Fix issues + Support template literal types as discriminants by ahejlsberg · Pull Request #46137 · microsoft/TypeScript · GitHub

We probably want to remove the export from this and the other line to keep around the declaration emit in the test.

Yeah, for sure.

Won't this flag foo-${boolean} and foo-${number} as possibly related? We still need to make sure the holes relate when this returns True, no?

Yes, it'll flag them as possibly related. Or, rather, not flag them as definitely unrelated. We could potentially check the placeholders that immediately follow identical prefix texts or immediately precede identical suffix texts, but honestly there's very little gained from it. In fact, probably only the boolean vs. number disqualification. I think what is here already is sufficient, and it impacts whether we flag operands as non-overlapping, which isn't super critical to get 100% right.

Before I forget, can we also get a copy of this test with a signature like

function ff1<T extends string>(x: `foo-${T}`, y: `${T}-bar`, z: `baz-${T}`) {

? I figure it should behave the same, but, y'know, coverage. Maybe it can just be more cases on this signature, since it already has a generic parameter for some reason~

Will do.

TypeScript Bot (@typescript-bot) test this
TypeScript Bot (@typescript-bot) user test this
TypeScript Bot (@typescript-bot) perf test this

Heya Daniel Rosenwasser (@DanielRosenwasser), I've started to run the perf test suite on this PR at e1907a2. You can monitor the build here.

Update: The results are in!

Heya Daniel Rosenwasser (@DanielRosenwasser), I've started to run the extended test suite on this PR at e1907a2. You can monitor the build here.

Heya Daniel Rosenwasser (@DanielRosenwasser), I've started to run the parallelized community code test suite on this PR at e1907a2. You can monitor the build here.

Daniel Rosenwasser (@DanielRosenwasser)
The results of the perf run you requested are in!

Here they are:

Comparison Report - main..46137

Metric	main	46137	Delta	Best	Worst
Angular - node (v10.16.3, x64)
Memory used	354,154k (± 0.02%)	354,131k (± 0.02%)	-23k (- 0.01%)	354,015k	354,332k
Parse Time	1.94s (± 0.49%)	1.94s (± 0.37%)	+0.01s (+ 0.36%)	1.93s	1.96s
Bind Time	0.84s (± 0.62%)	0.84s (± 0.98%)	+0.00s (+ 0.36%)	0.83s	0.86s
Check Time	5.46s (± 0.46%)	5.47s (± 0.50%)	+0.00s (+ 0.07%)	5.39s	5.52s
Emit Time	5.82s (± 0.33%)	5.86s (± 0.69%)	+0.04s (+ 0.67%)	5.78s	5.96s
Total Time	14.06s (± 0.25%)	14.12s (± 0.33%)	+0.05s (+ 0.38%)	14.01s	14.23s
Compiler-Unions - node (v10.16.3, x64)
Memory used	203,944k (± 0.02%)	203,852k (± 0.04%)	-91k (- 0.04%)	203,701k	203,992k
Parse Time	0.78s (± 0.97%)	0.79s (± 1.13%)	+0.00s (+ 0.26%)	0.77s	0.81s
Bind Time	0.52s (± 1.18%)	0.52s (± 1.28%)	+0.00s (+ 0.38%)	0.51s	0.54s
Check Time	7.94s (± 0.70%)	7.91s (± 0.57%)	-0.02s (- 0.29%)	7.82s	8.06s
Emit Time	2.47s (± 0.81%)	2.46s (± 0.81%)	-0.01s (- 0.45%)	2.41s	2.50s
Total Time	11.71s (± 0.57%)	11.68s (± 0.42%)	-0.03s (- 0.26%)	11.60s	11.82s
Monaco - node (v10.16.3, x64)
Memory used	342,054k (± 0.03%)	341,996k (± 0.02%)	-58k (- 0.02%)	341,871k	342,258k
Parse Time	1.48s (± 0.51%)	1.47s (± 0.71%)	-0.01s (- 0.34%)	1.44s	1.49s
Bind Time	0.75s (± 0.40%)	0.75s (± 0.77%)	-0.00s (- 0.27%)	0.74s	0.76s
Check Time	5.45s (± 0.65%)	5.44s (± 0.63%)	-0.01s (- 0.17%)	5.38s	5.53s
Emit Time	3.18s (± 0.69%)	3.19s (± 0.96%)	+0.00s (+ 0.13%)	3.10s	3.25s
Total Time	10.86s (± 0.33%)	10.85s (± 0.36%)	-0.01s (- 0.11%)	10.76s	10.94s
TFS - node (v10.16.3, x64)
Memory used	304,663k (± 0.02%)	304,672k (± 0.02%)	+9k (+ 0.00%)	304,503k	304,836k
Parse Time	1.19s (± 0.50%)	1.19s (± 0.44%)	-0.00s (- 0.25%)	1.18s	1.20s
Bind Time	0.71s (± 0.81%)	0.71s (± 0.78%)	-0.00s (- 0.28%)	0.69s	0.72s
Check Time	4.99s (± 0.66%)	4.96s (± 0.46%)	-0.03s (- 0.56%)	4.88s	5.00s
Emit Time	3.33s (± 1.65%)	3.33s (± 0.88%)	-0.01s (- 0.21%)	3.26s	3.40s
Total Time	10.22s (± 0.61%)	10.18s (± 0.42%)	-0.04s (- 0.40%)	10.05s	10.26s
material-ui - node (v10.16.3, x64)
Memory used	472,240k (± 0.02%)	472,270k (± 0.02%)	+30k (+ 0.01%)	472,053k	472,392k
Parse Time	1.77s (± 0.38%)	1.76s (± 0.50%)	-0.00s (- 0.23%)	1.75s	1.79s
Bind Time	0.66s (± 0.61%)	0.66s (± 0.55%)	-0.00s (- 0.46%)	0.65s	0.66s
Check Time	14.41s (± 0.65%)	14.38s (± 0.71%)	-0.03s (- 0.18%)	14.25s	14.68s
Emit Time	0.00s (± 0.00%)	0.00s (± 0.00%)	0.00s ( NaN%)	0.00s	0.00s
Total Time	16.83s (± 0.59%)	16.80s (± 0.64%)	-0.03s (- 0.17%)	16.66s	17.12s
Angular - node (v12.1.0, x64)
Memory used	332,038k (± 0.03%)	331,993k (± 0.02%)	-45k (- 0.01%)	331,833k	332,156k
Parse Time	1.93s (± 0.78%)	1.92s (± 0.45%)	-0.00s (- 0.21%)	1.90s	1.93s
Bind Time	0.82s (± 0.82%)	0.81s (± 0.90%)	-0.01s (- 0.61%)	0.80s	0.83s
Check Time	5.30s (± 0.37%)	5.30s (± 0.56%)	-0.00s (- 0.06%)	5.23s	5.36s
Emit Time	6.09s (± 0.61%)	6.14s (± 0.89%)	+0.04s (+ 0.72%)	6.06s	6.29s
Total Time	14.14s (± 0.39%)	14.18s (± 0.37%)	+0.03s (+ 0.23%)	14.03s	14.30s
Compiler-Unions - node (v12.1.0, x64)
Memory used	191,331k (± 0.11%)	191,264k (± 0.12%)	-66k (- 0.03%)	190,408k	191,506k
Parse Time	0.78s (± 0.85%)	0.78s (± 0.61%)	-0.00s (- 0.38%)	0.77s	0.79s
Bind Time	0.53s (± 0.98%)	0.53s (± 0.92%)	+0.00s (+ 0.95%)	0.52s	0.54s
Check Time	7.43s (± 0.66%)	7.39s (± 0.75%)	-0.04s (- 0.54%)	7.30s	7.51s
Emit Time	2.46s (± 1.03%)	2.48s (± 0.95%)	+0.02s (+ 0.69%)	2.43s	2.54s
Total Time	11.20s (± 0.61%)	11.18s (± 0.62%)	-0.02s (- 0.21%)	11.06s	11.33s
Monaco - node (v12.1.0, x64)
Memory used	325,116k (± 0.02%)	325,088k (± 0.02%)	-28k (- 0.01%)	324,925k	325,220k
Parse Time	1.45s (± 0.47%)	1.47s (± 0.81%)	+0.01s (+ 0.89%)	1.44s	1.49s
Bind Time	0.73s (± 0.47%)	0.73s (± 1.06%)	+0.00s (+ 0.55%)	0.72s	0.75s
Check Time	5.35s (± 0.65%)	5.31s (± 0.33%)	-0.03s (- 0.58%)	5.27s	5.35s
Emit Time	3.20s (± 0.70%)	3.17s (± 0.65%)	-0.03s (- 0.81%)	3.14s	3.22s
Total Time	10.73s (± 0.46%)	10.68s (± 0.36%)	-0.04s (- 0.39%)	10.60s	10.77s
TFS - node (v12.1.0, x64)
Memory used	289,416k (± 0.02%)	289,429k (± 0.02%)	+14k (+ 0.00%)	289,295k	289,617k
Parse Time	1.21s (± 0.71%)	1.21s (± 0.62%)	+0.00s (+ 0.08%)	1.20s	1.23s
Bind Time	0.69s (± 0.69%)	0.69s (± 0.81%)	+0.00s (+ 0.15%)	0.68s	0.70s
Check Time	4.90s (± 0.26%)	4.89s (± 0.31%)	-0.02s (- 0.33%)	4.86s	4.94s
Emit Time	3.39s (± 0.85%)	3.38s (± 0.92%)	-0.01s (- 0.35%)	3.31s	3.45s
Total Time	10.20s (± 0.33%)	10.17s (± 0.37%)	-0.03s (- 0.25%)	10.08s	10.23s
material-ui - node (v12.1.0, x64)
Memory used	450,955k (± 0.02%)	450,742k (± 0.08%)	-213k (- 0.05%)	449,797k	451,114k
Parse Time	1.77s (± 0.47%)	1.77s (± 0.57%)	0.00s ( 0.00%)	1.75s	1.80s
Bind Time	0.64s (± 1.32%)	0.64s (± 1.25%)	+0.00s (+ 0.16%)	0.62s	0.66s
Check Time	13.05s (± 0.49%)	13.00s (± 0.68%)	-0.05s (- 0.38%)	12.84s	13.26s
Emit Time	0.00s (± 0.00%)	0.00s (± 0.00%)	0.00s ( NaN%)	0.00s	0.00s
Total Time	15.47s (± 0.45%)	15.42s (± 0.58%)	-0.05s (- 0.32%)	15.26s	15.67s
Angular - node (v14.15.1, x64)
Memory used	330,452k (± 0.01%)	330,452k (± 0.01%)	-1k (- 0.00%)	330,419k	330,494k
Parse Time	1.95s (± 0.67%)	1.94s (± 0.49%)	-0.00s (- 0.26%)	1.93s	1.97s
Bind Time	0.86s (± 0.65%)	0.86s (± 0.84%)	+0.00s (+ 0.47%)	0.85s	0.88s
Check Time	5.35s (± 0.56%)	5.33s (± 0.58%)	-0.02s (- 0.36%)	5.26s	5.40s
Emit Time	6.22s (± 0.74%)	6.20s (± 0.70%)	-0.02s (- 0.32%)	6.10s	6.30s
Total Time	14.38s (± 0.51%)	14.33s (± 0.54%)	-0.04s (- 0.29%)	14.19s	14.48s
Compiler-Unions - node (v14.15.1, x64)
Memory used	193,187k (± 0.01%)	192,529k (± 0.49%)	-659k (- 0.34%)	189,961k	193,210k
Parse Time	0.81s (± 0.64%)	0.81s (± 0.90%)	-0.00s (- 0.37%)	0.80s	0.83s
Bind Time	0.56s (± 0.65%)	0.55s (± 0.40%)	-0.01s (- 0.90%)	0.55s	0.56s
Check Time	7.55s (± 0.73%)	7.49s (± 0.37%)	-0.07s (- 0.86%)	7.41s	7.55s
Emit Time	2.41s (± 0.48%)	2.44s (± 1.14%)	+0.04s (+ 1.45%)	2.39s	2.52s
Total Time	11.32s (± 0.50%)	11.29s (± 0.33%)	-0.03s (- 0.26%)	11.21s	11.39s
Monaco - node (v14.15.1, x64)
Memory used	323,989k (± 0.00%)	323,996k (± 0.01%)	+7k (+ 0.00%)	323,956k	324,046k
Parse Time	1.51s (± 0.51%)	1.51s (± 0.77%)	-0.00s (- 0.20%)	1.48s	1.53s
Bind Time	0.76s (± 0.48%)	0.75s (± 0.79%)	-0.00s (- 0.26%)	0.74s	0.77s
Check Time	5.30s (± 0.59%)	5.30s (± 0.49%)	+0.00s (+ 0.02%)	5.25s	5.39s
Emit Time	3.21s (± 0.87%)	3.22s (± 0.81%)	+0.02s (+ 0.50%)	3.18s	3.29s
Total Time	10.78s (± 0.45%)	10.79s (± 0.39%)	+0.01s (+ 0.13%)	10.69s	10.89s
TFS - node (v14.15.1, x64)
Memory used	288,334k (± 0.01%)	288,337k (± 0.01%)	+3k (+ 0.00%)	288,294k	288,407k
Parse Time	1.24s (± 0.47%)	1.23s (± 0.24%)	-0.01s (- 0.73%)	1.22s	1.23s
Bind Time	0.73s (± 0.84%)	0.73s (± 0.55%)	+0.00s (+ 0.00%)	0.72s	0.74s
Check Time	4.91s (± 0.38%)	4.93s (± 0.39%)	+0.01s (+ 0.28%)	4.88s	4.97s
Emit Time	3.48s (± 0.52%)	3.49s (± 0.87%)	+0.01s (+ 0.43%)	3.43s	3.56s
Total Time	10.36s (± 0.25%)	10.38s (± 0.31%)	+0.02s (+ 0.19%)	10.31s	10.44s
material-ui - node (v14.15.1, x64)
Memory used	448,958k (± 0.09%)	449,300k (± 0.00%)	+342k (+ 0.08%)	449,275k	449,351k
Parse Time	1.82s (± 0.61%)	1.81s (± 0.56%)	-0.01s (- 0.49%)	1.79s	1.84s
Bind Time	0.68s (± 1.03%)	0.68s (± 0.69%)	-0.00s (- 0.15%)	0.67s	0.69s
Check Time	13.23s (± 1.18%)	13.13s (± 0.55%)	-0.10s (- 0.73%)	12.97s	13.36s
Emit Time	0.00s (± 0.00%)	0.00s (± 0.00%)	0.00s ( NaN%)	0.00s	0.00s
Total Time	15.73s (± 1.04%)	15.63s (± 0.52%)	-0.11s (- 0.67%)	15.45s	15.89s

System

Machine Name	ts-ci-ubuntu
Platform	linux 4.4.0-210-generic
Architecture	x64
Available Memory	16 GB
Available Memory	8 GB
CPUs	4 × Intel(R) Core(TM) i7-4770 CPU @ 3.40GHz

Hosts

node (v10.16.3, x64)
node (v12.1.0, x64)
node (v14.15.1, x64)

Scenarios

Angular - node (v10.16.3, x64)
Angular - node (v12.1.0, x64)
Angular - node (v14.15.1, x64)
Compiler-Unions - node (v10.16.3, x64)
Compiler-Unions - node (v12.1.0, x64)
Compiler-Unions - node (v14.15.1, x64)
Monaco - node (v10.16.3, x64)
Monaco - node (v12.1.0, x64)
Monaco - node (v14.15.1, x64)
TFS - node (v10.16.3, x64)
TFS - node (v12.1.0, x64)
TFS - node (v14.15.1, x64)
material-ui - node (v10.16.3, x64)
material-ui - node (v12.1.0, x64)
material-ui - node (v14.15.1, x64)

Benchmark	Name	Iterations
Current	46137	10
Baseline	main	10

Developer Information:

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The user suite test run you requested has finished and failed. I've opened a PR with the baseline diff from master.

Tests look clean, perf is slightly improved if anything. I think this one is good to go!

Great! But will buitin string methods support this? For now, I'm forced to use these standalone functions:

const startsWith = <T extends string>(str: string, search: T): str is `${T}${string}` => str.startsWith(search);
const endsWith = <T extends string>(str: string, search: T): str is `${string}${T}` => str.endsWith(search);
const includes = <T extends string>(str: string, search: T): str is `${string}${T}${string}` => str.includes(search);

Anders Hejlsberg (ahejlsberg) added 2 commits September 29, 2021 13:42

Fix issues + Support template literal types in discriminants

7f34792

Add tests

e1907a2

TypeScript Bot (typescript-bot) assigned Anders Hejlsberg (ahejlsberg) Sep 29, 2021

TypeScript Bot (typescript-bot) added Author: Team For Milestone Bug PRs that fix a bug with a specific milestone labels Sep 29, 2021

Anders Hejlsberg (ahejlsberg) requested review from Daniel Rosenwasser (DanielRosenwasser) and Andrew Branch (andrewbranch) September 29, 2021 20:58

Wesley Wigham (weswigham) reviewed Sep 29, 2021

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Address CR feedback

955fda5

TypeScript Bot (typescript-bot) mentioned this pull request Sep 29, 2021

🤖 User test baselines have changed for fix46125 typescript-bot/TypeScript#207

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Wesley Wigham (weswigham) approved these changes Sep 29, 2021

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Andrew Branch (andrewbranch) approved these changes Sep 29, 2021

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Daniel Rosenwasser (DanielRosenwasser) merged commit 5ec836d into main Sep 30, 2021

Daniel Rosenwasser (DanielRosenwasser) deleted the fix46125 branch September 30, 2021 00:38

Daniel Rosenwasser (DanielRosenwasser) mentioned this pull request Sep 30, 2021

Design Meeting Notes, 9/29/2021 #46141

Closed

Vitaly (zardoy) mentioned this pull request Jan 31, 2022

feat: Improve startsWith, includes and endsWith typings for TS 4.5 selfrefactor/rambda#622

Merged

Ryan Cavanaugh (RyanCavanaugh) mentioned this pull request Mar 22, 2024

Improve comparable relation of template literal types to detect more impossible situations #57872

Open

Mateusz Burzyński (Andarist) mentioned this pull request Aug 5, 2025

Support contextual type discrimination based on template literal expressions #62203

Closed

Microsoft (microsoft) locked as resolved and limited conversation to collaborators Oct 21, 2025

+                          const targetEnd = target.texts[target.texts.length - 1];
+                          const startLen = Math.min(sourceStart.length, targetStart.length);
+                          const endLen = Math.min(sourceEnd.length, targetEnd.length);
+                          return sourceStart.slice(0, startLen) !== targetStart.slice(0, startLen) ||


		// Repro from #46125

		function ff1<T extends string>(x: `foo-${string}`, y: `${string}-bar`, z: `baz-${string}`) {

Fix issues + Support template literal types as discriminants by ahejlsberg · Pull Request #46137 · microsoft/TypeScript · GitHub

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Fix issues + Support template literal types as discriminants - #46137

Fix issues + Support template literal types as discriminants#46137Daniel Rosenwasser (DanielRosenwasser) merged 3 commits intomainmicrosoft/TypeScript:mainfrom fix46125microsoft/TypeScript:fix46125Copy head branch name to clipboard

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Fix issues + Support template literal types as discriminants#46137
Daniel Rosenwasser (DanielRosenwasser) merged 3 commits into
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fix46125

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