Fix System.Text.Json source generator constructor accessor for generic types with inaccessible constructors

[svick]

Summary

Fixes #133369.

The System.Text.Json source generator emitted an invalid [UnsafeAccessor(UnsafeAccessorKind.Constructor)] extern for a generic type with an inaccessible constructor (e.g. a private [JsonConstructor]). The extern named the closed generic type directly:

[UnsafeAccessor(UnsafeAccessorKind.Constructor)]
private static extern MyType<int> __ctor_MyTypeInt32(int value);

That compiles, but UnsafeAccessor does not resolve a constructor against a directly-named closed generic type, so deserialization threw System.MissingMethodException: Method not found: 'MyType`1..ctor' at run time. The member (get/set/field) accessors already handle generics correctly via a wrapper class; only the constructor accessor was missing that treatment.

Fix

Emit the constructor extern inside a generic wrapper class whose type parameters flow into the signature, and reference it through the closed type arguments at the call site:

private static class __GenericCtorAccessor_MyTypeInt32<T>
{
    [UnsafeAccessor(UnsafeAccessorKind.Constructor)]
    public static extern MyType<T> __ctor_MyTypeInt32(T p0);
}
// call site: __GenericCtorAccessor_MyTypeInt32<int>.__ctor_MyTypeInt32((int)args[0])

This mirrors the existing __GenericAccessors_<T> member-accessor wrapper (distinct name, so a type with both inaccessible members and an inaccessible constructor emits both wrappers without clashing). Non-generic types and the downlevel reflection fallback (net462) are unchanged.

Spec additions carrying the open-generic strings (incremental-pipeline safe): TypeGenerationSpec.{TypeParameterNames, OpenTypeFQN, TypeParameterConstraintClauses} and ParameterGenerationSpec.OpenParameterTypeFQN, populated only when the type is generic and generic UnsafeAccessors are supported (.NET 9+).

Tests

• New output-baseline case UnsafeAccessors_GenericTypeInaccessibleConstructor (netcoreapp UnsafeAccessor wrapper + net462 reflection fallback).
• New functional test JsonSerializerContextTests.GenericTypeWithInaccessibleConstructor_IsSupported that round-trips a generic type with a private [JsonConstructor]; it throws MissingMethodException before the fix and passes after.

Note

This PR description was generated by GitHub Copilot.

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