convert: Type information lost when converting unknown value to DynamicPseudoType

[apparentlymart]

The intended meaning of a conversion to DynamicPseudoType is to take no conversion action whatsoever and just return the given value verbatim.

This works for known values, but due to a special case for unknown values does not behave as expected for those:

go-cty/cty/convert/conversion.go

Lines 20 to 22 in 88fbe72

	if !in.IsKnown() {
	return cty.UnknownVal(out), nil
	}

This is correct behavior when the output type is known, since the conversion type selection logic already proved that a conversion between the in and out types is valid, but if the output type is DynamicPseudoType this causes the result to be DynamicVal, losing the original type information in the unknown value.

Instead, we should treat DynamicPseudoType as a further special case and just return the given unknown value verbatim if that is selected as the output type.

(Converting to dynamic is a strange thing to do specifically, but this behavior is intended as a convenience for callers that are doing a conversion on behalf of a transitive caller that is specifying a target type, so that such intermediate callers don't need to themselves handle this as a special case.)

[apparentlymart]

This was fixed by 3a34021.