Championed issue: dotnet/csharplang#4394
Spec: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-10.0/extended-property-patterns.md

Initial PR: #52139

Compilers

• Verify a conditional access is not allowed in place of the name
• IOperation and CFG (using { Prop1: { Prop2: pattern } } translation, see ExtendedPropertyPatterns_IOperation_*, ExtendedPropertyPatterns_IOperationOnMissing)
• Semantic Model (GetSymbolInfo, GetTypeInfo, see ExtendedPropertyPatterns_SymbolInfo_*)
• nullable analysis
  • do we learn from member accesses, i.e. not null state for nested members?
  • do we learn from pure null tests? (see OtherComparisonsAsPureNullTests_ExtendedProperties_PureNullTest)
  • nullable value types
• Pattern Explainer (do we give correct examples in a non-exhaustive match, see ExtendedPropertyPatterns_Explainer)
• definite assignment (mark read fields, see ExtendedPropertyPatterns_05)
• Verify all existing scenarios (where the name is optional or disallowed) work as expected with a member access (see ExtendedPropertyPatterns_ExpressionColonIn*)
• inaccessible properties/fields (see ExtendedPropertyPatterns_Inaccessible*)
• expression tree (see ExtendedPropertyPatterns_ExpressionTree)
• Language Version
• SyntaxNormalizer (see TestNormalizeExtendedPropertyPattern)
• Obsolete (see ExtendedPropertyPatterns_Obsolete*)
• Keep rejecting null-suppression and pointer access in patterns (issue Reject null-suppression operator in patterns #53484)
• Count checked fields as used (issue False warning: pattern matching on a field doesn't count as a use #52956)
• evaluation order
• static properties/fields (errors)
• API shape (unshipped, syntax)
• EnC (nothing special)
• exhaustiveness of switch expression
• spec: no formal commitment to only evaluate once

IDE

• Completion for nested members and patterns
  • Test completion with two possible bindings (see CompletionOnListPattern_FirstNested and test above)
• QuickInfo/GoToDef
• FindAllReferences on nested members
  • Add one highlight or rename test (see RenameExtendedProperty)
• formatting
  • Add an indentation test (see ExtendedPropertyPattern_Before and ExtendedPropertyPattern_After)
• TypeInferrer (see TestEnumInPatterns_SwitchStatement_ExtendedPropertyPattern)
• GenerateVariable (see TestPropertyPatternInCasePattern2)
• Add test for { A.B: MissingConstant$$ } (see TestConstantPatternInExtendedPropertyPattern)
• Offer to collapse pattern (is { P1: { P2: ... } } => is { P1.P2: ... }) (issue Offer to refactor into extended property pattern  #55545)