I-2-b: Dart does not throw a compile error if non-function type alias type argument does not conform to the bound.

[iarkh]

Dart SDK version: 2.12.0-33.0.dev (dev) (Wed Nov 11 17:34:11 2020 -0800) on "windows_x64"

Here is a sample code example:

class C<X> {}
typedef A<X extends C<X>> = C<X>;

class C1<X extends C1<X>> {}

void main() {
  A();
  C1();
}

It seems like both A() and C1() calls should throw compile errors because type argument does not conform to the bound in both cases.

However it does not happen for the non-function type alias, A() call just passes.

Sample output is:

$> dart --enable-experiment=nonfunction-type-aliases test.dart
test.dart:8:3: Error: Inferred type argument 'C1<Object?>' doesn't conform to the bound 'C1<X>' of the type variable 'X' on 'C1'.
 - 'C1' is from 'test.dart'.
 - 'Object' is from 'dart:core'.
Try specifying type arguments explicitly so that they conform to the bounds.
  C1();
  ^
test.dart:4:10: Context: This is the type variable whose bound isn't conformed to.
class C1<X extends C1<X>> {}
         ^

[leafpetersen]

@eernstg is this still an issue?

[eernstg]

Adjusting the labels: There is an error in the instance creation C1() because inference yields C1<C1<Object?>>(), and that is not a regular-bounded type. However, inference on A() yields A<C<Object?>>() which means C<C<Object?>>(), and that is not an error (C accepts any type argument whatsoever, so C<Object?> is OK, and so is C<C<Object?>>).

So there is no issue with the front end here.

However, there is a certain amount of confusion in the error messages emitted by the analyzer (931203b):

ERROR|COMPILE_TIME_ERROR|COULD_NOT_INFER|/usr/local/google/home/eernst/lang/dart/scratch/202102/n011.dart|8|3|2|Couldn't infer type parameter 'X'.\n\nTried to infer 'C1<Object?>' for 'X' which doesn't work:\n  Type parameter 'X' declared to extend 'C1<C1<Object?>>'.\n\nConsider passing explicit type argument(s) to the generic.\n\n
ERROR|COMPILE_TIME_ERROR|TYPE_ARGUMENT_NOT_MATCHING_BOUNDS|/usr/local/google/home/eernst/lang/dart/scratch/202102/n011.dart|8|3|2|'C1<Object?>' doesn't extend 'X extends C1<X>'.

Analyzing n011.dart...
  error • Couldn't infer type parameter 'X'.

Tried to infer 'C1<Object?>' for 'X' which doesn't work:
  Type parameter 'X' declared to extend 'C1<C1<Object?>>'.

Consider passing explicit type argument(s) to the generic.

 • n011.dart:8:3 • could_not_infer
  error • 'C1<Object?>' doesn't extend 'X extends C1<X>'. • n011.dart:8:3 • type_argument_not_matching_bounds
2 errors found.

I don't quite understand "'X' declared to extend 'C1<C1<Object?>>'", because X is actually declared to extend C1<X>, and in the current situation we require C1<Object?> <: C1<C1<Object?>>. Similarly, the wording "'C1<Object?>' doesn't extend 'X extends C1'" is confusing.

[eernstg]

@scheglov, can you take the next step?