special case unifying tuples as lists

List data is often contained within tuple types, either do to language
constructs (hcl tuple literals) or function return types. When presented
with only lists and tuples for unification, first check if the tuples
themselves unify as lists. If they do, run the unification again with
only the list types, and wrap the conversions as necessary.

cty/convert/unify.go

@@ -57,6 +57,15 @@ func unify(types []cty.Type, unsafe bool) (cty.Type, []Conversion) {
 			return unifyCollectionTypes(cty.Map, types, unsafe, dynamicCt > 0)
 		case listCt > 0 && (listCt+dynamicCt) == len(types):
 			return unifyCollectionTypes(cty.List, types, unsafe, dynamicCt > 0)
+		case listCt > 0 && (listCt+tupleCt+dynamicCt) == len(types):
+			// Tuples are often lists in disguise, and we may be able to
+			// unify them as such.
+			ty, convs := unifyTuplesAsList(types, unsafe)
+			// if we got a list back, we know the unification was successful.
+			// Otherwise we will fall back to the heterogeneous type codepath.
+			if ty.IsListType() {
+				return ty, convs
+			}
 		case setCt > 0 && (setCt+dynamicCt) == len(types):
 			return unifyCollectionTypes(cty.Set, types, unsafe, dynamicCt > 0)
 		case objectCt > 0 && (objectCt+dynamicCt) == len(types):
@@ -110,6 +119,63 @@ Preferences:
 	return cty.NilType, nil
 }
 
+// unifyTuplesAsList attempts to first see if the tuples unify as lists, then
+// re-unifies the given types with the list in place of the tuples.
+func unifyTuplesAsList(types []cty.Type, unsafe bool) (cty.Type, []Conversion) {
+	var tuples []cty.Type
+	var tupleIdxs []int
+	for i, t := range types {
+		if t.IsTupleType() {
+			tuples = append(tuples, t)
+			tupleIdxs = append(tupleIdxs, i)
+		}
+	}
+
+	ty, tupleConvs := unifyTupleTypesToList(tuples, unsafe)
+	if !ty.IsListType() {
+		return cty.NilType, nil
+	}
+
+	// the tuples themselves unified as a list, get the overall
+	// unification with this list type instead of the tuple.
+	// make a copy of the types, so we can fallback to the standard
+	// codepath if something went wrong
+	listed := make([]cty.Type, len(types))
+	copy(listed, types)
+	for _, idx := range tupleIdxs {
+		listed[idx] = ty
+	}
+
+	newTy, convs := unify(listed, unsafe)
+	if !newTy.IsListType() {
+		return cty.NilType, nil
+	}
+
+	// we have a good conversion, wrap the nested tuple conversions.
+	// We know the tuple conversion is not nil, because we went from tuple to
+	// list
+	for i, idx := range tupleIdxs {
+		listConv := convs[idx]
+		tupleConv := tupleConvs[i]
+
+		if listConv == nil {
+			convs[idx] = tupleConv
+			continue
+		}
+
+		convs[idx] = func(in cty.Value) (out cty.Value, err error) {
+			out, err = tupleConv(in)
+			if err != nil {
+				return out, err
+			}
+
+			return listConv(in)
+		}
+	}
+
+	return newTy, convs
+}
+
 func unifyCollectionTypes(collectionType func(cty.Type) cty.Type, types []cty.Type, unsafe bool, hasDynamic bool) (cty.Type, []Conversion) {
 	// If we had any dynamic types in the input here then we can't predict
 	// what path we'll take through here once these become known types, so