Previously, the front end recovered from an invalid assignment by
producing an "as" expression, e.g.:
int x;
String y;
x = y;
Would produce an "invalid assignment" error, as well as this kernel
code:
int x;
String y;
x = y as{TypeError} int;
The rationale was that the "as" check was guaranteed to fail, so this
code would produce a runtime error at the correct location. However,
there were two problems:
1. (Minor problem) the "as" doesn't actually fail if the RHS is null.
2. (Major problem) for type inference, the type of an assignment
expression is defined to be the type of the RHS. This means that if
the invalid assignment gets used for type inference, we generate
mal-typed kernel code, e.g.:
int x;
String y;
var z = (x = y);
Gets compiled to:
int x;
String y;
String z = (x = y as{TypeError} int);
This CL addresses both problems by changing the kernel representation
so that it evaluates the RHS and then throws an exception. Since a
"throw" expression has type Bottom, the kernel representation is
guaranteed to be properly typed. This also ensures that an exception
gets thrown if the RHS is null. Finally, as a side bonus, it makes
the error-recovered kernel code more similar to the kernel code we
generate in other error recovery scenarios.
Change-Id: Iac74e0c726ce029ac0560d271413e85c15bfec5c
Reviewed-on: https://dart-review.googlesource.com/24140
Reviewed-by: Kevin Millikin <kmillikin@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>
Paul Berry
c800a1cc29