[analyzer] An explicitly instantiated top level function tearoff is parsed as an instance creation expression

[stereotype441]

With the constructor-tearoffs experiment enabled, the following code is accepted by the CFE but rejected by the analyzer:

typedef X<_> = Class;

class Class {}

int f2<X, Y>([_]) => 0;

extension <T extends Object?> on T {
  T get self => this;
  Object? call<R, S>([_]) => null;
}

main() {
  f2<X, X>.self();
}

(Note: this code is based on https://dart-review.googlesource.com/c/sdk/+/211420)

The error message (at the line f2<X, X>.self();) is The name 'f2' isn't a class. (creation_with_non_type).

It seems like the analyzer is treating f2<X, X>.self() as an instance creation expression (understandable, because it looks like one), but in fact it is an explicitly instantiated tearoff of the top level function f2, followed by an invocation of the extension getter self, followed by an implicit invocation of the extension method call. This should all be allowed provided that the constructor-tearoffs experiment is enabled.

Note: this seems similar to #46721 (which is marked as fixed), but the thing being torn off is a top level function rather than a method.

[eernstg]

Here is another issue which is similar: #46887.

That issue is area-front-end, and has been fixed in ce4e770, but the analyzer (from 7959f03) reports an error in the following program as well:

String f(a, [b]) => "$a, $b";

String a<T1, T2>(int x) {
  return "a<$T1, $T2>($x)";
}

typedef b = int;
typedef c = String;

main() {
  print('The following should be equal:');
  print('  ${a<b, c>}, null');
  print('  ${f(a<b, c>.toString())}');
}

ERROR|COMPILE_TIME_ERROR|CREATION_WITH_NON_TYPE|/usr/local/google/home/eernst/lang/dart/scratch/202108/n029.dart|13|16|1|The name 'a' isn't a class.

Cf. https://github.com/dart-lang/co19/blob/aff1a334d7875e465c2f43e1b38774512fbb4ba4/LanguageFeatures/Constructor-tear-offs/ambiguities_A08_t02.dart, where this construct occurs.