Incorrect type warning on "symbol in object" expression

[rconnamacher]

Closure Compiler's type warnings show an incorrect type error when using a symbol on the left side of an in expression:

src/js/SourceFile:100: WARNING - left side of 'in'
found   : symbol
required: string
        if ( SYMBOL in record ) {
              ^^^^^^^^^

This should be updated to allow either a string or symbol on the left side of in.

(Using --compilation_level=ADVANCED_OPTIMIZATIONS, --language_out=ECMASCRIPT5_STRICT, and Closure Compiler version 20180805

[blickly]

This is easy to repro:
https://closure-compiler-debugger.appspot.com/#input0%3D%252F**%2520%2540type%2520%257Bsymbol%257D%2520*%252F%250Avar%2520sym%253B%250A%250Aif%2520(sym%2520in%2520%257B%257D)%2520%257B%257D%26input1%26conformanceConfig%26externs%26refasterjs-template%26CHECK_TYPES%3Dtrue%26TRANSPILE%3Dtrue%26CLOSURE_PASS%3Dtrue%26PRESERVE_TYPE_ANNOTATIONS%3Dtrue%26PRETTY_PRINT%3Dtrue

@brad4d, who's knows about a bunch of the work on supporting ES6+ features in the typechecker, do you know what the status is of typechecking symbol in this way?

[concavelenz]

I added support for symbol as a primitive type to the type system and type checker, but I clearly missed "in". It should be a trivial fix.

[concavelenz]

Thanks for the report. I have a fix out for review.