Expression trees can't represent some new C# syntax elements

[BillWagner]

See #17988 (comment)

Some of the new language elements aren't translated into expression trees. The null forgiving operator ! is one. The null propagating operator ?. is another. I think pattern matching switch expressions are also affected.

We should update this section to include unsupported language elements, and guidance on general use.

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Document Details

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• ID: 67627978-50fb-0575-fc71-d61e124fd9db
• Version Independent ID: fef5fa5e-1fc3-d1d3-d27d-2084ec41d17f
• Content: Expression Trees
• Content Source: docs/csharp/expression-trees.md
• Product: dotnet-csharp
• Technology: csharp-advanced-concepts
• GitHub Login: @BillWagner
• Microsoft Alias: wiwagn

[AntonLapounov]

Expression trees have many restrictions, see all those Error() calls in https://github.com/dotnet/roslyn/blob/master/src/Compilers/CSharp/Portable/Lowering/DiagnosticsPass_ExpressionTrees.cs.

[Joe4evr]

Related: dotnet/csharplang#158

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