Ambigious call in PLinq Aggregate method due to C#10 lambda type inference

[akoeplinger]

Version Used: 4.0.0-5.21453.15 (2bbf85b)

Steps to Reproduce:

1. Compile https://github.com/dotnet/runtime/blob/91f43669bbd6bdf1ba760a6eaa4e42c2794c5f08/src/libraries/System.Linq.Parallel/tests/QueryOperators/AggregateTests.cs

Expected Behavior: No errors.

Actual Behavior:

AggregateTests.cs(320,59): error CS0121: The call is ambiguous between the following methods or properties: 'ParallelEnumerable.Aggregate<TSource, TAccumulate, TResult>(ParallelQuery<TSource>, Func<TAccumulate>, Func<TAccumulate, TSource, TAccumulate>, Func<TAccumulate, TAccumulate, TAccumulate>, Func<TAccumulate, TResult>)' and 'ParallelEnumerable.Aggregate<TSource, TAccumulate, TResult>(ParallelQuery<TSource>, TAccumulate, Func<TAccumulate, TSource, TAccumulate>, Func<TAccumulate, TAccumulate, TAccumulate>, Func<TAccumulate, TResult>)' [/Users/alexander/dev/runtime/src/libraries/System.Linq.Parallel/tests/System.Linq.Parallel.Tests.csproj]
AggregateTests.cs(333,93): error CS0121: The call is ambiguous between the following methods or properties: 'ParallelEnumerable.Aggregate<TSource, TAccumulate, TResult>(ParallelQuery<TSource>, Func<TAccumulate>, Func<TAccumulate, TSource, TAccumulate>, Func<TAccumulate, TAccumulate, TAccumulate>, Func<TAccumulate, TResult>)' and 'ParallelEnumerable.Aggregate<TSource, TAccumulate, TResult>(ParallelQuery<TSource>, TAccumulate, Func<TAccumulate, TSource, TAccumulate>, Func<TAccumulate, TAccumulate, TAccumulate>, Func<TAccumulate, TResult>)' [/Users/alexander/dev/runtime/src/libraries/System.Linq.Parallel/tests/System.Linq.Parallel.Tests.csproj]
AggregateTests.cs(338,97): error CS0121: The call is ambiguous between the following methods or properties: 'ParallelEnumerable.Aggregate<TSource, TAccumulate, TResult>(ParallelQuery<TSource>, Func<TAccumulate>, Func<TAccumulate, TSource, TAccumulate>, Func<TAccumulate, TAccumulate, TAccumulate>, Func<TAccumulate, TResult>)' and 'ParallelEnumerable.Aggregate<TSource, TAccumulate, TResult>(ParallelQuery<TSource>, TAccumulate, Func<TAccumulate, TSource, TAccumulate>, Func<TAccumulate, TAccumulate, TAccumulate>, Func<TAccumulate, TResult>)' [/Users/alexander/dev/runtime/src/libraries/System.Linq.Parallel/tests/System.Linq.Parallel.Tests.csproj]
AggregateTests.cs(360,103): error CS0121: The call is ambiguous between the following methods or properties: 'ParallelEnumerable.Aggregate<TSource, TAccumulate, TResult>(ParallelQuery<TSource>, Func<TAccumulate>, Func<TAccumulate, TSource, TAccumulate>, Func<TAccumulate, TAccumulate, TAccumulate>, Func<TAccumulate, TResult>)' and 'ParallelEnumerable.Aggregate<TSource, TAccumulate, TResult>(ParallelQuery<TSource>, TAccumulate, Func<TAccumulate, TSource, TAccumulate>, Func<TAccumulate, TAccumulate, TAccumulate>, Func<TAccumulate, TResult>)' [/Users/alexander/dev/runtime/src/libraries/System.Linq.Parallel/tests/System.Linq.Parallel.Tests.csproj]
AggregateTests.cs(362,119): error CS0121: The call is ambiguous between the following methods or properties: 'ParallelEnumerable.Aggregate<TSource, TAccumulate, TResult>(ParallelQuery<TSource>, Func<TAccumulate>, Func<TAccumulate, TSource, TAccumulate>, Func<TAccumulate, TAccumulate, TAccumulate>, Func<TAccumulate, TResult>)' and 'ParallelEnumerable.Aggregate<TSource, TAccumulate, TResult>(ParallelQuery<TSource>, TAccumulate, Func<TAccumulate, TSource, TAccumulate>, Func<TAccumulate, TAccumulate, TAccumulate>, Func<TAccumulate, TResult>)' [/Users/alexander/dev/runtime/src/libraries/System.Linq.Parallel/tests/System.Linq.Parallel.Tests.csproj]
AggregateTests.cs(363,121): error CS0121: The call is ambiguous between the following methods or properties: 'ParallelEnumerable.Aggregate<TSource, TAccumulate, TResult>(ParallelQuery<TSource>, Func<TAccumulate>, Func<TAccumulate, TSource, TAccumulate>, Func<TAccumulate, TAccumulate, TAccumulate>, Func<TAccumulate, TResult>)' and 'ParallelEnumerable.Aggregate<TSource, TAccumulate, TResult>(ParallelQuery<TSource>, TAccumulate, Func<TAccumulate, TSource, TAccumulate>, Func<TAccumulate, TAccumulate, TAccumulate>, Func<TAccumulate, TResult>)' [/Users/alexander/dev/runtime/src/libraries/System.Linq.Parallel/tests/System.Linq.Parallel.Tests.csproj]

@CyrusNajmabadi asked me to open an issue for discussion.

/cc @333fred @cston

[cston]

The ambiguities should be resolved in dotnet/runtime#58953.

[CyrusNajmabadi]

Note: using @AlekseyTs 's formalization from yesterday, i don't think this would be a break. We would still bind with the original meaning, and only fall back to the new meaning if htat fails.

[CyrusNajmabadi]

@cston i don't like that approach as i think the original code they have is reasonable and they shouldn't have to add explicit types to workaround it. The particular example is somewhat egregious as i think the language is sliding backwards here. Specifically, given teh type Func<int> it seems wrong to me that an instantiation of T -> Func<int> is treated as good as an instantiation of Func<T> -> Func<int>. The latter seems to intuitively be what someone would want and expect. Even if T -> Func<int> is a possible inference, it should not be as good as Func<T> -> Func<int>. I think we should revisit this and not have people have to provide parametr types in cases like these.

Note that linq is a high pri api that we should endeavor to keep working IMO.