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NoSuchMethodError.toString should handle _Type.call() case #27651

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Hixie opened this issue Oct 22, 2016 · 4 comments
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NoSuchMethodError.toString should handle _Type.call() case #27651

Hixie opened this issue Oct 22, 2016 · 4 comments
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area-core-library SDK core library issues (core, async, ...); use area-vm or area-web for platform specific libraries. library-core

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@Hixie
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Hixie commented Oct 22, 2016

...or maybe _Type.call() itself should be implemented and throw a NoSuchMethodError with an appropriate message.

This situation occurs when you have code that omits the "new" in a call like this:

  var x = Timer();

Right now it says:

NoSuchMethodError: Class '_Type' has no instance method 'call'.
Receiver: Type: class 'Timer'
Tried calling: call()

This is highly unclear and we've seen experienced Dart engineers in usability studies get mildly confused by it. We should instead say something like:

NoSuchMethodError: Attempted to use type 'Timer' as a function. Since types do not define
a method 'call', this is not possible. Did you intend to call the Timer constructor and
forget the 'new' operator?
Receiver: Type: class 'Timer'
Tried calling: Timer()

See also #27571.
@vsmenon vsmenon added area-core-library SDK core library issues (core, async, ...); use area-vm or area-web for platform specific libraries. library-core labels Oct 24, 2016
@vsmenon
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vsmenon commented Oct 24, 2016

BTW, this should always be a static error (and a static error during AOT compilation, not a runtime one). Are you not seeing a static error?

@lrhn
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lrhn commented Oct 24, 2016 •
edited

The current Dart specification is a little lacking about this particular case.
The syntax C() is an "unqualified invocation". If C is a class name visible in the current scope, then none of the special cases for an unqualified invocation applies (it's not a prefix, nor any kind of function, getter or variable), so it is equivalent to this.C().
Luckily nobody actually implements that (see, e.g.: https://dartpad.dartlang.org/ae8fe2acde8378d00e90dc4c825af7c5).

The implementations are not treating it correctly as a "function expression invocation" either. If they did, it would be rewritten into C.call() which would call a static function named call on C if it's there. That doesn't happen either.

The implementations instead convert it to (C).call() and then fail when the type object doesn't have a call method. There is nothing in the spec documenting the behavior, but it is pretty reasonable, and we should probably adapt the spec to match - or just make it a compile-time error.

That does mean that the spec doesn't make it a compile-time error yet. It should be a warning (and in dartpad, it is: "C is not a function").

@Hixie
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Hixie commented Oct 24, 2016

I totally agree that the analyzer should do a better job of reporting this as well, but this issue is specifically about the exception that is thrown.

@fsc8000
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fsc8000 commented Nov 15, 2016

https://codereview.chromium.org/2507493003/ improves the error message: e.g.

Unhandled exception:
NoSuchMethodError: Attempted to use type 'A' as a function. Since types do not define a method 'call', this is not possible. Did you intend to call the A constructor and forget the 'new' operator?
Receiver: Type: class 'A'
Tried calling: A(1, 2, 4)

@fsc8000 fsc8000 self-assigned this Nov 16, 2016
lrhn added a commit that referenced this issue Nov 17, 2016
@lrhn
Make C(), where C is a class name, a compile-time error.
de8e04d 
Fixers issue #27651
BUG= http://dartbug.com/27651
R=eernst@google.com, floitsch@google.com, fschneider@google.com

Review URL: https://codereview.chromium.org/2444843002 .
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