FARKLE1009: Cannot infer types of production builder factory arguments

This warning is emitted when the production builder factory source generator fails to infer the type of some of the arguments to a call to Production.Build.

To understand when this situation can occur, consider the following grammar:

[module: UseEnhancedSyntax]

var number = Terminals.Double("Number");

var function = Nonterminal.Create("Function",
    Production.Build("sqrt").FinishConstant(Math.Sqrt),
    Production.Build("sin").FinishConstant(Math.Sin));

var expression = Nonterminal.Create("Expression",
    Production.Build("nan").FinishConstant(double.NaN),
    Production.Build("infinity").FinishConstant(double.PositiveInfinity),
    Production.Build(function, number).Finish((f, n) => f(n)));

var statement = Nonterminal.Create("Statement",
    // The following line will emit FARKLE1009 as well as other compiler errors.
    Production.Build("print", expression).Finish(x => {
        Console.WriteLine(x);
        return (object?)null;
    }));

Before the production builder factory source generator runs, all calls to Production.Build are bound to a placeholder overload that accepts params object[] and returns an untyped ProductionBuilder. Under this view, the third production of Expression has a syntax error, because Production.Build(function, number) returns ProductionBuilder, which does not have a Finish method that accepts a delegate with two parameters. This makes the compiler unable to infer the type of expression, so when we try to use it in the production of Statement, the source generator becomes unable to determine the type of expression, and does not generate a specific overload. This causes a mysterious syntax error on the Finish call, and we also emit FARKLE1009 to make it clearer what went wrong.

In this case, you can fix the problem by explicitly specifying the type of expression:

[module: UseEnhancedSyntax]

var number = Terminals.Double("Number");

var function = Nonterminal.Create("Function",
    Production.Build("sqrt").FinishConstant(Math.Sqrt),
    Production.Build("sin").FinishConstant(Math.Sin));

IGrammarSymbol<double> expression = Nonterminal.Create("Expression",
    Production.Build("nan").FinishConstant(double.NaN),
    Production.Build("infinity").FinishConstant(double.PositiveInfinity),
    Production.Build(function, number).Finish((f, n) => f(n)));

var statement = Nonterminal.Create("Statement",
    Production.Build("print", expression).Finish(x => {
        Console.WriteLine(x);
        return (object?)null;
    }));

Tip

Temporarily removing the [UseEnhancedSyntax] attribute might help you fix these problems, because it will give you the source generator's view of your code before it runs. In the example above, the use of the expression variable in the production of Statement will raise a FARKLE1006 error with the message Argument 1: cannot convert from '?' to 'string' or 'Farkle.Builder.IGrammarSymbol'.

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FARKLE1009: Cannot infer types of production builder factory arguments

Tip