Skip to content

Add support for nonnull constraints #830

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Closed
wants to merge 4 commits into from
Closed

Add support for nonnull constraints #830

Changes from all commits
Commits
Show all changes
4 commits
Select commit Hold shift + click to select a range
5bdd0c3
add support for nonnull constraints
RexJaeschke Jun 30, 2023
1ece5c4
Update classes.md
RexJaeschke Jun 30, 2023
bf52b41
Correct example annotation header
RexJaeschke Oct 23, 2023
1c77891
Merge branch 'draft-v8' into Rex-nonnull-constraint
RexJaeschke Nov 13, 2024
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
47 changes: 45 additions & 2 deletions standard/classes.md
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -416,6 +416,7 @@ primary_constraint
| 'class'
| 'struct'
| 'unmanaged'
| 'notnull'
;

secondary_constraints
Expand All @@ -434,7 +435,7 @@ Each *type_parameter_constraints_clause* consists of the token `where`, followed

The list of constraints given in a `where` clause can include any of the following components, in this order: a single primary constraint, one or more secondary constraints, and the constructor constraint, `new()`.

A primary constraint can be a class type, the ***reference type constraint*** `class`, the ***value type constraint*** `struct`, or the ***unmanaged type constraint*** `unmanaged`.
A primary constraint can be a class type, the ***reference type constraint*** `class`, the ***value type constraint*** `struct`, or the ***nonullable type constraint*** `notnull`.

A secondary constraint can be a *type_parameter* or *interface_type*.

Expand Down Expand Up @@ -490,7 +491,49 @@ Any constraints shall be consistent among dependent type parameters. If type par

It is valid for `S` to have the value type constraint and `T` to have the reference type constraint. Effectively this limits `T` to the types `System.Object`, `System.ValueType`, `System.Enum`, and any interface type.

If the `where` clause for a type parameter includes a constructor constraint (which has the form `new()`), it is possible to use the `new` operator to create instances of the type ([§12.8.17.2](expressions.md#128172-object-creation-expressions)). Any type argument used for a type parameter with a constructor constraint shall be a value type, a non-abstract class having a public parameterless constructor, or a type parameter having the value type constraint or constructor constraint.
The token `notnull` is neither a keyword nor a contextual keyword. When it is encountered, it will either:
Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

As per #1246, we believe it is a contextual keyword.


- Bind to a type named `notnull`
Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

@gafter pointed out it could be an alias - we can just strike "named notnull", or see @Nigel-Ecma's wording in #1246.

- Bind to no type, in which case, it is interpreted as the nonnullable type constraint.

The nonnullable type constraint specifies that a type argument used for the type parameter shall be a non-nullable value type or non-nullable reference type.

Generic declarations that include the `notnull` constraint may be used in a nullable oblivious context; however, that constraint is ignored, and a warning shall be generated.

> *Example*: Consider the following:
>
> <!-- Example: {template:"standalone-lib-without-using", name:"TypeParameterConstraints0", expectedWarnings:["CS8714","CS8714","CS8631"], ignoredWarnings:["CS0168"]} -->
> ```csharp
> #nullable enable
> public class C { }
> public class A<T> where T : notnull { }
> public class B1<T> where T : C { }
> public class B2<T> where T : C? { }
> class Test
> {
> static void M()
> {
> // nonnull constraint allows nonnullable struct type argument
> A<int> x1;
> // warning: nonnull constraint prohibits nullable struct type argument
> A<int?> x2;
> // nonnullconstraint allows nonnullable class type argument
> A<C> x3;
> // warning: nonnull constraint prohibits nullable class type argument
> A<C?> x4;
> // nonnullable base class requirement allows nonnullable class type argument
> B1<C> x5;
> // warning: nonnullable base class requirement prohibits nullable class type argument
> B1<C?> x6;
> // nullable base class requirement allows nonnullable class type argument
> B2<C> x7;
> // nullable base class requirement allows nullable class type argument
> B2<C?> x8;
> }
> }
> ```

If the `where` clause for a type parameter includes a constructor constraint (which has the form `new()`), it is possible to use the `new` operator to create instances of the type ([§12.8.16.2](expressions.md#128162-object-creation-expressions)). Any type argument used for a type parameter with a constructor constraint shall be a value type, a non-abstract class having a public parameterless constructor, or a type parameter having the value type constraint or constructor constraint.

It is a compile-time error for *type_parameter_constraints* having a *primary_constraint* of `struct` or `unmanaged` to also have a *constructor_constraint*.

Expand Down
Loading