Can you supply an example of when this message is shown.

I've tried:

void main () {
    test();
}

Test test () {
    var a = new Mismatch();
    return a;
}

class Test {
}

class Mismatch {
}

but this produces "Return: Cannot convert from `Mismatch' to `Test'"

Try implementing an interface.

Created attachment 335309 [details]
example of the error message from a class implementing an interface

Error message is:

class_with_interface.vala:7.2-7.21: error: overriding method `Example.test' is incompatible with base method `TestOfReturnType.test': incompatible return type. Expected Test but got Mismatch.

Created attachment 335310 [details]
example of the error message from a class as a subtype, with the class's supertype also being a subtype

Error message is:

class_subtype.vala:7.2-7.30: error: overriding method `Example.test' is incompatible with base method `TestParent.test': incompatible return type. Expected Test but got Mismatch.

The base method shows the correct base type.

Review of attachment 332697 [details] [review]:

Patch appears to work in all the right places.

All I would suggest is making the message one sentence instead of two. So "incompatible return type. Expected Test but got Mismatch." would be something like "expected return type of `Test' but got `Mismatch'" This makes the full message more readable: "error: overriding method `Example.test' is incompatible with base method `TestOfReturnType.test': expected return type of `Test' but got `Mismatch'"

Also note it appears to be the convention that identifiers are in `' quotes. Although personally I prefer '' quotes.

Created attachment 339171 [details] [review]
method: Print encountered type if return types mismatch

Review of attachment 339171 [details] [review]:

Patch applies and vala compiles fine.

Messages produces are good. From the examples above the new messages are:

class_with_interface.vala:7.2-7.21: error: overriding method `Example.test' is incompatible with base method `TestOfReturnType.test': Base method expected return type `Test', but `Mismatch' was provided.

class_subtype.vala:7.2-7.30: error: overriding method `Example.test' is incompatible with base method `TestParent.test': Base method expected return type `Test', but `Mismatch' was provided.

I'm not sure about the full stop at the end because this doesn't appear in other error messages, but that's a trivial point.

Thanks for the patch

Attachment 339171 [details] pushed as faa7dc1 - method: Print encountered type if return types mismatch