[dart2wasm] Getting illegal cast, only when compiled with -O0

[eyebrowsoffire]

This is a minimum reproducible case, distilled from some code in the flutter framework that was causing this problem.

Steps to reproduce:

(1) Compile the following code with dart compile wasm -O0 --extra-compiler-option=--enable-asserts:

void main() {
  final Map<String, Map<String, String>> nestedMap = <String, Map<String, String>>{};
  nestedMap?['hello'] ??= <String, String>{};
  nestedMap?['hello']!['world'] = 'foo';

  print('map: $nestedMap');
}

bool get isDebug => !bool.fromEnvironment('dart.vm.product') && !bool.fromEnvironment('dart.vm.profile');

Map<String, Map<String, String>>? getNestedMap() {
  return isDebug ? <String, Map<String, String>>{} : null;
}

(2) Instantiate and run the output in a browser.

Results:
This crashes, with the following appearing in console:

assert_text.wasm:0xbb2e Uncaught RuntimeError: illegal cast
    at main (assert_text.wasm:0xbb2e)
    at main tear-off trampoline (assert_text.wasm:0xbb87)
    at _invokeMain (assert_text.wasm:0xbd73)
    at invoke (assert_text.mjs:227:28)
    at (index):7:9

Note: This seems to not occur when running at -O1 or higher, so something in the wasm-opt transformations somehow smooths out whatever issue is occurring here.

[osa1]

Slightly smaller repro:

void main() {
  final Map<String, Map<String, String>> nestedMap = <String, Map<String, String>>{};
  nestedMap?['hello'] ??= <String, String>{};
  print('map: $nestedMap');
}

Kernel: (simplified)

static method main() → void {
  final Map<String, Map<String, String>> nestedMap = <String, Map<String, String>>{};

  let final Map<String, Map<String, String>> #t1 = nestedMap in
    #t1 == null
      ?{Map<String, String>?} null
      : let final String #t2 = "hello" in
          #t1.{Map::[]}(#t2){(Object?) → Map<String, String>?} == null
            ?{Map<String, String>} #t1.{Map::[]=}(#t2, <String, String>{}){(String, Map<String, String>) → void}
            : null;

  print("map: ${nestedMap}");
}

When compiling conditional with the Map.[] call we try to cast the return type to ref Object because that's the expectedType argument passed to the compiler (I don't know when compiling which library function), but the else branch returns null.

[johnniwinther]

I think we set the static type of the last conditional expression wrongly. From the language perspective it is correct that the whole expression is known to return Map<String, String> but since we see that the value is not used we use null as the false case of the conditional expression. We should either not optimize for this case (and use the value of nestedMap['hello'] as the result) or update the static type of the ConditionalExpression to be nullable.

@chloestefantsova What do you think is the best approach?

[chloestefantsova]

@chloestefantsova What do you think is the best approach?

Intuitively it feels that adjusting the type of the conditional expression should be an easier route. Let me look into it a bit more close.

[chloestefantsova]

I'm experimenting with a potential solution at https://dart-review.googlesource.com/c/sdk/+/360865.

[akalankavinda]

Hello.
I am simply trying to pass a string to dart function.

@pragma("wasm:export")
void onKeyup(String elemntId) {
print(elemntId);
}

but it fails with the following error on the browser.

can you suggest me a fix for this please ?