Championed proposal: dotnet/csharplang#6420
Speclet: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-13.0/partial-properties.md
Feature branch: https://github.com/dotnet/roslyn/tree/features/partial-properties

Compiler

• LangVersion (LangVersion_01)
• A missing or extra declaration or implementation (MissingDeclaration_*, DuplicateDeclaration_*)
• Accessor mismatch (get/set missing in either declaration or implementation) (MissingAccessor_*, UnexpectedAccessor_*)
• set vs. init (AccessorKind_*)
• readonly vs. not (ModifierDifference_Readonly_Accessors)
• Accessors split across two declarations or implementations (DuplicateDeclaration_08, split implementation but not split definition)
• extern part is considered implementation
• Differences in return or parameter types (TypeDifference_*, NullableDifference_*, Indexers_*)
  • tuple names
  • nullability (warning regardless of nullability context and warning wave)
  • ref kind (RefKindDifference_*)
  • alias usage (AliasDifference)
  • params (ParamsDifference_*)
  • optional parameters (only definition counts, implementation warns, OptionalParameters_*)
  • parameter names (warning, IndexerParameterNameDifference)
• static vs. non-static (StaticDifference)
• virtual/override/sealed differences (ExtendedDifference_01)
• abstract is disallowed (Abstract)
• unsafe (UnsafeDifference_*)
• required (Required_*)
• Different accessibilities, explicit vs. default accessibility (ModifierDifference_Accessibility_*)
• scoped (ScopedDifference_*, Semantics_Scoped)
• explicit interface implementation is disallowed (ExplicitImplementation)
• only in partial type (NotInPartialType)
• Attributes (Attributes_*)
  • Attributes on both parts
  • field-targeted attribute on declaration
  • nullability attribute differences
  • caller attributes
  • obsolete (Obsolete_*)
  • UnmanagedCallersOnly
• public API (PartialDefinitionPart, PartialImplementationPart and IsPartialDefinition on property symbol, AssociatedSymbol on accessors, GetDeclaredSymbol) (GetDeclaredSymbol_*)
• nullability analysis of accessor bodies against correct signature (AllowNull_*, NullableDifference_Analysis)
• xml docs (PartialProperties_*)
• Overrides (Semantics_ExtendedModifier)
• Override a property where the base has a modopt Partial properties: merging declarations and emit of simple cases #72999 (comment)
• EnC (EnC: Implement support for partial properties #74494)

Productivity

• IDE minbar (completion, QuickInfo, formatting, colorization, no crashes in manual test pass)
• GoToDefinition/GoToImplementation