Improve narrowing of generic types in control flow analysis by ahejlsberg · Pull Request #43183 · microsoft/TypeScript · GitHub

In #15576 we improved control flow analysis for references of generic types with nullable constraints when those references occur in constraint positions (locations where it is known we'll fetch the constraint).

This PR expands the concept further. Specifically, when the type of a reference is or contains a generic type with a union type constraint, and when the reference is in a constraint position or has a contextual type containing no generic types (meaning constraints will determine assignability), we substitute constraints for all generic types in the type of the reference to give control flow analysis an opportunity to narrow it further. For example:

function f1<T extends string | undefined>(x: T): string {
    if (x) {
        x.length;  // x narrowed to string
        return x;  // x narrowed to string (error before this PR)
    }
}

Above, in x.length we substitute string | undefined for T in the type of x because x occurs in a constraint position. Similarly in return x we substitute string | undefined for T in the type of x because x has the contextual type string (due to the return type annotation on the containing function) which contains no generic types. Following substitution, control flow analysis is then able to narrow string | undefined to string based on the truthiness check in the containing if statement.

Some additional examples:

declare function takeA(a: 'a'): void;
declare function takeB(b: 'b'); void;

function f2<T extends 'a' | 'b'>(x: T) {
    if (x === 'a') {
        takeA(x);  // x narrowed to 'a' (error before this PR)
    }
    else {
        takeB(x);  // x narrowed to 'b' (error before this PR)
    }
}

type A = { kind: 'a', value: string };
type B = { kind: 'b', value: number };

function f3<T extends A | B>(x: T) {
    if (x.kind === 'a') {
        x.value;  // Narrowed to string (wasn't narrowed before this PR)
    }
    else {
        x.value;  // Narrowed to number (wasn't narrowed before this PR)
    }
}

Fixes #13995.
Fixes #20375.
Fixes #42939.

…al type

…l type

TypeScript Bot (@typescript-bot) test this
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Heya Anders Hejlsberg (@ahejlsberg), I've started to run the extended test suite on this PR at a0fbf5c. You can monitor the build here.

Heya Anders Hejlsberg (@ahejlsberg), I've started to run the perf test suite on this PR at a0fbf5c. You can monitor the build here.

Update: The results are in!

Heya Anders Hejlsberg (@ahejlsberg), I've started to run the parallelized Definitely Typed test suite on this PR at a0fbf5c. You can monitor the build here.

Heya Anders Hejlsberg (@ahejlsberg), I've started to run the abridged perf test suite on this PR at a0fbf5c. You can monitor the build here.

Update: The results are in!

Heya Anders Hejlsberg (@ahejlsberg), I've started to run the parallelized community code test suite on this PR at a0fbf5c. You can monitor the build here.

Anders Hejlsberg (@ahejlsberg)
The results of the perf run you requested are in!

Here they are:

Comparison Report - master..43183

Metric	master	43183	Delta	Best	Worst
Angular - node (v10.16.3, x64)
Memory used	348,602k (± 0.03%)	348,691k (± 0.03%)	+89k (+ 0.03%)	348,517k	348,893k
Parse Time	1.92s (± 0.52%)	1.92s (± 0.58%)	-0.00s (- 0.16%)	1.90s	1.94s
Bind Time	0.83s (± 0.71%)	0.83s (± 0.36%)	-0.01s (- 0.72%)	0.82s	0.83s
Check Time	5.03s (± 0.56%)	5.05s (± 0.36%)	+0.02s (+ 0.44%)	5.01s	5.09s
Emit Time	5.92s (± 0.38%)	5.92s (± 0.77%)	-0.00s (- 0.07%)	5.83s	6.02s
Total Time	13.70s (± 0.34%)	13.71s (± 0.39%)	+0.01s (+ 0.07%)	13.65s	13.85s
Compiler-Unions - node (v10.16.3, x64)
Memory used	204,777k (± 0.04%)	203,960k (± 0.04%)	-817k (- 0.40%)	203,739k	204,132k
Parse Time	0.78s (± 0.86%)	0.77s (± 0.44%)	-0.00s (- 0.51%)	0.77s	0.78s
Bind Time	0.53s (± 0.90%)	0.52s (± 1.18%)	-0.01s (- 1.14%)	0.51s	0.53s
Check Time	7.45s (± 0.66%)	7.39s (± 0.64%)	-0.06s (- 0.78%)	7.29s	7.49s
Emit Time	2.61s (± 1.12%)	2.59s (± 1.12%)	-0.02s (- 0.69%)	2.52s	2.65s
Total Time	11.36s (± 0.52%)	11.27s (± 0.55%)	-0.08s (- 0.74%)	11.13s	11.38s
Monaco - node (v10.16.3, x64)
Memory used	356,547k (± 0.01%)	356,594k (± 0.03%)	+47k (+ 0.01%)	356,373k	356,783k
Parse Time	1.56s (± 0.44%)	1.55s (± 0.57%)	-0.01s (- 0.58%)	1.53s	1.56s
Bind Time	0.74s (± 0.68%)	0.73s (± 0.65%)	-0.00s (- 0.27%)	0.72s	0.74s
Check Time	5.20s (± 0.38%)	5.17s (± 0.78%)	-0.04s (- 0.71%)	5.07s	5.24s
Emit Time	3.12s (± 0.82%)	3.12s (± 0.75%)	-0.01s (- 0.16%)	3.08s	3.18s
Total Time	10.61s (± 0.39%)	10.56s (± 0.51%)	-0.05s (- 0.46%)	10.43s	10.69s
TFS - node (v10.16.3, x64)
Memory used	308,965k (± 0.02%)	308,965k (± 0.02%)	-0k (- 0.00%)	308,817k	309,142k
Parse Time	1.22s (± 0.67%)	1.21s (± 0.43%)	-0.01s (- 0.66%)	1.20s	1.22s
Bind Time	0.69s (± 0.69%)	0.69s (± 0.72%)	+0.00s (+ 0.29%)	0.69s	0.71s
Check Time	4.70s (± 0.67%)	4.67s (± 0.41%)	-0.03s (- 0.55%)	4.64s	4.72s
Emit Time	3.28s (± 1.06%)	3.25s (± 0.80%)	-0.03s (- 0.92%)	3.19s	3.30s
Total Time	9.88s (± 0.62%)	9.82s (± 0.34%)	-0.06s (- 0.61%)	9.76s	9.91s
material-ui - node (v10.16.3, x64)
Memory used	501,568k (± 0.01%)	501,610k (± 0.01%)	+41k (+ 0.01%)	501,435k	501,729k
Parse Time	1.98s (± 0.38%)	1.98s (± 0.44%)	-0.00s (- 0.05%)	1.96s	2.00s
Bind Time	0.65s (± 1.29%)	0.65s (± 1.40%)	+0.00s (+ 0.62%)	0.63s	0.67s
Check Time	14.26s (± 0.80%)	14.18s (± 0.40%)	-0.09s (- 0.60%)	14.06s	14.29s
Emit Time	0.00s (± 0.00%)	0.00s (± 0.00%)	0.00s ( NaN%)	0.00s	0.00s
Total Time	16.89s (± 0.66%)	16.81s (± 0.37%)	-0.08s (- 0.50%)	16.69s	16.94s
Angular - node (v12.1.0, x64)
Memory used	326,249k (± 0.02%)	326,236k (± 0.02%)	-13k (- 0.00%)	326,061k	326,374k
Parse Time	1.91s (± 0.46%)	1.91s (± 0.80%)	0.00s ( 0.00%)	1.87s	1.93s
Bind Time	0.81s (± 0.83%)	0.81s (± 0.49%)	+0.00s (+ 0.25%)	0.80s	0.82s
Check Time	4.93s (± 0.41%)	4.92s (± 0.45%)	-0.01s (- 0.28%)	4.86s	4.98s
Emit Time	5.97s (± 0.59%)	5.96s (± 0.74%)	-0.01s (- 0.25%)	5.90s	6.08s
Total Time	13.61s (± 0.46%)	13.59s (± 0.48%)	-0.03s (- 0.21%)	13.42s	13.75s
Compiler-Unions - node (v12.1.0, x64)
Memory used	191,506k (± 0.07%)	190,446k (± 0.07%)	-1,061k (- 0.55%)	189,942k	190,613k
Parse Time	0.77s (± 0.84%)	0.77s (± 0.76%)	-0.00s (- 0.13%)	0.76s	0.78s
Bind Time	0.52s (± 0.65%)	0.52s (± 0.77%)	-0.00s (- 0.76%)	0.51s	0.53s
Check Time	6.90s (± 0.67%)	6.89s (± 0.69%)	-0.01s (- 0.17%)	6.78s	6.99s
Emit Time	2.56s (± 0.93%)	2.55s (± 0.92%)	-0.01s (- 0.47%)	2.50s	2.61s
Total Time	10.76s (± 0.45%)	10.73s (± 0.53%)	-0.03s (- 0.26%)	10.57s	10.85s
Monaco - node (v12.1.0, x64)
Memory used	339,016k (± 0.01%)	339,053k (± 0.02%)	+37k (+ 0.01%)	338,923k	339,192k
Parse Time	1.53s (± 0.55%)	1.52s (± 0.57%)	-0.01s (- 0.59%)	1.50s	1.54s
Bind Time	0.72s (± 0.86%)	0.71s (± 0.51%)	-0.01s (- 0.97%)	0.71s	0.72s
Check Time	5.03s (± 0.48%)	4.97s (± 0.41%)	-0.06s (- 1.27%)	4.93s	5.01s
Emit Time	3.12s (± 0.58%)	3.09s (± 0.59%)	-0.03s (- 0.83%)	3.06s	3.14s
Total Time	10.40s (± 0.28%)	10.31s (± 0.27%)	-0.10s (- 0.94%)	10.24s	10.37s
TFS - node (v12.1.0, x64)
Memory used	293,124k (± 0.02%)	293,068k (± 0.01%)	-57k (- 0.02%)	293,011k	293,141k
Parse Time	1.21s (± 1.19%)	1.22s (± 0.73%)	+0.01s (+ 0.66%)	1.19s	1.23s
Bind Time	0.68s (± 0.87%)	0.68s (± 0.70%)	-0.00s (- 0.15%)	0.67s	0.69s
Check Time	4.61s (± 0.46%)	4.55s (± 0.50%)	-0.06s (- 1.30%)	4.49s	4.59s
Emit Time	3.16s (± 0.82%)	3.15s (± 0.86%)	-0.01s (- 0.44%)	3.06s	3.20s
Total Time	9.66s (± 0.45%)	9.59s (± 0.32%)	-0.06s (- 0.67%)	9.53s	9.69s
material-ui - node (v12.1.0, x64)
Memory used	479,599k (± 0.01%)	479,336k (± 0.07%)	-263k (- 0.05%)	478,393k	479,758k
Parse Time	2.00s (± 0.43%)	1.98s (± 0.38%)	-0.01s (- 0.70%)	1.97s	2.00s
Bind Time	0.64s (± 0.52%)	0.64s (± 0.97%)	-0.00s (- 0.16%)	0.63s	0.65s
Check Time	12.84s (± 0.66%)	13.00s (± 1.15%)	+0.15s (+ 1.18%)	12.71s	13.31s
Emit Time	0.00s (± 0.00%)	0.00s (± 0.00%)	0.00s ( NaN%)	0.00s	0.00s
Total Time	15.48s (± 0.56%)	15.62s (± 0.99%)	+0.14s (+ 0.88%)	15.34s	15.94s
Angular - node (v14.15.1, x64)
Memory used	324,912k (± 0.01%)	324,931k (± 0.01%)	+19k (+ 0.01%)	324,872k	325,042k
Parse Time	1.90s (± 0.29%)	1.91s (± 0.86%)	+0.01s (+ 0.42%)	1.88s	1.96s
Bind Time	0.84s (± 0.40%)	0.85s (± 0.76%)	+0.01s (+ 1.07%)	0.84s	0.86s
Check Time	4.91s (± 0.35%)	4.92s (± 0.24%)	+0.00s (+ 0.08%)	4.89s	4.94s
Emit Time	6.28s (± 0.68%)	6.27s (± 0.55%)	-0.01s (- 0.13%)	6.22s	6.39s
Total Time	13.93s (± 0.39%)	13.95s (± 0.27%)	+0.01s (+ 0.09%)	13.89s	14.07s
Compiler-Unions - node (v14.15.1, x64)
Memory used	191,073k (± 0.02%)	190,058k (± 0.01%)	-1,015k (- 0.53%)	190,003k	190,115k
Parse Time	0.79s (± 0.62%)	0.80s (± 0.86%)	+0.00s (+ 0.25%)	0.78s	0.81s
Bind Time	0.55s (± 1.01%)	0.55s (± 0.00%)	-0.00s (- 0.36%)	0.55s	0.55s
Check Time	6.97s (± 0.52%)	6.94s (± 0.20%)	-0.03s (- 0.49%)	6.91s	6.97s
Emit Time	2.54s (± 0.60%)	2.53s (± 0.89%)	-0.00s (- 0.08%)	2.49s	2.58s
Total Time	10.85s (± 0.41%)	10.81s (± 0.21%)	-0.04s (- 0.39%)	10.77s	10.87s
Monaco - node (v14.15.1, x64)
Memory used	338,095k (± 0.00%)	338,053k (± 0.01%)	-42k (- 0.01%)	338,011k	338,095k
Parse Time	1.55s (± 0.43%)	1.55s (± 0.44%)	+0.00s (+ 0.26%)	1.54s	1.57s
Bind Time	0.75s (± 0.89%)	0.75s (± 0.67%)	-0.00s (- 0.67%)	0.74s	0.76s
Check Time	4.94s (± 0.50%)	4.92s (± 0.15%)	-0.02s (- 0.45%)	4.90s	4.93s
Emit Time	3.15s (± 0.70%)	3.14s (± 0.46%)	-0.01s (- 0.35%)	3.12s	3.19s
Total Time	10.39s (± 0.41%)	10.36s (± 0.19%)	-0.03s (- 0.31%)	10.33s	10.42s
TFS - node (v14.15.1, x64)
Memory used	292,227k (± 0.01%)	292,252k (± 0.01%)	+25k (+ 0.01%)	292,214k	292,306k
Parse Time	1.25s (± 0.93%)	1.24s (± 1.16%)	-0.01s (- 0.96%)	1.21s	1.27s
Bind Time	0.72s (± 0.97%)	0.71s (± 0.70%)	-0.00s (- 0.56%)	0.71s	0.73s
Check Time	4.56s (± 0.39%)	4.57s (± 0.33%)	+0.01s (+ 0.18%)	4.54s	4.60s
Emit Time	3.24s (± 0.70%)	3.24s (± 0.62%)	-0.00s (- 0.06%)	3.21s	3.30s
Total Time	9.78s (± 0.35%)	9.77s (± 0.23%)	-0.01s (- 0.07%)	9.72s	9.82s
material-ui - node (v14.15.1, x64)
Memory used	477,936k (± 0.00%)	477,719k (± 0.07%)	-217k (- 0.05%)	476,751k	478,019k
Parse Time	2.06s (± 0.69%)	2.06s (± 0.55%)	+0.00s (+ 0.15%)	2.04s	2.08s
Bind Time	0.70s (± 0.83%)	0.70s (± 0.52%)	-0.00s (- 0.14%)	0.69s	0.70s
Check Time	12.99s (± 0.70%)	12.97s (± 0.79%)	-0.02s (- 0.17%)	12.76s	13.17s
Emit Time	0.00s (± 0.00%)	0.00s (± 0.00%)	0.00s ( NaN%)	0.00s	0.00s
Total Time	15.74s (± 0.64%)	15.72s (± 0.65%)	-0.02s (- 0.14%)	15.54s	15.93s

System

Machine Name	ts-ci-ubuntu
Platform	linux 4.4.0-198-generic
Architecture	x64
Available Memory	16 GB
Available Memory	5 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	43183	10
Baseline	master	10

Developer Information:

Download Benchmark

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

Anders Hejlsberg (@ahejlsberg)
The results of the perf run you requested are in!

Here they are:

Comparison Report - master..43183

Metric	master	43183	Delta	Best	Worst
Angular - node (v14.15.1, x64)
Memory used	324,912k (± 0.01%)	324,935k (± 0.01%)	+23k (+ 0.01%)	324,842k	325,009k
Parse Time	1.90s (± 0.29%)	1.91s (± 0.43%)	+0.01s (+ 0.47%)	1.89s	1.93s
Bind Time	0.84s (± 0.40%)	0.85s (± 0.76%)	+0.01s (+ 0.59%)	0.84s	0.86s
Check Time	4.91s (± 0.35%)	4.91s (± 0.65%)	-0.00s (- 0.08%)	4.85s	4.99s
Emit Time	6.28s (± 0.68%)	6.27s (± 0.40%)	-0.01s (- 0.19%)	6.21s	6.34s
Total Time	13.93s (± 0.39%)	13.93s (± 0.35%)	-0.00s (- 0.02%)	13.82s	14.04s
Compiler-Unions - node (v14.15.1, x64)
Memory used	191,073k (± 0.02%)	190,043k (± 0.01%)	-1,030k (- 0.54%)	189,997k	190,118k
Parse Time	0.79s (± 0.62%)	0.80s (± 0.86%)	+0.00s (+ 0.25%)	0.78s	0.81s
Bind Time	0.55s (± 1.01%)	0.56s (± 0.89%)	+0.00s (+ 0.72%)	0.55s	0.57s
Check Time	6.97s (± 0.52%)	7.02s (± 0.76%)	+0.05s (+ 0.75%)	6.94s	7.20s
Emit Time	2.54s (± 0.60%)	2.54s (± 0.39%)	+0.00s (+ 0.12%)	2.52s	2.56s
Total Time	10.85s (± 0.41%)	10.91s (± 0.55%)	+0.06s (+ 0.55%)	10.82s	11.11s
Monaco - node (v14.15.1, x64)
Memory used	338,095k (± 0.00%)	338,063k (± 0.01%)	-32k (- 0.01%)	338,015k	338,109k
Parse Time	1.55s (± 0.43%)	1.56s (± 0.73%)	+0.01s (+ 0.52%)	1.54s	1.59s
Bind Time	0.75s (± 0.89%)	0.75s (± 0.80%)	-0.00s (- 0.53%)	0.74s	0.76s
Check Time	4.94s (± 0.50%)	4.93s (± 0.42%)	-0.01s (- 0.18%)	4.88s	4.97s
Emit Time	3.15s (± 0.70%)	3.15s (± 0.46%)	-0.00s (- 0.03%)	3.12s	3.18s
Total Time	10.39s (± 0.41%)	10.38s (± 0.26%)	-0.01s (- 0.07%)	10.31s	10.43s
TFS - node (v14.15.1, x64)
Memory used	292,227k (± 0.01%)	292,248k (± 0.01%)	+21k (+ 0.01%)	292,192k	292,278k
Parse Time	1.25s (± 0.93%)	1.26s (± 0.89%)	+0.01s (+ 0.56%)	1.24s	1.30s
Bind Time	0.72s (± 0.97%)	0.71s (± 1.06%)	-0.01s (- 0.70%)	0.70s	0.74s
Check Time	4.56s (± 0.39%)	4.57s (± 0.35%)	+0.00s (+ 0.09%)	4.53s	4.60s
Emit Time	3.24s (± 0.70%)	3.25s (± 0.56%)	+0.00s (+ 0.09%)	3.21s	3.28s
Total Time	9.78s (± 0.35%)	9.79s (± 0.38%)	+0.01s (+ 0.11%)	9.73s	9.92s
material-ui - node (v14.15.1, x64)
Memory used	477,936k (± 0.00%)	477,953k (± 0.00%)	+16k (+ 0.00%)	477,885k	477,980k
Parse Time	2.06s (± 0.69%)	2.06s (± 0.60%)	+0.01s (+ 0.34%)	2.04s	2.10s
Bind Time	0.70s (± 0.83%)	0.69s (± 1.19%)	-0.00s (- 0.43%)	0.68s	0.71s
Check Time	12.99s (± 0.70%)	12.88s (± 0.34%)	-0.11s (- 0.82%)	12.79s	12.97s
Emit Time	0.00s (± 0.00%)	0.00s (± 0.00%)	0.00s ( NaN%)	0.00s	0.00s
Total Time	15.74s (± 0.64%)	15.64s (± 0.29%)	-0.10s (- 0.65%)	15.52s	15.73s

System

Machine Name	ts-ci-ubuntu
Platform	linux 4.4.0-198-generic
Architecture	x64
Available Memory	16 GB
Available Memory	5 GB
CPUs	4 × Intel(R) Core(TM) i7-4770 CPU @ 3.40GHz

Hosts

node (v14.15.1, x64)

Scenarios

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

Benchmark	Name	Iterations
Current	43183	10
Baseline	master	10

Developer Information:

Download Benchmark

Suggested change

      
                        // has a contextual type containing no instantiables (meaning constraints will determine assignability),
          
                        // has a contextual type containing no top-level instantiables (meaning constraints will determine assignability),

or

Suggested change

      
                        // has a contextual type containing no instantiables (meaning constraints will determine assignability),
          
                        // has a contextual type containing no immediate instantiables (meaning constraints will determine assignability),

TypeScript Bot (@typescript-bot) pack this

Heya Daniel Rosenwasser (@DanielRosenwasser), I've started to run the tarball bundle task on this PR at 1a695e9. You can monitor the build here.

Hey Daniel Rosenwasser (@DanielRosenwasser), I've packed this into an installable tgz. You can install it for testing by referencing it in your package.json like so:

{
    "devDependencies": {
        "typescript": "https://typescript.visualstudio.com/cf7ac146-d525-443c-b23c-0d58337efebc/_apis/build/builds/98224/artifacts?artifactName=tgz&fileId=266017171922858A31CD3EFD83E393EF6E56EB48D53ED8C01365E5A0113A6F5602&fileName=/typescript-4.3.0-insiders.20210310.tgz"
    }
}

and then running npm install.

There is also a playground for this build and an npm module you can use via "typescript": "npm:@typescript-deploys/pr-build@4.3.0-pr-43183-11".;

Anders Hejlsberg (@ahejlsberg) might be worth adding in the following test cases - f2 now works correctly.

interface Box<T> {
    item: T;
}

declare function isBox(x: any): x is Box<unknown>;
declare function isUndefined(x: unknown): x is undefined;
declare function unbox<T>(x: Box<T>): T;

function f1<T extends Box<T> | undefined>(x: T) {
    if (isBox(x)) {
        unbox(x);
    }
}

function f2<T extends Box<T> | undefined>(x: T) {
    if (!isUndefined(x)) {
        unbox(x);
    }
}

function f3<T extends Box<T> | undefined>(x: T) {
    if (!isBox(x)) {
        // error
        unbox(x);
    }
}

function f4<T extends Box<T> | undefined>(x: T) {
    if (isUndefined(x)) {
        // error
        unbox(x);
    }
}

Daniel Rosenwasser (@DanielRosenwasser) Will do.

Performance looks to be unaffected. RWC test suite looks good too, only change is a couple of error messages that now show constraint types instead of generic types (as expected).

…ction microsoft/TypeScript#43183

TS 4.7 catches a class of error which 4.6 and earlier did not, courtesy of microsoft/TypeScript#43183: if the type passed does not implement `toString()`, it notices, because it no longer defaults to falling back to `{}`. Fixes #330

TS 4.7 catches a class of error which 4.6 and earlier did not, courtesy of microsoft/TypeScript#43183: if the type passed does not implement `toString()`, it notices, because it no longer defaults to falling back to `{}`. To resolve this, *loosen* the constraints on what is acceptable as input to `toString`, by making the implementation itself more robust. Doing so also fixes a bug with the output for `toString` when working with strings: previously if you had `Maybe("a string")`, the `toString` output would be `'Maybe(a string)'`; it is now `'Maybe("a string")'`. Fixes #330

(cherry picked from commit 25adb1a) --- Original message: Add support for TypeScript 4.7 TS 4.7 catches a class of error which 4.6 and earlier did not, courtesy of microsoft/TypeScript#43183: if the type passed does not implement `toString()`, it notices, because it no longer defaults to falling back to `{}`. To resolve this, *loosen* the constraints on what is acceptable as input to `toString`, by making the implementation itself more robust. Doing so also fixes a bug with the output for `toString` when working with strings: previously if you had `Maybe("a string")`, the `toString` output would be `'Maybe(a string)'`; it is now `'Maybe("a string")'`. Fixes #330

Anders Hejlsberg (ahejlsberg) added 4 commits March 8, 2021 16:25

Narrow type variables with union constraints when merited by contextu…

a3028a2

…al type

Narrow generics with union type constraints as indicated by contextua…

df7d7be

…l type

Accept new baselines

13cf64e

Add tests

a0fbf5c

TypeScript Bot (typescript-bot) assigned Anders Hejlsberg (ahejlsberg) Mar 10, 2021

TypeScript Bot (typescript-bot) added the Author: Team label Mar 10, 2021

TypeScript Bot (typescript-bot) assigned Daniel Rosenwasser (DanielRosenwasser) Mar 10, 2021

TypeScript Bot (typescript-bot) added the For Milestone Bug PRs that fix a bug with a specific milestone label Mar 10, 2021

Anders Hejlsberg (ahejlsberg) requested review from Daniel Rosenwasser (DanielRosenwasser) and Ryan Cavanaugh (RyanCavanaugh) March 10, 2021 17:59

TypeScript Bot (typescript-bot) mentioned this pull request Mar 10, 2021

🤖 User test baselines have changed for fix13995 typescript-bot/TypeScript#161

Open

Fix circularity for JSX elements

1a695e9

Daniel Rosenwasser (DanielRosenwasser) reviewed Mar 10, 2021

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Anders Hejlsberg (ahejlsberg) added 2 commits March 10, 2021 16:23

Remove unnecessary isConstraintPosition information from flow cache key

a05ff0b

Update comment

0e76720

Brian Jenkins (brianjenkins94) mentioned this pull request Apr 28, 2021

Generic type unnarrowed by switch...case #43873

Open

Zeno Wu (zen0wu) mentioned this pull request Apr 30, 2021

Suggestion for Dependent-Type-Like Functions: Conservative Narrowing of Generic Indexed Access Result Type #33014

Open

5 tasks

Nathan Shively-Sanders (sandersn) mentioned this pull request May 7, 2021

Type narrowing for object literal being defined and returned in place behaves differently with variable indirection #43966

Closed

Anders Hejlsberg (ahejlsberg) mentioned this pull request May 17, 2021

Regression: Rest parameter typed as union of tuple types behaves differently in 4.3 #44093

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Tom Peters (TomPeters) mentioned this pull request Jun 3, 2021

Union types are not being narrowed correctly in generic functions in 4.3 #44404

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Hugo Raynal (RaynalHugo) added a commit to labelflow/labelflow that referenced this pull request Jul 23, 2021

Update typescript to solve narrowing generic type issue in list fun…

ec70c45

…ction microsoft/TypeScript#43183

Andrew Branch (andrewbranch) mentioned this pull request Jul 23, 2021

Type guard of union with conditional type not working since 4.3 #44382

Closed

Nikita Golubov (nekitk) mentioned this pull request Jul 28, 2021

refactor(CollapseGroup): get rid of additional generics in component types consta-design-system/uikit#1377

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13 tasks

Joe Calzaretta (jcalz) mentioned this pull request Jul 29, 2021

Contextual narrowing not narrowing on generic map. #45234

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Nathan Shively-Sanders (sandersn) mentioned this pull request Oct 4, 2021

Maximum call stack size exceeded when compiling tsx #46021

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Anders Hejlsberg (ahejlsberg) mentioned this pull request Oct 25, 2021

Type 'keyof T' cannot be used to index type 'T extends <type>' with unions in v4.4.4 and newer #46495

Closed

Gabriela Araujo Britto (gabritto) mentioned this pull request Jan 10, 2022

Fix destructuring and narrowing interaction #47337

Merged

Daniel Rosenwasser (DanielRosenwasser) mentioned this pull request Mar 29, 2022

Generic type no longer narrowed as expected without extends object #48468

Closed

Chris Krycho (chriskrycho) mentioned this pull request Mar 30, 2022

Failing on TS 4.7 nightlies true-myth/true-myth#330

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Long Zheng (longzheng) mentioned this pull request Apr 14, 2022

Generic cannot be narrowed to satisfy NonNullable #48691

Closed

Chris Krycho (chriskrycho) mentioned this pull request Apr 22, 2022

Add support for TypeScript 4.7 true-myth/true-myth#349

Merged

whzx5byb mentioned this pull request Aug 6, 2022

CFA doesn't work when using generics to associate two function arguments #50205

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Adrien HARNAY (adrienharnay) mentioned this pull request Sep 6, 2022

Type narrowing of generic types in control flow analysis #50652

Open

Mateusz Burzyński (Andarist) mentioned this pull request Jan 25, 2023

satisfies changes the generic type, "resolves" it immediately #52394

Open

Jake Bailey (jakebailey) mentioned this pull request Jul 11, 2023

undefined not assignable error within "if" statement, when member of Readonly<T> #28997

Closed

Anders Hejlsberg (ahejlsberg) mentioned this pull request Oct 2, 2023

Narrowed type not used when indexing #55922

Closed

Joe Calzaretta (jcalz) mentioned this pull request Mar 16, 2024

TS doesn't narrow generic parameter T to null, despite an explicit comparison #57803

Open

Aly Thobani (alythobani) mentioned this pull request May 18, 2024

Allow type variables to be constrained singleton, causing lookup into a non-generic mapped type to substitute #25879

Open

4 tasks

Mateusz Burzyński (Andarist) mentioned this pull request Aug 22, 2024

Generic type narrowing for truthy check has inconsistent / incorrect behavior #59717

Open

milkcask (milkcask) mentioned this pull request Oct 5, 2024

Type is not referred correctly in the while loop #59715

Open

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

+                      function getConstraintForReference(type: Type, reference: Identifier | ElementAccessExpression | PropertyAccessExpression | QualifiedName, checkMode: CheckMode | undefined) {
+                          // When the type of a reference is or contains an instantiable type with a union type constraint, and
+                          // when the reference is in a constraint position (where it is known we'll obtain the apparent type) or
+                          // has a contextual type containing no instantiables (meaning constraints will determine assignability),

	// has a contextual type containing no instantiables (meaning constraints will determine assignability),
	// has a contextual type containing no top-level instantiables (meaning constraints will determine assignability),

Improve narrowing of generic types in control flow analysis by ahejlsberg · Pull Request #43183 · microsoft/TypeScript · GitHub

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Improve narrowing of generic types in control flow analysis - #43183

Improve narrowing of generic types in control flow analysis#43183Anders Hejlsberg (ahejlsberg) merged 15 commits intomastermicrosoft/TypeScript:masterfrom fix13995microsoft/TypeScript:fix13995Copy head branch name to clipboard

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Improve narrowing of generic types in control flow analysis#43183
Anders Hejlsberg (ahejlsberg) merged 15 commits into
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