interpreter: fails to parse a valid-looking wat

[nagisa]

(module
  (type (func (param i32) (result i32)))
  (func (type 0) (param i32) (result i32)
    i32.const 2
    i32.const 3
    i32.const 0
    i32.const 1
    local.get 0
    select
    select
  )
)

is a different, but equivalent, way to write https://github.com/WebAssembly/spec/blob/937fc7d63efb9e18f36334a9e761a8a040ac44b7/test/core/select.wast#L47-L49

but it fails to parse with the following error when fed to the reference interpreter

11.3-11.4: syntax error: unexpected token

If we parenthesize the last select, the parse succeeds. This appears to be the only such function in the spec-test suite where such an equivalent rewrite fails to parse in the test suite, as far as I can tell.

[rossberg]

Darn, this is due to the hack necessary to parse the optional result signature. In fact, call_indirect has the same problem, none of the following work:

(func select select)
(func select call_indirect)
(func call_indirect select)
(func call_indirect call_indirect)

It works if you add another instruction at the end.

Unfortunately, the fix is rather non-obvious, it is easy to get shift/reduce conflicts here. Will have a look.

[rossberg]

Please see #1567.