hclsyntax: Allow the splat operators to be applied to sets

We automatically convert from set to list in many other situations, so for
consistency we should accept sets here too and just treat them as
unordered sequences.

This closes #30.

hcl/hclsyntax/expression.go

@@ -1170,7 +1170,7 @@ func (e *SplatExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
 	// both to tuples/lists and to other values, and in the latter case
 	// the value will be treated as an implicit single-value list. We'll
 	// deal with that here first.
-	if !(sourceVal.Type().IsTupleType() || sourceVal.Type().IsListType()) {
+	if !(sourceVal.Type().IsTupleType() || sourceVal.Type().IsListType() || sourceVal.Type().IsSetType()) {
 		sourceVal = cty.ListVal([]cty.Value{sourceVal})
 	}
 

hcl/hclsyntax/expression_test.go

@@ -733,6 +733,22 @@ upper(
 			}),
 			0,
 		},
+		{
+			`set.*.name`,
+			&hcl.EvalContext{
+				Variables: map[string]cty.Value{
+					"set": cty.SetVal([]cty.Value{
+						cty.ObjectVal(map[string]cty.Value{
+							"name": cty.StringVal("Steve"),
+						}),
+					}),
+				},
+			},
+			cty.TupleVal([]cty.Value{
+				cty.StringVal("Steve"),
+			}),
+			0,
+		},
 		{
 			`["hello", "goodbye"].*`,
 			nil,