Get rid of ambiguous OutputType::NONE value

Based on suggestion by Pieter Wuille <pieter.wuille@gmail.com> at
https://github.com/bitcoin/bitcoin/pull/12119#issuecomment-357982763

After #12119, the NONE output type was overloaded to refer to either an output
type that couldn't be parsed, or to an automatic change output mode.  This
change drops the NONE enum and uses a simple bool indicate parse failure, and a
new CHANGE_AUTO enum to refer the change output type.

This change is almost a pure refactoring except it makes RPCs reject empty
string ("") address types instead of treating them like they were unset. This
simplifies the parsing code a little bit and could prevent RPC usage mistakes.
It's noted in the release notes.

1 files changed, 12 insertions, 16 deletions

diff --git a/src/wallet/wallet.cpp b/src/wallet/wallet.cpp
index dbc1760c80..965eb067cc 100644
--- a/src/wallet/wallet.cpp
+++ b/src/wallet/wallet.cpp
@@ -2648,7 +2648,7 @@ bool CWallet::FundTransaction(CMutableTransaction& tx, CAmount& nFeeRet, int& nC
 OutputType CWallet::TransactionChangeType(OutputType change_type, const std::vector<CRecipient>& vecSend)
 {
     // If -changetype is specified, always use that change type.
-    if (change_type != OutputType::NONE) {
+    if (change_type != OutputType::CHANGE_AUTO) {
         return change_type;
     }
 
@@ -4022,16 +4022,12 @@ CWallet* CWallet::CreateWalletFromFile(const std::string& name, const fs::path&
         }
     }
 
-    walletInstance->m_default_address_type = ParseOutputType(gArgs.GetArg("-addresstype", ""), DEFAULT_ADDRESS_TYPE);
-    if (walletInstance->m_default_address_type == OutputType::NONE) {
+    if (!gArgs.GetArg("-addresstype", "").empty() && !ParseOutputType(gArgs.GetArg("-addresstype", ""), walletInstance->m_default_address_type)) {
         InitError(strprintf("Unknown address type '%s'", gArgs.GetArg("-addresstype", "")));
         return nullptr;
     }
 
-    // If changetype is set in config file or parameter, check that it's valid.
-    // Default to OutputType::NONE if not set.
-    walletInstance->m_default_change_type = ParseOutputType(gArgs.GetArg("-changetype", ""), OutputType::NONE);
-    if (walletInstance->m_default_change_type == OutputType::NONE && !gArgs.GetArg("-changetype", "").empty()) {
+    if (!gArgs.GetArg("-changetype", "").empty() && !ParseOutputType(gArgs.GetArg("-changetype", ""), walletInstance->m_default_change_type)) {
         InitError(strprintf("Unknown change type '%s'", gArgs.GetArg("-changetype", "")));
         return nullptr;
     }
@@ -4219,19 +4215,19 @@ static const std::string OUTPUT_TYPE_STRING_LEGACY = "legacy";
 static const std::string OUTPUT_TYPE_STRING_P2SH_SEGWIT = "p2sh-segwit";
 static const std::string OUTPUT_TYPE_STRING_BECH32 = "bech32";
 
-OutputType ParseOutputType(const std::string& type, OutputType default_type)
+bool ParseOutputType(const std::string& type, OutputType& output_type)
 {
-    if (type.empty()) {
-        return default_type;
-    } else if (type == OUTPUT_TYPE_STRING_LEGACY) {
-        return OutputType::LEGACY;
+    if (type == OUTPUT_TYPE_STRING_LEGACY) {
+        output_type = OutputType::LEGACY;
+        return true;
     } else if (type == OUTPUT_TYPE_STRING_P2SH_SEGWIT) {
-        return OutputType::P2SH_SEGWIT;
+        output_type = OutputType::P2SH_SEGWIT;
+        return true;
     } else if (type == OUTPUT_TYPE_STRING_BECH32) {
-        return OutputType::BECH32;
-    } else {
-        return OutputType::NONE;
+        output_type = OutputType::BECH32;
+        return true;
     }
+    return false;
 }
 
 const std::string& FormatOutputType(OutputType type)