Front end: Crash during inference.

[eernstg]

Consider the following program:

class A<X> {
  R f<R>(R Function<X>(A<X>) f) => f<X>(this);
}

main() {
  A<num> a = A<int>();
  a.f(<X>(_) => 42);
}

Compilation of this program crashes, and the crash depends on R (change the signature of f to f(Function<X>(A<X>) f) and there is no crash).

With the dart from c4af35c, the response starts as follows:

Crash when compiling file:///usr/local/google/home/eernst/lang/dart/scratch/201906/n007.dart,
at character offset 68:
NoSuchMethodError: The method 'accept' was called on null.
Receiver: null
Tried calling: accept(Instance of '_TypeSchemaEliminationVisitor')
#0      Object.noSuchMethod (dart:core-patch/object_patch.dart:50:5)
#1      _TypeSchemaEliminationVisitor.run (package:front_end/src/fasta/type_inference/type_schema_elimination.dart:126:25)
#2      greatestClosure (package:front_end/src/fasta/type_inference/type_schema_elimination.dart:26:35)
#3      new ClosureContext (package:front_end/src/fasta/type_inference/type_inferrer.dart:215:9)
#4      TypeInferrerImpl.inferLocalFunction (package:front_end/src/fasta/type_inference/type_inferrer.dart:1455:41)
#5      InferenceVisitor.visitFunctionNodeJudgment (package:front_end/src/fasta/kernel/inference_visitor.dart:449:21)
#6      InferenceVisitor.visitFunctionExpression (package:front_end/src/fasta/kernel/inference_visitor.dart:464:27)
...

Similarly, dart2js from c4af35c responds as follows:

n007.dart:
Internal Error: The compiler crashed when compiling this element.

The compiler is broken.

When compiling the above element, the compiler crashed. It is not
possible to tell if this is caused by a problem in your program or
not. Regardless, the compiler should not crash.

The Dart team would greatly appreciate if you would take a moment to
report this problem at http://dartbug.com/new.

Please include the following information:

* the name and version of your operating system,

* the Dart SDK build number (build number could not be determined), and

* the entire message you see here (including the full stack trace
  below as well as the source location above).

The compiler crashed: Crash when compiling file:///usr/local/google/home/eernst/lang/dart/scratch/201906/n007.dart,
at character offset 68:
'package:front_end/src/fasta/type_inference/type_inferrer.dart': Failed assertion: line 213 pos 12: 'returnContext != null': is not true.
#0      _AssertionError._doThrowNew (dart:core-patch/errors_patch.dart:40:39)
#1      _AssertionError._throwNew (dart:core-patch/errors_patch.dart:36:5)
#2      new ClosureContext (package:front_end/src/fasta/type_inference/type_inferrer.dart:213:12)
#3      TypeInferrerImpl.inferLocalFunction (package:front_end/src/fasta/type_inference/type_inferrer.dart:1455:41)
#4      InferenceVisitor.visitFunctionNodeJudgment (package:front_end/src/fasta/kernel/inference_visitor.dart:449:21)
#5      InferenceVisitor.visitFunctionExpression (package:front_end/src/fasta/kernel/inference_visitor.dart:464:27)

I've labeled this crash as a front end issue because of the call sites, and because the problem seems to be shared by several tools (even though the error reports are not identical, they are quite similar).

Note the related issue #37382.

[mraleph]

Dima, could you check this one out?

[chloestefantsova]

It's a nice somewhat fractal test case!

Proposed fix is here: https://dart-review.googlesource.com/c/sdk/+/107505.