Incorrect nullable annotations on most MemoryExtensions methods

[stephentoub]

Most of our MemoryExtensions methods have a shape like:

public static bool Contains<T>(this Span<T> span, T value) where T : IEquatable<T>

where the T is constrained to be IEquatable<T>. The questionable nature of the constraint in general aside (we can use EqualityComparer<T>.Default), the constraint is too strict with regards to nullable reference types. The constraint should be where T : IEquatable<T>? rather than where T : IEquatable<T>; otherwise, it's prohibiting nullable Ts, e.g. this erroneously warns:

Span<string?> objects = new string?[] { "hello", null, "world" };
Console.WriteLine(objects.Contains(null));

with:

warning CS8631: The type 'string?' cannot be used as type parameter 'T' in the generic type or method 'MemoryExtensions.Contains<T>(Span<T>, T)'. Nullability of type argument 'string?' doesn't match constraint type 'System.IEquatable<string?>'.

We need to audit all such constraints and fix them.

cc: @terrajobst, @bartonjs, @jeffhandley

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Issue Details

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Most of our MemoryExtensions methods have a shape like:

public static bool Contains<T>(this Span<T> span, T value) where T : IEquatable<T>

where the T is constrained to be IEquatable<T>. The questionable nature of the constraint in general aside (I don't see a good reason to constrain to IEquatable<T> here), the constraint is too strict with regards to nullable reference types. The constraint should be where T : IEquatable<T>? rather than where T : IEquatable<T>; otherwise, it's prohibiting nullable Ts, e.g. this erroneously warns:

Span<string?> objects = new string?[] { "hello", null, "world" };
Console.WriteLine(objects.Contains(null));

with:

warning CS8631: The type 'string?' cannot be used as type parameter 'T' in the generic type or method 'MemoryExtensions.Contains<T>(Span<T>, T)'. Nullability of type argument 'string?' doesn't match constraint type 'System.IEquatable<string?>'.

We need to audit all such constraints and fix them.

cc: @terrajobst, @bartonjs, @jeffhandley

Author:	stephentoub
Assignees:	-
Labels:	area-System.Memory
Milestone:	7.0.0