Illegal cast in switch statement for dynamic values on WASM

[thomasostfeld]

A switch statement on a dynamic value tries to cast the value to a type, when all (non-default) cases are matching that type.

Reproducable Code:

void main() {
  dynamic number = 42;
  switch (number) {
    case 'a':
      print('a');
    case 'b':
      print('b');
    default:
      print('default');
  }
}

This code works on web with JS, but fails when running with --wasm.

Expected behaviour:

prints out default.

Actual behaviour:

Uncaught RuntimeError: illegal cast

When adding a case of any other type than String it works.

I thought this would be fixed by #56321, but this is still happening.

dart --version output:

Dart SDK version: 3.6.0 (stable) (Thu Dec 5 07:46:24 2024 -0800) on "macos_arm64"

[lrhn]

For refutable patterns, the static type of the matched value shouldn't matter, so something is being cast unnecessarily here. (And for these patterns, the type shouldn't matter at all, they're equivalent to 'a' == $v which works for any type of object.

The only case where the static matched value type being dynamic should mater for a refutable pattern is when you use the relational operators, like case == expr or case < expr, where the operator invocation is a dynamic invocation and its result is subject to implicit downcast to bool (which doesn't really matter for ==).
And bound variables like var x could get the type dynamic, but that doesn't matter inside the pattern.

[biggs0125]

Dart2wasm tries to follow the spec for dynamic expression switches and it uses the appropriate == based on the types of the case expressions. However, in order to dispatch to the == checks it first tries to cast to the relevant types. This breaks when the type of the switch expression doesn't match the types of the case expressions.

We should be adding a type guard before checking against the case expressions to ensure the type of the expression even matches. If it doesn't then we can run the default case or skip all the cases.