Championed issue: dotnet/csharplang#125
PR: #44476

Compiler:

• LangVer
• A data flow test would be good (a test in the vein of RegionAnalysisTests.AnalysisOfExpressionInTupleEquality).
• Let's check the semantic model in nested scenario (see VerifyModelForDeconstructionLocal which covers declaration, and add checks for GetTypeInfo and GetSymbolInfo on variables used in mixed declarations).
• Add an IOperation test

IDE:

• EnC: EnC: validate mixed declarations and variables in deconstruction #50901