Counterintuitive interaction of null-conditional and null-forgiving operators

[AntonLapounov]

According to the documentation, the null-forgiving operator has no effect at run time. However, it has a surprising side effect of breaking a null_conditional_operations chain at compile time, which may change evaluation of the expression. Consider this example:

int[] a = null;

// s1 is null: ToString() is not called at all
string s1 = a?.First().ToString();

// s2 is the empty string: ToString() is called on default(int?)
string s2 = a?.First()!.ToString();

Adding the ! operator effectively changes parsing of the expression to (a?.First())!.ToString(). Since this behavior is surprising for many developers (e.g., dotnet/docs#17988), I am wondering whether not allowing ! in null_conditional_operations was due to omission rather than intentional design. In the former case we might consider fixing this, in the latter — I would like to learn arguments that led to this decision.

[AntonLapounov]

According to the null-conditional and null-forgiving operator specifications, a?.b! is not even a legal expression, because ! may follow only a primary_expression and not a null_conditional_expression:

null_conditional_expression
    : primary_expression null_conditional_operations
    ;

primary_expression
    : ...
    | null_forgiving_expression
    ;
    
null_forgiving_expression
    : primary_expression '!'
    ;

I guess that could be fixed by removing the break in ParsePostFixExpression:

roslyn/src/Compilers/CSharp/Portable/Parser/LanguageParser.cs

Lines 9540 to 9550 in 45d9c60

	case SyntaxKind.QuestionToken:
	if (CanStartConsequenceExpression(this.PeekToken(1).Kind))
	{
	var qToken = this.EatToken();
	var consequence = ParseConsequenceSyntax();
	expr = _syntaxFactory.ConditionalAccessExpression(expr, qToken, consequence);
	expr = CheckFeatureAvailability(expr, MessageID.IDS_FeatureNullPropagatingOperator);
	break;
	}
	goto default;

[jcouv]

LDM decided to push this into C# 9 and settled on syntax/semantics (9/14/2020).