Fix inference of type arguments when source is 'awaited'

[rbuckton]

This changes inference behavior when inferring a type argument that is awaited:

function f<T>(a: T): Promise<awaited T> {
    // will now infer `Promise<T>` instead of `Promise<awaited T>`
    return new Promise(r => r(a));
}

Fixes #37526

[ahejlsberg]

Hmm, I'm not sure I follow the rationale here. Just because T is used in an awaited T type constructor somewhere doesn't mean that we want to change inference behavior everywhere for T.

Can you provide a bit more context on what you're trying to accomplish and why?

[ahejlsberg]

Is a correct summary that you intend for inference from awaited S to a type parameter T, where T only occurs in awaited T type constructors, to infer S instead of awaited S?

[rbuckton]

At the end of the day, awaited's main purpose is to give you the right type for the value parameter of the onfulfilled callback of the then method of a Promise (or Promise/A+-compatible promise-like implementation), as that is the only place where a value with that type is actually produced.

Consider this minimal definition for something promise-like:

interface Promise<T> {
  then(onfulfilled: (value: awaited T) => any): any
                     ~~~~~
}

The type of value will be the same for all of the following instantiations:

promise	value
Promise<T>	awaited T
Promise<Promise<T>>	awaited T
Promise<Promise<Promise<T>>>	awaited T
Promise<awaited T>	awaited T
Promise<awaited awaited T>	awaited T
Promise<awaited Promise<T>>	awaited T
Promise<awaited Promise<awaited T>>	awaited T
Promise<T | awaited T>	awaited T
Promise<T | Promise<T>>	awaited T
Promise<awaited T | Promise<T>>	awaited T

In addition, awaited shares some of same difficulties as conditional types when it comes to working with the type in expression-space. This is apparent if you look at #37526, since there's no way to resolve a promise with a T when it is instantiated with an awaited T, despite the fact that the resulting value will have the same type. Since both awaited T and T will resolve to awaited T, resolving it with a T should be permitted. However, we don't want to make T assignable to awaited T (or vice versa), so generally we want to unwrap any inference to awaited T to become just T as that's more useful.

It is possible this proposed heuristic may be too loose. In reality we may only want to perform this inference when the awaited T is only referenced as the type of value, above. If that is the case I would welcome suggestions on a better way to enforce that.

[rbuckton]

Is a correct summary that you intend for inference from awaited S to a type parameter T, where T only occurs in awaited T type constructors, to infer S instead of awaited S?

I believe that would be the case, yes. At least, in cases where the T is produced as a value to expression-space. For example, the finally method returns Promise<T> which would be a reference to T that is not awaited. In addition, the TResult1 type parameter for then uses T as a default without necessarily wrapping it in an awaited.

[ahejlsberg]

I did a little experiment where I changed inference between references to the same generic type to always proceed structurally if any type parameters have variances marked unreliable (as is the case with T in Promise<T>). This causes the original example to just work as expected. What's happening in the example is that the new Promise(r => r(a)) is contextually typed by Promise<awaited T>. As we proceed with structural inference, we end up with an awaited (awaited T) for the value parameter in the contextual type, which reduces to awaited T, and we then make the correct inference T.

[rbuckton]

Do we want inference to proceed structurally for all "unreliable" variances or just cases where awaited is used? I'm happy to close this PR if that turns out to be a better solution.

[ahejlsberg]

@rbuckton Just pushed the changes we talked about. We now perform structural type inference for any generic type with a type parameter that occurs in an awaited T type constructor.

Only change in baselines is that we flatten inferences from Promise<Promise<number>> which actually seems desirable.

[rbuckton]

@typescript-bot perf test
@typescript-bot run dt
@typescript-bot test this
@typescript-bot user test this

[typescript-bot]

Heya @rbuckton, I've started to run the parallelized Definitely Typed test suite on this PR at 70f92ff. You can monitor the build here.

[typescript-bot]

Heya @rbuckton, I've started to run the parallelized community code test suite on this PR at 70f92ff. You can monitor the build here.

[typescript-bot]

Heya @rbuckton, I've started to run the perf test suite on this PR at 70f92ff. You can monitor the build here.

Update: The results are in!

[typescript-bot]

Heya @rbuckton, I've started to run the extended test suite on this PR at 70f92ff. You can monitor the build here.

[typescript-bot]

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

[typescript-bot]

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

Here they are:

Comparison Report - master..37540

Metric	master	37540	Delta	Best	Worst
Angular - node (v10.16.3, x64)
Memory used	328,158k (± 0.02%)	328,414k (± 0.01%)	+257k (+ 0.08%)	328,291k	328,470k
Parse Time	1.62s (± 0.37%)	1.62s (± 0.76%)	-0.00s (- 0.12%)	1.60s	1.66s
Bind Time	0.89s (± 0.53%)	0.89s (± 0.92%)	-0.00s (- 0.34%)	0.87s	0.90s
Check Time	4.76s (± 0.38%)	4.80s (± 0.40%)	+0.05s (+ 1.03%)	4.75s	4.85s
Emit Time	5.32s (± 0.45%)	5.32s (± 0.79%)	+0.00s (+ 0.08%)	5.25s	5.45s
Total Time	12.59s (± 0.23%)	12.63s (± 0.53%)	+0.05s (+ 0.36%)	12.54s	12.81s
Monaco - node (v10.16.3, x64)
Memory used	327,136k (± 0.02%)	327,094k (± 0.01%)	-43k (- 0.01%)	327,042k	327,168k
Parse Time	1.26s (± 0.49%)	1.26s (± 0.40%)	-0.00s (- 0.32%)	1.25s	1.27s
Bind Time	0.77s (± 0.80%)	0.78s (± 0.48%)	+0.00s (+ 0.52%)	0.77s	0.78s
Check Time	4.75s (± 0.40%)	4.73s (± 0.25%)	-0.02s (- 0.36%)	4.71s	4.76s
Emit Time	2.94s (± 0.45%)	2.91s (± 0.76%)	-0.02s (- 0.85%)	2.85s	2.96s
Total Time	9.72s (± 0.16%)	9.68s (± 0.23%)	-0.04s (- 0.44%)	9.62s	9.72s
TFS - node (v10.16.3, x64)
Memory used	291,989k (± 0.02%)	291,982k (± 0.02%)	-7k (- 0.00%)	291,843k	292,162k
Parse Time	0.95s (± 0.49%)	0.95s (± 0.38%)	-0.00s (- 0.42%)	0.94s	0.95s
Bind Time	0.74s (± 0.66%)	0.74s (± 0.75%)	-0.00s (- 0.27%)	0.73s	0.75s
Check Time	4.28s (± 0.46%)	4.26s (± 0.38%)	-0.02s (- 0.47%)	4.22s	4.30s
Emit Time	3.04s (± 0.60%)	3.05s (± 0.90%)	+0.01s (+ 0.43%)	2.97s	3.09s
Total Time	9.01s (± 0.35%)	9.00s (± 0.29%)	-0.01s (- 0.12%)	8.91s	9.04s
material-ui - node (v10.16.3, x64)
Memory used	453,294k (± 0.01%)	453,051k (± 0.02%)	-242k (- 0.05%)	452,854k	453,206k
Parse Time	1.78s (± 0.52%)	1.78s (± 0.54%)	-0.00s (- 0.11%)	1.76s	1.80s
Bind Time	0.69s (± 0.72%)	0.68s (± 0.70%)	-0.00s (- 0.44%)	0.67s	0.69s
Check Time	13.68s (± 0.47%)	13.71s (± 0.61%)	+0.03s (+ 0.24%)	13.57s	13.91s
Emit Time	0.00s (± 0.00%)	0.00s (± 0.00%)	0.00s ( NaN%)	0.00s	0.00s
Total Time	16.14s (± 0.38%)	16.17s (± 0.54%)	+0.03s (+ 0.19%)	16.03s	16.39s
Angular - node (v12.1.0, x64)
Memory used	303,839k (± 0.02%)	304,013k (± 0.06%)	+174k (+ 0.06%)	303,268k	304,289k
Parse Time	1.57s (± 0.78%)	1.57s (± 0.63%)	-0.00s (- 0.13%)	1.55s	1.60s
Bind Time	0.88s (± 0.66%)	0.87s (± 1.08%)	-0.01s (- 0.80%)	0.85s	0.89s
Check Time	4.64s (± 0.19%)	4.68s (± 0.83%)	+0.03s (+ 0.73%)	4.63s	4.82s
Emit Time	5.45s (± 0.79%)	5.44s (± 0.58%)	-0.01s (- 0.17%)	5.38s	5.51s
Total Time	12.54s (± 0.45%)	12.56s (± 0.52%)	+0.02s (+ 0.16%)	12.48s	12.79s
Monaco - node (v12.1.0, x64)
Memory used	307,053k (± 0.01%)	307,098k (± 0.02%)	+45k (+ 0.01%)	306,941k	307,343k
Parse Time	1.21s (± 0.69%)	1.21s (± 0.55%)	+0.00s (+ 0.08%)	1.19s	1.22s
Bind Time	0.74s (± 0.90%)	0.74s (± 0.66%)	+0.00s (+ 0.54%)	0.73s	0.75s
Check Time	4.55s (± 0.41%)	4.55s (± 0.48%)	-0.00s (- 0.02%)	4.50s	4.60s
Emit Time	2.95s (± 0.76%)	2.96s (± 0.71%)	+0.01s (+ 0.31%)	2.90s	3.00s
Total Time	9.45s (± 0.34%)	9.46s (± 0.39%)	+0.01s (+ 0.10%)	9.34s	9.51s
TFS - node (v12.1.0, x64)
Memory used	274,251k (± 0.02%)	274,261k (± 0.01%)	+9k (+ 0.00%)	274,210k	274,354k
Parse Time	0.93s (± 0.81%)	0.93s (± 0.97%)	-0.01s (- 1.07%)	0.91s	0.94s
Bind Time	0.71s (± 0.97%)	0.71s (± 1.35%)	+0.00s (+ 0.14%)	0.69s	0.73s
Check Time	4.18s (± 0.42%)	4.18s (± 0.34%)	+0.00s (+ 0.10%)	4.15s	4.21s
Emit Time	3.09s (± 0.44%)	3.08s (± 0.74%)	-0.01s (- 0.23%)	3.03s	3.12s
Total Time	8.91s (± 0.33%)	8.90s (± 0.26%)	-0.01s (- 0.13%)	8.86s	8.96s
material-ui - node (v12.1.0, x64)
Memory used	430,726k (± 0.01%)	430,528k (± 0.01%)	-198k (- 0.05%)	430,435k	430,644k
Parse Time	1.75s (± 0.34%)	1.75s (± 0.42%)	-0.00s (- 0.23%)	1.73s	1.76s
Bind Time	0.63s (± 0.54%)	0.63s (± 1.05%)	+0.00s (+ 0.48%)	0.62s	0.65s
Check Time	12.12s (± 0.56%)	12.15s (± 0.78%)	+0.03s (+ 0.26%)	12.01s	12.43s
Emit Time	0.00s (± 0.00%)	0.00s (± 0.00%)	0.00s ( NaN%)	0.00s	0.00s
Total Time	14.50s (± 0.50%)	14.53s (± 0.70%)	+0.03s (+ 0.21%)	14.37s	14.82s
Angular - node (v8.9.0, x64)
Memory used	323,163k (± 0.02%)	323,531k (± 0.01%)	+368k (+ 0.11%)	323,426k	323,629k
Parse Time	2.12s (± 0.45%)	2.11s (± 0.16%)	-0.00s (- 0.09%)	2.11s	2.12s
Bind Time	0.92s (± 0.87%)	0.91s (± 0.54%)	-0.00s (- 0.44%)	0.91s	0.93s
Check Time	5.48s (± 0.47%)	5.52s (± 0.86%)	+0.05s (+ 0.88%)	5.38s	5.63s
Emit Time	6.22s (± 0.62%)	6.23s (± 1.40%)	+0.01s (+ 0.10%)	5.98s	6.44s
Total Time	14.73s (± 0.46%)	14.78s (± 0.45%)	+0.04s (+ 0.31%)	14.59s	14.90s
Monaco - node (v8.9.0, x64)
Memory used	325,520k (± 0.02%)	325,529k (± 0.01%)	+9k (+ 0.00%)	325,462k	325,648k
Parse Time	1.54s (± 0.58%)	1.54s (± 0.34%)	-0.00s (- 0.19%)	1.53s	1.55s
Bind Time	0.90s (± 1.06%)	0.89s (± 0.53%)	-0.01s (- 0.67%)	0.88s	0.90s
Check Time	5.38s (± 0.45%)	5.35s (± 0.52%)	-0.03s (- 0.56%)	5.29s	5.41s
Emit Time	3.51s (± 0.63%)	3.53s (± 0.39%)	+0.02s (+ 0.63%)	3.50s	3.57s
Total Time	11.32s (± 0.44%)	11.31s (± 0.34%)	-0.02s (- 0.16%)	11.22s	11.38s
TFS - node (v8.9.0, x64)
Memory used	291,362k (± 0.01%)	291,356k (± 0.01%)	-6k (- 0.00%)	291,276k	291,475k
Parse Time	1.25s (± 0.72%)	1.25s (± 0.42%)	0.00s ( 0.00%)	1.24s	1.27s
Bind Time	0.75s (± 0.53%)	0.75s (± 0.49%)	-0.00s (- 0.40%)	0.74s	0.75s
Check Time	4.89s (± 1.47%)	4.88s (± 1.90%)	-0.01s (- 0.18%)	4.77s	5.17s
Emit Time	3.29s (± 1.80%)	3.28s (± 3.00%)	-0.00s (- 0.06%)	3.04s	3.55s
Total Time	10.18s (± 0.37%)	10.16s (± 0.74%)	-0.01s (- 0.11%)	10.02s	10.36s
material-ui - node (v8.9.0, x64)
Memory used	455,829k (± 0.01%)	455,631k (± 0.02%)	-198k (- 0.04%)	455,503k	455,895k
Parse Time	2.11s (± 0.83%)	2.10s (± 0.29%)	-0.01s (- 0.52%)	2.09s	2.11s
Bind Time	0.80s (± 1.25%)	0.81s (± 0.95%)	+0.00s (+ 0.12%)	0.79s	0.82s
Check Time	17.81s (± 0.51%)	17.71s (± 0.73%)	-0.10s (- 0.56%)	17.41s	18.00s
Emit Time	0.00s (± 0.00%)	0.00s (± 0.00%)	0.00s ( NaN%)	0.00s	0.00s
Total Time	20.72s (± 0.45%)	20.61s (± 0.63%)	-0.11s (- 0.52%)	20.32s	20.92s
Angular - node (v8.9.0, x86)
Memory used	185,946k (± 0.03%)	186,168k (± 0.02%)	+222k (+ 0.12%)	186,095k	186,280k
Parse Time	2.07s (± 1.04%)	2.05s (± 0.60%)	-0.02s (- 0.92%)	2.03s	2.09s
Bind Time	1.07s (± 0.71%)	1.07s (± 0.63%)	-0.01s (- 0.65%)	1.06s	1.09s
Check Time	5.01s (± 0.31%)	5.06s (± 0.44%)	+0.05s (+ 1.02%)	5.03s	5.13s
Emit Time	6.12s (± 1.45%)	6.03s (± 0.36%)	-0.09s (- 1.50%)	5.98s	6.08s
Total Time	14.27s (± 0.74%)	14.22s (± 0.22%)	-0.06s (- 0.41%)	14.16s	14.29s
Monaco - node (v8.9.0, x86)
Memory used	185,294k (± 0.02%)	185,291k (± 0.03%)	-3k (- 0.00%)	185,173k	185,409k
Parse Time	1.60s (± 1.34%)	1.59s (± 0.60%)	-0.01s (- 0.50%)	1.57s	1.61s
Bind Time	0.77s (± 0.89%)	0.76s (± 0.73%)	-0.00s (- 0.52%)	0.75s	0.78s
Check Time	5.42s (± 0.56%)	5.41s (± 0.60%)	-0.01s (- 0.13%)	5.37s	5.53s
Emit Time	2.87s (± 0.24%)	2.86s (± 1.08%)	-0.02s (- 0.59%)	2.79s	2.94s
Total Time	10.66s (± 0.42%)	10.63s (± 0.56%)	-0.03s (- 0.31%)	10.52s	10.80s
TFS - node (v8.9.0, x86)
Memory used	166,793k (± 0.03%)	166,771k (± 0.02%)	-22k (- 0.01%)	166,713k	166,864k
Parse Time	1.29s (± 0.43%)	1.28s (± 0.64%)	-0.00s (- 0.31%)	1.27s	1.31s
Bind Time	0.71s (± 0.81%)	0.71s (± 0.70%)	+0.00s (+ 0.28%)	0.71s	0.73s
Check Time	4.62s (± 0.40%)	4.60s (± 0.30%)	-0.02s (- 0.41%)	4.57s	4.63s
Emit Time	3.00s (± 2.06%)	2.99s (± 1.92%)	-0.01s (- 0.37%)	2.90s	3.20s
Total Time	9.62s (± 0.67%)	9.59s (± 0.69%)	-0.03s (- 0.29%)	9.48s	9.84s
material-ui - node (v8.9.0, x86)
Memory used	257,880k (± 0.02%)	257,733k (± 0.02%)	-146k (- 0.06%)	257,655k	257,848k
Parse Time	2.18s (± 0.38%)	2.17s (± 0.35%)	-0.00s (- 0.05%)	2.16s	2.19s
Bind Time	0.68s (± 1.21%)	0.69s (± 0.72%)	+0.00s (+ 0.29%)	0.68s	0.70s
Check Time	16.30s (± 0.71%)	16.21s (± 0.53%)	-0.09s (- 0.56%)	16.03s	16.38s
Emit Time	0.00s (± 0.00%)	0.00s (± 0.00%)	0.00s ( NaN%)	0.00s	0.00s
Total Time	19.16s (± 0.59%)	19.07s (± 0.45%)	-0.09s (- 0.45%)	18.88s	19.26s

System

Machine Name	ts-ci-ubuntu
Platform	linux 4.4.0-166-generic
Architecture	x64
Available Memory	16 GB
Available Memory	1 GB
CPUs	4 × Intel(R) Core(TM) i7-4770 CPU @ 3.40GHz

Hosts

• node (v10.16.3, x64)
• node (v12.1.0, x64)
• node (v8.9.0, x64)
• node (v8.9.0, x86)

Scenarios

• Angular - node (v10.16.3, x64)
• Angular - node (v12.1.0, x64)
• Angular - node (v8.9.0, x64)
• Angular - node (v8.9.0, x86)
• Monaco - node (v10.16.3, x64)
• Monaco - node (v12.1.0, x64)
• Monaco - node (v8.9.0, x64)
• Monaco - node (v8.9.0, x86)
• TFS - node (v10.16.3, x64)
• TFS - node (v12.1.0, x64)
• TFS - node (v8.9.0, x64)
• TFS - node (v8.9.0, x86)
• material-ui - node (v10.16.3, x64)
• material-ui - node (v12.1.0, x64)
• material-ui - node (v8.9.0, x64)
• material-ui - node (v8.9.0, x86)

Benchmark	Name	Iterations
Current	37540	10
Baseline	master	10

[rbuckton]

This seems to have improved things in vscode: https://github.com/typescript-bot/TypeScript/pull/9/files#diff-0e62d568d28b82cee9e66b2b8fafd290R7, however there seem to be a larger number of errors now in office-ui-fabric (https://github.com/typescript-bot/TypeScript/pull/9/files#diff-976ec9fe54f7b841cd49ec069e0303a9L135), though that may not be related to this change.

[rbuckton]

Closing this in favor of #37610 for now while we continue to investigate the impact of awaited.