Parser exception when tearing off a property of a thing via a cascade

[srawlins]

This code throws an exception, with constructor tearoffs enabled:

class A {
  void foo<T>(T a) {}
}

bar(A a) {
  a..foo<int>;
}

I think it is valid, but it should not throw, even if it is invalid. Stack trace when run as an analyzer test case:

'package:_fe_analyzer_shared/src/parser/parser_impl.dart': Failed assertion: line 5163 pos 16: 'optional('(', next)': is not true.                                                            
  #0      _AssertionError._doThrowNew (dart:core-patch/errors_patch.dart:46:39)                                                                                                                 
  #1      _AssertionError._throwNew (dart:core-patch/errors_patch.dart:36:5)                                                                                                                    
  #2      Parser.parseCascadeExpression (package:_fe_analyzer_shared/src/parser/parser_impl.dart:5163:16)                                                                                       
  #3      Parser._parsePrecedenceExpressionLoop (package:_fe_analyzer_shared/src/parser/parser_impl.dart:4881:19)                                                                               
  #4      Parser.parsePrecedenceExpression (package:_fe_analyzer_shared/src/parser/parser_impl.dart:4857:12)                                                                                    
  #5      Parser.parseExpression (package:_fe_analyzer_shared/src/parser/parser_impl.dart:4780:13)                                                                                              
  #6      Parser.parseExpressionStatement (package:_fe_analyzer_shared/src/parser/parser_impl.dart:4745:13)                                                                                     
  #7      Parser.parseExpressionStatementOrDeclarationAfterModifiers (package:_fe_analyzer_shared/src/parser/parser_impl.dart:6635:16)                                                          
  #8      Parser.parseStatementX (package:_fe_analyzer_shared/src/parser/parser_impl.dart:4541:14)                                                                                              
  #9      Parser.parseStatement (package:_fe_analyzer_shared/src/parser/parser_impl.dart:4531:20)                                                                                               
  #10     Parser.parseFunctionBody (package:_fe_analyzer_shared/src/parser/parser_impl.dart:4436:15)                                                                                            
  #11     Parser.parseTopLevelMethod (package:_fe_analyzer_shared/src/parser/parser_impl.dart:2950:13)                                                                                          
  #12     Parser.parseTopLevelMemberImpl (package:_fe_analyzer_shared/src/parser/parser_impl.dart:2729:14)                                                                                      
  #13     Parser.parseTopLevelDeclarationImpl (package:_fe_analyzer_shared/src/parser/parser_impl.dart:502:14)                                                                                  
  #14     Parser.parseUnit (package:_fe_analyzer_shared/src/parser/parser_impl.dart:357:15)                                                                                                     
  #15     Parser.parseCompilationUnit2 (package:analyzer/src/generated/parser.dart:195:32)                                                                                                      
  #16     Parser.parseCompilationUnit (package:analyzer/src/generated/parser.dart:191:12)                                                                                                       
  #17     FileState._parse (package:analyzer/src/dart/analysis/file_state.dart:601:23)                                                                                                          
  #18     FileState.parse (package:analyzer/src/dart/analysis/file_state.dart:404:14)                                                                                                           
  #19     FileState._getUnlinkedBytes (package:analyzer/src/dart/analysis/file_state.dart:546:16)                                                                                               
  #20     FileState.refresh (package:analyzer/src/dart/analysis/file_state.dart:451:17)                                                                                                         
  #21     FileSystemState.getFileForPath (package:analyzer/src/dart/analysis/file_state.dart:857:12)                                                                                            
  #22     AnalysisDriver._computeAnalysisResult (package:analyzer/src/dart/analysis/driver.dart:1323:31)                                                                                        
  #23     AnalysisDriver.performWork (package:analyzer/src/dart/analysis/driver.dart:992:22)                                                                                                    
  #24     AnalysisDriverScheduler._run (package:analyzer/src/dart/analysis/driver.dart:2105:24)                                                                                                 
  <asynchronous suspension>

[srawlins]

cc @stereotype441

[eernstg]

The feature specification for 'constructor-tearoffs' does not cover cascades, but the spec_parser grammar Dart.g was extended with support for this feature in https://dart-review.googlesource.com/c/sdk/+/197161. It makes <typeArguments> a selector, and that enables a..foo<int>.

Of course, it's not that useful because it computes a value in a computation that can't have side-effects, and then the result is discarded. So we'll probably never have it anywhere outside language tests, and it might even be targeted by a lint. ;-)

Cf. dart-lang/language#1746.