Bug 165477 – Functions with a return type of ExceptionOr<String> cannot "return emptyString();"

• Status: NEW
• Component: Tools / Tests

[Jer Noble]

Attempting to do so generates the following error:

 error: no viable conversion from returned value of type 'const WTF::String' to function return type 'ExceptionOr<WTF::String>'

With the following notes:

ExceptionOr.h:35:37: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'const WTF::String' to 'const WebCore::ExceptionOr<WTF::String> &' for 1st argument
ExceptionOr.h:37:5: note: candidate constructor not viable: no known conversion from 'const WTF::String' to 'WebCore::Exception &&' for 1st argument
ExceptionOr.h:38:5: note: candidate constructor not viable: 1st argument ('const WTF::String') would lose const qualifier
ExceptionOr.h:39:88: note: candidate template ignored: disabled by 'enable_if' [with OtherType = WTF::String]

[Darin Adler]

The workaround is to write this:

    return String { emptyString() };

I’m not sure we need to fix this.

[Darin Adler]

Let me know if the workaround works or not.

I’m sure we can come up with a way to make the syntax more elegant if this comes up a lot.

[Darin Adler]

I think this came up for me one time: in Internals::captionsStyleSheetOverride.

[Jer Noble]

(In reply to comment #2)
> Let me know if the workaround works or not.

It does!

> I’m sure we can come up with a way to make the syntax more elegant if this
> comes up a lot.

I understand the motivation for not allowing "initialization-by-copy-constructor" by default, but I think this pattern may become more common. Enum values in WebIDL are turn into Strings in the bindings, and I've often defined "static const String& enumValue();" methods in tho bound classes to represent each enumerated value. A getter that can either return an enum value or throw an exception will likely hit this quirk.  It may be worth it to make an exception (hah!) for ExceptionOr<String> or ExceptionOr<AtomicString>.