Merge pull request bitcoin#111 from sipa/201910_whynocmssuccess

Explain why CMS is not turned into SUCCESSx

Author: sipa

bip-tapscript.mediawiki

@@ -67,7 +67,7 @@ Validation of such inputs must be equivalent to performing the following steps i
 ===Script execution===
 
 The execution rules for tapscript are based on those for P2WSH according to BIP141, including the <code>OP_CHECKLOCKTIMEVERIFY</code> and <code>OP_CHECKSEQUENCEVERIFY</code> opcodes defined in BIP65 and BIP112, but with the following modifications:
-* '''Disabled script opcodes''' The following script opcodes are disabled in tapscript: <code>OP_CHECKMULTISIG</code> and <code>OP_CHECKMULTISIGVERIFY</code>. The disabled opcodes behave in the same way as <code>OP_RETURN</code>, by failing and terminating the script immediately when executed, and being ignored when found in unexecuted branch.
+* '''Disabled script opcodes''' The following script opcodes are disabled in tapscript: <code>OP_CHECKMULTISIG</code> and <code>OP_CHECKMULTISIGVERIFY</code><ref>'''Why are <code>OP_CHECKMULTISIG</code> and <code>OP_CHECKMULTISIGVERIFY</code> disabled, and not turned into OP_SUCCESSx?''' This is a precaution to make sure people who accidentally keep using <code>OP_CHECKMULTISIG</code> in Tapscript notice a problem immediately. It also avoids the complication of script disassemblers needing to become context-dependent.</ref>. The disabled opcodes behave in the same way as <code>OP_RETURN</code>, by failing and terminating the script immediately when executed, and being ignored when found in unexecuted branch.
 * '''Consensus-enforced MINIMALIF''' The MINIMALIF rules, which are only a standardness rule in P2WSH, are consensus enforced in tapscript. This means that the input argument to the <code>OP_IF</code> and <code>OP_NOTIF</code> opcodes must be either exactly 0 (the empty vector) or exactly 1 (the one-byte vector with value 1)<ref>'''Why make MINIMALIF consensus?''' This makes it considerably easier to write non-malleable scripts that take branch information from the stack.</ref>.
 * '''OP_SUCCESSx opcodes''' As listed above, some opcodes are renamed to <code>OP_SUCCESSx</code>, and make the script unconditionally valid.
 * '''Signature opcodes'''. The <code>OP_CHECKSIG</code> and <code>OP_CHECKSIGVERIFY</code> are modified to operate on Schnorr public keys and signatures (see bip-schnorr) instead of ECDSA, and a new opcode <code>OP_CHECKSIGADD</code> is added.