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Allow null literals to contribute nullability to type inference #56022

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merged 7 commits into from
Sep 9, 2021
Merged

Allow null literals to contribute nullability to type inference #56022

merged 7 commits into from
Sep 9, 2021

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RikkiGibson
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@RikkiGibson RikkiGibson commented Aug 30, 2021 •
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Closes #43536

We skip removing conversions from call arguments when the conversion is predefined and applied to a null literal. Note that other kinds of constant-null expressions we do expect to have a natural type, so we do remove their conversion.

Changed approach to instead add special "nullable bounds" to method type inference per #56022 (comment). Will update the PR title prior to merge to avoid messing up email threading.

@RikkiGibson RikkiGibson marked this pull request as ready for review August 31, 2021 15:08
@RikkiGibson RikkiGibson requested a review from a team as a code owner August 31, 2021 15:08
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jcouv commented Aug 31, 2021 •
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From our offline chat, I'm not sure that's the right approach. I'd suggest reading through https://github.com/dotnet/csharplang/blob/main/proposals/csharp-8.0/nullable-reference-types-specification.md#generic-type-inference
Then it feels like the solution would be to collect a bound that doesn't contribute a type but does contribute an annotation, whereas today if the argument doesn't have a type then it contributes no bound (see MakeExplicitParameterTypeInferences).

In reply to: 909400174

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jcouv reviewed Aug 31, 2021
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Done with review pass (iteration 3)

@jcouv jcouv self-assigned this Aug 31, 2021
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@dotnet/roslyn-compiler Please review

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@jaredpar FYI I tried building dotnet/runtime with this change and I wasn't able to observe any new nullable warnings. I wasn't able to build everything because of weird variant of dotnet/runtime#42397 but I was able to build ~427 C# projects successfully.

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jcouv reviewed Sep 1, 2021
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src/Compilers/CSharp/Portable/Binder/Semantics/OverloadResolution/MethodTypeInference.cs
{
var ordinal = ((TypeParameterSymbol)target.Type).Ordinal;
var typeWithAnnotations = _extensions.GetTypeWithAnnotations(argument);
_nullableAnnotationLowerBounds[ordinal] = _nullableAnnotationLowerBounds[ordinal].Join(typeWithAnnotations.NullableAnnotation);
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@jcouv jcouv Sep 1, 2021 •
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nit: Given that we're now only using Join and the starting values are NotAnnotated. I'm wonder if we should just store a boolean. I'll leave that for consideration for you and second reviewer. I don't feel strongly (seems equivalent). #Closed

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I also don't feel strongly about it. This did inspire some additional tests for 'null!' and 'null' in a disabled context, so thanks :)

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jcouv approved these changes Sep 1, 2021
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LGTM Thanks (iteration 5)

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@dotnet/roslyn-compiler for a second review.

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@dotnet/roslyn-compiler Please review

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@dotnet/roslyn-compiler Please review

@RikkiGibson RikkiGibson changed the title Preserve conversions on null literals before nullable type argument reinference Allow null literals to contribute nullability to type inference Sep 9, 2021
@RikkiGibson RikkiGibson merged commit 861b427 into dotnet:main Sep 9, 2021
@ghost ghost added this to the Next milestone Sep 9, 2021
@RikkiGibson RikkiGibson deleted the null-type-arg-inference branch September 9, 2021 22:00
@Cosifne Cosifne modified the milestones: Next, 17.0.P5 Sep 27, 2021
@jcouv jcouv mentioned this pull request Sep 28, 2021
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@jcouv jcouv jcouv approved these changes

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Area-Compilers New Language Feature - Nullable Reference Types Nullable Reference Types
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Type inference handles null and default for reference types differently
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@RikkiGibson @jcouv @cston @Cosifne