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Fix small spec bugs regarding nullable reference type parameters #1389

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Fix small spec bugs regarding nullable reference type parameters #1389

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0b3ee5d
Not every constrained type parameter may be annotated
jnm2 Aug 9, 2025
8a1de22
Mention type parameters in '§8.2 Reference types' the same as in '§8.…
jnm2 Aug 9, 2025
918e022
Remove line referring to impossible situation in C# 8
jnm2 Aug 9, 2025
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7 changes: 4 additions & 3 deletions standard/classes.md
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Expand Up @@ -395,7 +395,7 @@
: 'where' type_parameter ':' type_parameter_constraints
;

type_parameter_constraints

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MDC032::Line length 83 > maximum 81
: primary_constraint (',' secondary_constraints)? (',' constructor_constraint)?
| secondary_constraints (',' constructor_constraint)?
| constructor_constraint
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> *Note*: To specify that a type argument is a nullable reference type, don’t add the nullable type annotation as a constraint (use `T : class` or `T : BaseClass`), but use `T?` throughout the generic declaration to indicate the corresponding nullable reference type for the type argument. *end note*

<!-- Remove in C# 9, when this is allowed -->
The nullable type annotation, `?`, can’t be used on an unconstrained type argument.
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@jnm2 jnm2 Aug 9, 2025 •
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Here is an example of a type parameter that is not unconstrained, and thus was not disallowed by the original statement, which is still not allowed to use ?:

#nullable enable

class C
{
    void M<T>(T? test) where T : System.IDisposable
    {
    }
}

Roslyn:

A nullable type parameter must be known to be a value type or non-nullable reference type unless language version '9.0' or greater is used. Consider changing the language version or adding a 'class', 'struct', or type constraint.

<!-- Remove in C# 9, when `?` is allowed on any type parameter. -->
The nullable type annotation, `?`, can only be used on a type parameter that has the value type constraint, the reference type constraint without the *nullable_type_annotation*, or a class type constraint without the *nullable_type_annotation*.

For a type parameter `T` when the type argument is a nullable reference type `C?`, instances of `T?` are interpreted as `C?`, not `C??`.
<!-- Add in C# 9, when `?` is allowed on nullable reference type parameters. -->
<!-- For a type parameter `T` when the type argument is a nullable reference type `C?`, instances of `T?` are interpreted as `C?`, not `C??`. -->
Comment on lines +448 to +449
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@jnm2 jnm2 Aug 9, 2025 •
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Instances of T? do not exist when the type argument is a nullable reference type C? in C# 8, e.g.:

#nullable enable

class C
{
    void M<T>(T? test) where T : class?
    {
    }
}

Roslyn:

A nullable type parameter must be known to be a value type or non-nullable reference type unless language version '9.0' or greater is used. Consider changing the language version or adding a 'class', 'struct', or type constraint.


> *Example*: The following examples show how the nullability of a type argument impacts the nullability of a declaration of its type parameter:
>
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> static void M()
> {
> // nonnull constraint allows nonnullable struct type argument
> A<int> x1;

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> // possible warning: nonnull constraint prohibits nullable struct type argument
> A<int?> x2;
> // nonnull constraint allows nonnullable class type argument
> A<C> x3;

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> // possible warning: nonnull constraint prohibits nullable class type argument
> A<C?> x4;

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> // nonnullable base class requirement allows nonnullable class type argument
> B1<C> x5;

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> // possible warning: nonnullable base class requirement prohibits nullable class type argument
> B1<C?> x6;
> // nullable base class requirement allows nonnullable class type argument
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> static void Main()
> {
> field = 10;
> Console.WriteLine(Property); // Prints 10

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> Property = 20; // This invokes the get accessor, then assigns
> // via the resulting variable reference
> Console.WriteLine(field); // Prints 20
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: '!'
;

overloadable_unary_operator

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: '+' | '-' | logical_negation_operator | '~' | '++' | '--' | 'true' | 'false'
;

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2 changes: 1 addition & 1 deletion standard/types.md
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Expand Up @@ -27,7 +27,7 @@

### 8.2.1 General

A reference type is a class type, an interface type, an array type, a delegate type, or the `dynamic` type. For each non-nullable reference type, there is a corresponding nullable reference type noted by appending the `?` to the type name.
A reference type is a class type, an interface type, an array type, a delegate type, the `dynamic` type, or any type parameter that is constrained to be a reference type (that is, any type parameter with the reference type constraint or a class type constraint ([§15.2.5](classes.md#1525-type-parameter-constraints))). For each non-nullable reference type, there is a corresponding nullable reference type noted by appending the `?` to the type name.
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@jnm2 jnm2 Aug 9, 2025 •
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This borrows the existing language which appears in https://github.com/dotnet/csharpstandard/blob/draft-v8/standard/types.md#8312-nullable-value-types:

Conversely, a non-nullable value type is any value type other than System.Nullable<T> and its shorthand T? (for any T), plus any type parameter that is constrained to be a non-nullable value type (that is, any type parameter with a value type constraint (§15.2.5)).


```ANTLR
reference_type
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> public void M(string s)
> {
> int length = s.Length; // No warning. s is not null
>

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> _ = s == null; // Null check by testing equality. The null state of s is maybe null
> length = s.Length; // Warning, and changes the null state of s to not null
>

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> _ = s?.Length; // The ?. is a null check and changes the null state of s to maybe null
> if (s.Length > 4) // Warning. Changes null state of s to not null
> {

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> _ = s?[4]; // ?[] is a null check and changes the null state of s to maybe null
> _ = s.Length; // Warning. s is maybe null
> }
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> {
> var t = new Test();
> if (t.DisappearingProperty != null)
> {

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> int len = t.DisappearingProperty.Length; // No warning. A compiler can assume property is stateful
> }
> }
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