Test plan for Dictionary Expressions

[cston]

Proposal: dictionary-expressions.md
Proposal: collection-expression-arguments.md

Compiler

• semantic model on with and the arguments (check conversions), and to observe the selected method
• arguments:
  • nullability analysis of with
  • ref safety analysis of with
  • __arglist
  • semantic model: GetSymbolInfo()
  • IOperation
• IReadOnlyDictionary implementation IsReadOnly returns true
• public API review
• check ConvertedType on elements, including those with parts-wise conversion

IDE support

• completion of with keyword
• completion of with arguments (SignatureHelp)
  • arguments for collection builder factory method do not include implicit ReadOnlySpan<T> elements
• GoToDefinition on with?
• FAR on constructor (should find collection expressions that use it)
• refactoring to add names to arguments