`Function` is a built-in identifier, but no error is emitted when it is used to name a type

[eernstg]

Consider the following programs:

class Function {}
class C<Function> {}
void main() {}

typedef Function = int;
typedef F<Function> = int;
void main() {}

extension Function on List {}
extension E<Function> on List<Function> {}
void main() {}

mixin Function {}
mixin M<Function> implements List<Function> {}
void main() {}

The word Function is a built-in identifier, but it is not consistently reported as an error when this word is used as the name of a type (that is, a class, type parameter, type alias, etc.).

No errors or other diagnostics are emitted except for the following:

• The analyzer hints that a class named Function is deprecated.
• The analyzer and the CFE emit an error "'Function' can't be used as an identifier because it's a keyword" on typedef Function = int;.

All declaring occurrences of Function should be marked as a compile-time error.

[jensjoha]

The link tries to point to line 16025 in a file with 14154 lines in it... Could you provide a better link?

[lrhn]

The link is correct, but Github sometimes doesn't show the whole file (it's 21899 lines). Try reloading.
(It's also not a particularly interesting link, it just points to the line saying \alt \FUNCTION{} in the list of built-in identifiers.)

[jensjoha]

Hah, fantastic! Thanks!

[scheglov]

There is class Function in lib/core/function.dart, and another in dart-sdk/lib/_internal/vm/lib/function_patch.dart.
Do we make an exception for SDK?

[lrhn]

As I remember it (big caveat), we did not choose to make Function a built-in identifier. Instead we treated Function followed by ( or < as a context-sensitive keyword instead of an identifier.

One of the reasons was that Function is a type name already, and you need to be able to use it as one, which you can't with built-in identifiers. Did we change the approach to "built-in identifier but can be used as type" instead?

[bwilkerson]

Section 17.37 Identifier Reference, as of May 6, lists Function as a built-in identifier. I don't know whether that's as intended.

@eernstg

[lrhn]

Nope, my bad. Function has apparently been a built-in identifier since Dart 2.0. Don't think I ever remembered that.
Introduced in https://dart-review.googlesource.com/c/sdk/+/41360/9/docs/language/dartLangSpec.tex

In which case the platform "class" declaration for Function must be special cased (just like FutureOr, because it's not really class, or at least it doesn't have to be).

[eernstg]

Yes, Function was made a built-in identifier Feb 19 2018 in cec672c. I don't remember the exact discussions about this change. We already have another built-in identifier which typically denotes a type: dynamic. So in that sense Function is not alone. I don't see a problem in the fact that a system library contains a class Function ... — that declaration is given special treatment by the analyzer/compilers anyway.

We do get an error for a type alias, for the CFE as well as the analyzer:

// Store in `n014.dart`.
typedef Function = int;
void main() {}
/*
> dart n014.dart
n014.dart:1:9: Error: 'Function' can't be used as an identifier because it's a keyword.
Try renaming this to be an identifier that isn't a keyword.
typedef Function = int;
        ^^^^^^^^
> dartanalyzer n014.dart
Analyzing n014.dart...
  error • 'Function' can't be used as an identifier because it's a keyword. • n014.dart:1:9 • expected_identifier_but_got_keyword
1 error found.
*/

We do not get that error for a class or a mixin, but the analyzer reports that it is "deprecated" to use the name Function for a class. In other words, it's not fully implemented today.

So do we wish to make it a regular identifier in the spec, given that the error is not emitted for classes an mixins? I don't think it will create any serious parsing difficulties because Function must be handled when it occurs as a type anyway, but I doubt that it will be helpful in a software engineering sense to have a user-written class or mixin named Function.

[bwilkerson]

I'll just note that there is a CL to start generating diagnostics in analyzer in these cases (https://dart-review.googlesource.com/c/sdk/+/195760).

Does the fact that we use Function for function types, such as int Function(String), impact whether it's a built-in identifier or a normal identifier?

[scheglov]

I started this in https://dart-review.googlesource.com/c/sdk/+/200080, but it seems that there are more changes to CFE than I can do on my own.

[lrhn]

Does the fact that we use Function for function types, such as int Function(String), impact whether it's a built-in identifier or a normal identifier?

No, when followed by < or (, the Function token is effectively a contextual keyword instead of an identifier. It's not resolved against the environment at all.