type mismatch only for JS target

[flyx]

This is an error I get when trying to compile NimYAML to JS. It works with the C backend, which is strange, because it should not be backend-specific. To reproduce:

git clone https://github.com/flyx/NimYAML.git && cd NimYAML
git checkout 2f3a8097387a80c4e9bebbb3f3e21676610e873c
nim js test/tests

Output:

/Users/flyx/Projects/NimYAML/private/lex.nim(1144, 7)
    template/generic instantiation from here
/Users/flyx/Projects/NimYAML/private/lex.nim(1118, 29)
    template/generic instantiation from here
/Users/flyx/Projects/NimYAML/private/lex.nim(480, 31)
    template/generic instantiation from here
/Users/flyx/Projects/NimYAML/private/lex.nim(701, 44)
    template/generic instantiation from here
/Users/flyx/Projects/NimYAML/private/lex.nim(802, 35)
    Error: type mismatch: got (proc (lex: YamlLexer): bool{.gcsafe, locks: 0.})
    but expected 'LexerState = proc (lex: YamlLexer): bool{.closure, gcsafe, locks: 0.}'

This work with nim c test/tests.

[flyx]

This has been a mismatch in raises declarations.