/// <summary>
/// Similar to <see cref="ISyntaxFacts.GetStandaloneExpression(SyntaxNode)"/>, this gets the containing
/// expression that is actually a language expression and not just typed as an ExpressionSyntax for convenience.
/// However, this goes beyond that that method in that if this expression is the RHS of a conditional access
/// (i.e. <c>a?.b()</c>) it will also return the root of the conditional access expression tree.
/// <para/> The intuition here is that this will give the topmost expression node that could realistically be
/// replaced with any other expression. For example, with <c>a?.b()</c> technically <c>.b()</c> is an
/// expression. But that cannot be replaced with something like <c>(1 + 1)</c> (as <c>a?.(1 + 1)</c> is not
/// legal). However, in <c>a?.b()</c>, then <c>a</c> itself could be replaced with <c>(1 + 1)?.b()</c> to form
/// a legal expression.
/// </summary>
public static SyntaxNode GetRootStandaloneExpression(this ISyntaxFacts syntaxFacts, SyntaxNode node)
{
// First, make sure we're on a construct the language things is a standalone expression.
var standalone = syntaxFacts.GetStandaloneExpression(node);
// Then, if this is the RHS of a `?`, walk up to the top of that tree to get the final standalone expression.
return(syntaxFacts.GetRootConditionalAccessExpression(standalone) ?? standalone);
}