[NNBD] Analyzer and CFE behave differently for implicit return

[sgrekhov]

The following code produces an error in VM and no issues in analyzer (both with --enable-experiment=non-nullable)

main() {
  (x) {
    if (x)
      return 1;
  } (true);
}

VM output is

D:\co19\Language\Statements\Return>dart --enable-experiment=non-nullable,nonfunction-type-aliases many_return_statements_t04.dart
many_return_statements_t04.dart:21:3: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
(x) {                                                                                                                                              
^

Dart VM version: 2.9.0-20.0.dev (dev) (Wed Jul 1 11:29:16 2020 +0200) on "windows_x64"

@eernstg please clarify what is an expected behavior here

[sgrekhov]

See also #39476

[eernstg]

Blocked on dart-lang/language#1063: the current specification makes the function literal an error, and I'm proposing that we keep it like that even though it means that the analyzer will reject more programs.

[mit-mit]

@eernstg did we determine what the right behavoir is?

[eernstg]

Yes, when the end of a function with inferred return type can be reached (and null is implicitly returned), the inferred return type is made nullable and there is no error.

Implementation issue for CFE, #42743, is closed. The analyzer already did this.