Add comment clarifying why the given test cases are failing

hclsyntax/expression_test.go

@@ -1959,6 +1959,10 @@ func TestExpressionErrorMessages(t *testing.T) {
  "The true and false result expressions must have consistent types. The 'false' value includes object attribute \"b\", which is absent in the 'true' value.",
  },
  {
+ // Failing cases for automatic collection conversions. HCL and cty
+ // will attempt to unify tuples into lists. We have to make sure
+ // the tuple inner types have no common base type, so we mix and
+ // match booleans and numbers and validate the error messages.
  "true ? listOf2Tuple : listOf1Tuple",
  &hcl.EvalContext{
  Variables: map[string]cty.Value{