0.86%)	~	2.01s	2.06s	p=0.373 n=6
Check Time	41.17s (± 0.26%)	41.05s (± 0.19%)	~	40.94s	41.17s	p=0.093 n=6
Emit Time	4.12s (± 4.15%)	4.00s (± 1.06%)	~	3.95s	4.07s	p=0.107 n=6
Total Time	53.51s (± 0.35%)	53.30s (± 0.27%)	~	53.06s	53.47s	p=0.066 n=6
self-compiler - node (v18.15.0, x64)
Errors	0	0	~	~	~	p=1.000 n=6
Symbols	255,950	255,952	+2 (+ 0.00%)	~	~	p=0.001 n=6
Types	104,957	104,964	+7 (+ 0.01%)	~	~	p=0.001 n=6
Memory used	427,965k (± 0.04%)	427,955k (± 0.04%)	~	427,739k	428,298k	p=0.936 n=6
Parse Time	3.35s (± 0.70%)	3.36s (± 0.69%)	~	3.33s	3.39s	p=0.624 n=6
Bind Time	1.32s (± 0.88%)	1.31s (± 1.42%)	~	1.29s	1.33s	p=0.445 n=6
Check Time	17.98s (± 0.45%)	18.00s (± 0.39%)	~	17.90s	18.06s	p=0.809 n=6
Emit Time	1.65s (± 1.04%)	1.64s (± 1.37%)	~	1.62s	1.67s	p=0.682 n=6
Total Time	24.31s (± 0.27%)	24.32s (± 0.33%)	~	24.21s	24.39s	p=0.810 n=6
ts-pre-modules - node (v18.15.0, x64)
Errors	35	35	~	~	~	p=1.000 n=6
Symbols	224,931	224,931	~	~	~	p=1.000 n=6
Types	94,146	94,146	~	~	~	p=1.000 n=6
Memory used	370,148k (± 0.04%)	370,101k (± 0.04%)	~	369,914k	370,319k	p=0.630 n=6
Parse Time	2.76s (± 0.78%)	2.76s (± 0.42%)	~	2.74s	2.77s	p=0.625 n=6
Bind Time	1.58s (± 1.09%)	1.58s (± 1.30%)	~	1.55s	1.61s	p=0.514 n=6
Check Time	15.67s (± 0.23%)	15.71s (± 0.21%)	~	15.65s	15.75s	p=0.053 n=6
Emit Time	0.00s	0.00s	~	~	~	p=1.000 n=6
Total Time	20.01s (± 0.13%)	20.05s (± 0.07%)	+0.04s (+ 0.19%)	20.03s	20.07s	p=0.024 n=6
vscode - node (v18.15.0, x64)
Errors	11	11	~	~	~	p=1.000 n=6
Symbols	2,986,839	2,986,839	~	~	~	p=1.000 n=6
Types	1,027,468	1,027,468	~	~	~	p=1.000 n=6
Memory used	3,110,626k (± 0.00%)	3,110,688k (± 0.00%)	~	3,110,618k	3,110,747k	p=0.128 n=6
Parse Time	13.87s (± 0.52%)	13.84s (± 0.40%)	~	13.76s	13.93s	p=0.687 n=6
Bind Time	4.35s (± 2.77%)	4.32s (± 2.02%)	~	4.28s	4.50s	p=0.624 n=6
Check Time	79.38s (± 0.42%)	79.18s (± 0.30%)	~	78.91s	79.54s	p=0.298 n=6
Emit Time	20.52s (± 0.37%)	20.50s (± 0.76%)	~	20.23s	20.63s	p=1.000 n=6
Total Time	118.12s (± 0.34%)	117.84s (± 0.29%)	~	117.37s	118.35s	p=0.298 n=6
webpack - node (v18.15.0, x64)
Errors	0	0	~	~	~	p=1.000 n=6
Symbols	267,567	267,567	~	~	~	p=1.000 n=6
Types	109,077	109,077	~	~	~	p=1.000 n=6
Memory used	412,438k (± 0.01%)	412,463k (± 0.02%)	~	412,378k	412,539k	p=0.689 n=6
Parse Time	3.80s (± 0.86%)	3.82s (± 0.59%)	~	3.78s	3.85s	p=0.744 n=6
Bind Time	1.70s (± 0.64%)	1.71s (± 0.86%)	~	1.68s	1.72s	p=0.131 n=6
Check Time	16.85s (± 0.38%)	16.82s (± 0.43%)	~	16.74s	16.94s	p=0.471 n=6
Emit Time	0.00s	0.00s	~	~	~	p=1.000 n=6
Total Time	22.36s (± 0.31%)	22.34s (± 0.43%)	~	22.26s	22.52s	p=0.574 n=6
xstate-main - node (v18.15.0, x64)
Errors	0	0	~	~	~	p=1.000 n=6
Symbols	511,807	511,807	~	~	~	p=1.000 n=6
Types	162,090	162,090	~	~	~	p=1.000 n=6
Memory used	449,073k (± 0.08%)	449,313k (± 0.03%)	~	449,121k	449,516k	p=0.298 n=6
Parse Time	3.16s (± 0.51%)	3.16s (± 0.62%)	~	3.13s	3.18s	p=0.618 n=6
Bind Time	1.16s (± 1.01%)	1.16s (± 0.35%)	~	1.15s	1.16s	p=0.787 n=6
Check Time	17.16s (± 0.33%)	17.18s (± 0.50%)	~	17.07s	17.33s	p=0.748 n=6
Emit Time	0.00s	0.00s	~	~	~	p=1.000 n=6
Total Time	21.48s (± 0.29%)	21.50s (± 0.43%)	~	21.35s	21.63s	p=0.936 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

Anders Hejlsberg (@ahejlsberg) Here are the results of running the top 400 repos with tsc comparing main and refs/pull/59437/merge:

Something interesting changed - please have a look.

Details

ag-grid/ag-grid

31 of 110 projects failed to build with the old tsc and were ignored

compiler-explorer/compiler-explorer

4 of 6 projects failed to build with the old tsc and were ignored

static/tsconfig.json

error TS18049: 'a' is possibly 'null' or 'undefined'.
- shared/common-utils.ts#L55
- shared/common-utils.ts#L57
error TS18049: 'b' is possibly 'null' or 'undefined'.
- shared/common-utils.ts#L55
- shared/common-utils.ts#L57

desktop/desktop

1 of 4 projects failed to build with the old tsc and were ignored

tsconfig.json

error TS18049: 'x' is possibly 'null' or 'undefined'.
error TS18049: 'y' is possibly 'null' or 'undefined'.

sequelize/sequelize

13 of 16 projects failed to build with the old tsc and were ignored

packages/utils/tsconfig.json

error TS18049: 'a' is possibly 'null' or 'undefined'.
- packages/utils/src/common/comparators/basic-comparator.ts#L21
- packages/utils/src/common/comparators/basic-comparator.ts#L25
error TS18049: 'b' is possibly 'null' or 'undefined'.
- packages/utils/src/common/comparators/basic-comparator.ts#L21
- packages/utils/src/common/comparators/basic-comparator.ts#L25

The new errors in the top 400 repos tests are all the same: Functions that compare values of unconstrained generic types. Our existing behavior is to permit such comparisons--we only error when the types have constraints that explicitly include null or undefined (which obviously is inconsistent).

We should decide if we think erroring on generic operands that are possibly null or undefined is the right thing to do since it basically amounts to a breaking change. The semantics of comparisons involving null and undefined are well defined (they act like NaN) and don't cause exceptions, yet it's probably rare for such comparisons to be intended.

We're going to wait for 5.7 on this one since it is technically a breaking change.

5.7 happened, and we're now at 5.8... should we do this? 😄

With 6.0 out as the final release vehicle for this codebase, we're closing all PRs that don't fit the merge criteria for post-6.0 patches. If you think this was a mistake and this PR fits the post-6.0 patch criteria, please post to the 6.0 iteration issue with details (specifically, which PR and which patch criteria it satisfies).

Next steps for PRs:

For crash bugfixes or language service improvements, PRs are currently accepted at the typescript-go repo
Changes to type system behavior should wait until after 7.0, at which point mainline TypeScript development will resume in this repository with the Go codebase
Library file updates (lib.d.ts etc) continue to live in this repo or the DOM Generator repo as appropriate

Anders Hejlsberg (ahejlsberg) added 6 commits July 24, 2024 15:16

In obj[key], omit non-nullable check when obj and key are both generic

7cba33c

Add tests

b93a10b

Accept new baselines

b9b07c4

Nullable errors on unconstrained generic types in strictNullChecks mode

9aec49b

Accept new baselines

1f5d7a3

Include tests from #59352

2e02dc4

TypeScript Bot (typescript-bot) assigned Anders Hejlsberg (ahejlsberg) Jul 26, 2024

TypeScript Bot (typescript-bot) added the Author: Team label Jul 26, 2024

TypeScript Bot (typescript-bot) assigned Isabel Duan (iisaduan) Jul 26, 2024

TypeScript Bot (typescript-bot) added the For Milestone Bug PRs that fix a bug with a specific milestone label Jul 26, 2024

Anders Hejlsberg (ahejlsberg) mentioned this pull request Jul 26, 2024

consistently check expressions with >, >=, <, <= for unconstrained types in strictNullChecks #59352

Closed

Anders Hejlsberg (ahejlsberg) mentioned this pull request Jul 29, 2024

No error on unconstrained type parameter in > comparison #50603

Open

Anders Hejlsberg (ahejlsberg) requested a review from Ryan Cavanaugh (RyanCavanaugh) July 29, 2024 21:25

TypeScript Bot (typescript-bot) closed this Mar 24, 2026

github-project-automation Bot moved this from Not started to Done in PR Backlog Mar 24, 2026

Fix generic nullable checks by ahejlsberg · Pull Request #59437 · microsoft/TypeScript · GitHub

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Fix generic nullable checks - #59437

Fix generic nullable checks#59437
Anders Hejlsberg (ahejlsberg) wants to merge 6 commits into
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fixGenericNullableChecks

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Fix generic nullable checks by ahejlsberg · Pull Request #59437 · microsoft/TypeScript · GitHub

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Fix generic nullable checks#59437Anders Hejlsberg (ahejlsberg) wants to merge 6 commits intomainmicrosoft/TypeScript:mainfrom fixGenericNullableChecksmicrosoft/TypeScript:fixGenericNullableChecksCopy head branch name to clipboard

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Fix generic nullable checks#59437
Anders Hejlsberg (ahejlsberg) wants to merge 6 commits into
mainfrom
fixGenericNullableChecks

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