formatlist: a DynamicVal arg may be a list

[jbardin]

When formatlist encounters a DynamicVal argument, it must return an
unknown result, as it cannot determine if that will be a single value
argument resulting in an unknown string, or a list to be formatted into
an unknown number of strings.

[apparentlymart]

I'm kinda surprised to see this work at all because (as I think your comment here is noting) Function.Call is supposed to guarantee to return a value that conforms to whatever Function.ReturnTypeForValues would've returned given the same arguments, and cty.DynamicVal doesn't conform to cty.List(cty.String). 🤔

go-cty/cty/function/function.go

Lines 304 to 311 in aba65fb

	// Returned value must conform to what the Type function expected, to
	// protect callers from having to deal with inconsistencies.
	if errs := retVal.Type().TestConformance(expectedType); errs != nil {
	panic(fmt.Errorf(
	"returned value %#v does not conform to expected return type %#v: %s",
	retVal, expectedType, errs[0],
	))
	}

I'm fine with leaving that bug to be solved in a separate PR, but was wondering if you did any investigation here that revealed why exactly this case returns the wrong type. If you don't know then I'm happy to go digging for that myself, but asking just in case you already did the research here. 😀

[jbardin]

Nope, I didn't dig into the Function.Call implementation yet. If I recall when I read through it for this bug, it looked like returning cty.DynamicVal was simply the easy path, but there was room for more accurate type checking which we could evaluate in a separate PR.