dotnet/roslyn
class? constraint combines poorly with non-nullable interface constraint
Aberta
#49.571 aberto em 23 de nov. de 2020
Area-CompilersBugFeature - Nullable Reference Typeshelp wanted
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Description
Version Used: dc6f316a5e4f8eb3a0d36ab356c5c12686db00a9
Steps to Reproduce:
#nullable enable
using System;
public class C {
public void M<T>(T t) where T : class?, IEquatable<T> {
t.ToString(); // no warning?
}
}
Expected Behavior: Variable of type T has a maybe-null initial state
Actual Behavior: Variable has not-null initial state
Remarks: This may be fine and by-design, however the combination of these particular constraints seems to produce a contradictory/unintuitive result, so maybe there should be a warning to change class? to class, or to change IEquatable<T> to IEquatable<T>?.
We should report a warning under the following condition:
If the collection of interface, base class and the
classconstraints on a type parameter contain a single one which is "non-nullable" then we should warn if any of the others are "nullable".