Passing an exception into ccall as string sends empty string

[SoylentGraham]

Version of emscripten/emsdk: 3.1.20-git (from homebrew)

This is user-error, but feels like it should have been caught higher up (maybe in the string/stack alloc?), and should be a simple UX fix

I passed an exception into ccall;

	const ReturnType = null;
	const ArgTypes = ['number','string'];
	const ArgValues = [Handle,Error];
	try
	{
		Module.ccall('OnStreamingDownloadFinished',ReturnType,ArgTypes,ArgValues);
	}
	catch(e)
	{...}

Didn't twig it wasn't a string for a while as chrome's debugger shows it as a string, the type convertor doesn't error, (and debugger even makes it look like a string), and it manages to allocate & copy a string (of length 1) because
var len = (str.length << 2) + 1; -->
var len = (undefined << 2) + 1; == 1

Feels to me like either
a) convertor should throw if object has no length property (or just isn't a string!) thereby forcing users to pass strings if they said they would
b) force the input value to be a string str = ${str}` (so type takes precedent)

(Screenshot to highlight UX)

[sbc100]

Certainly seems like we could use an assert about the type of str there. If you have time perhaps you would like to open PR to add that?

[SoylentGraham]

I haven't tried building the compiler before; Is this the code I need to change? (I assume this isn't auto-generated code?)

emscripten/src/library_ccall.js

Line 26 in 5667b03

$ccall: function(ident, returnType, argTypes, args, opts) {

[sbc100]

Yes, that is the where the ccall code lives.

[SoylentGraham]

Also, is a) your preferred resolution? (stop users making errors, rather than possibly invoking unexpected string conversions - which I probably better for unit tests too)

[sbc100]

I think an assert is the right solution here.