Championed issue: dotnet/csharplang#6065
Speclet: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-11.0/numeric-intptr.md

Compiler

• Public API:
  • IsNativeIntegerType
  • CreateNativeIntegerTypeSymbol()
• Built-in conversions are standard implicit and explicit (see PR Treat some nint/nuint conversions as standard conversions #61034)
• Add TargetFramework.Net70 and use that for our tests (issue Add TargetFramework.Net70 #61463)
• spec (PR)
• Tied to runtime feature flag
• Not tied to -langversion
• No [NativeInteger] attributes emitted
• Members defined on IntPtr/UIntPtr are available from IntPtr/UIntPtr and nint/nuint:
  • interfaces
  • static members
  • instance members
  • constructors
  • implicit and explicit conversion operators
  • overridden implementations: ToString(), GetHashCode(), Equals()
• Equivalence between nint/IntPtr and nuint/UIntPtr for:
  • interface implementations
  • overloads
  • partial methods
  • generic type constraints
  • type inference
  • type unification
• Retargeting when a compilation reference that uses wrapped nint is referenced in a compilation that uses .NET 7 (see RetargetingFromNonNumericToNumericIntPtrCorlib and RetargetingFromNumericIntPtrToNonNumericCorlib)
• Built-in operators for nint/nuint apply to IntPtr/UIntPtr (and IntPtr?/UIntPtr?)
  • built-in conversions
  • built-in unary and binary operators
• Constants (in int/uint range) supported for IntPtr/UIntPtr:
  • const allowed
  • constant folding
  • parameter default values
  • switch value
• fix better conversion rules (issue) and update specs accordingly
• Verify with >>> (see UnsignedRightShift)
• Breaking changes:
  • for checked operator+ and operator- with IntPtr/UIntPtr
  • breaking changes documented
• Other scenarios called out by [Proposal]: First class native integer support csharplang#4385

BCL

• Add runtime feature flag (tracked by [API Proposal]: Add RuntimeFeature field to identify support for numeric IntPtr type runtime#67966, done in Expose the RuntimeFeature.NumericIntPtr member for C# 11 runtime#69322)
• Modify System.IntPtr and System.UIntPtr (needs to include >>>) (not applicable)

Productivity

• QuickInfo (validated manually in PR Unify nint and IntPtr #60913, but can't be unittested because IDE tests run on .NET 6 BCL)
• Classification (SemanticClassifierTests.NativeInteger)
• MetadataAsSource (MetadataAsSource.MetadataAsSourceTests+CSharp.TestNativeInteger)
• Prefer keyword over full type name (UseExplicitTypeTests.InNativeIntIntrinsicType, this should follow the preference for built-in types)
• Simplify type name
  • using x = System.IntPtr; should not simplify to nint
• Update simplification defaults to suggest updating IntPtr to nint if that option is set
  • In cref: <see cref="nint"/>, <see cref="System.IntPtr"/> (completion issue, should offer to simplify to nint)
• Simplify nint x = default(nint); to either nint x = default; or nint x = 0;

Relates to test plan for native ints: #38821
Relates to statics in interfaces and numerics design: https://github.com/dotnet/designs/blob/main/accepted/2021/statics-in-interfaces/README.md