miniscript: make 'd:' have the 'u' property under Tapscript context

In Tapscript MINIMALIF is a consensus rule, so we can rely on the fact
that the `DUP IF [X] ENDIF` will always put an exact 1 on the stack upon
satisfaction.

Author: darosior

src/script/miniscript.cpp

@@ -33,7 +33,8 @@ Type SanitizeType(Type e) {
     return e;
 }
 
-Type ComputeType(Fragment fragment, Type x, Type y, Type z, const std::vector<Type>& sub_types, uint32_t k, size_t data_size, size_t n_subs, size_t n_keys) {
+Type ComputeType(Fragment fragment, Type x, Type y, Type z, const std::vector<Type>& sub_types, uint32_t k,
+                 size_t data_size, size_t n_subs, size_t n_keys, MiniscriptContext ms_ctx) {
     // Sanity check on data
     if (fragment == Fragment::SHA256 || fragment == Fragment::HASH256) {
         assert(data_size == 32);
@@ -116,7 +117,8 @@ Type ComputeType(Fragment fragment, Type x, Type y, Type z, const std::vector<Ty
             "e"_mst.If(x << "f"_mst) | // e=f_x
             (x & "ghijk"_mst) | // g=g_x, h=h_x, i=i_x, j=j_x, k=k_x
             (x & "ms"_mst) | // m=m_x, s=s_x
-            // NOTE: 'd:' is not 'u' under P2WSH as MINIMALIF is only a policy rule there.
+            // NOTE: 'd:' is 'u' under Tapscript but not P2WSH as MINIMALIF is only a policy rule there.
+            "u"_mst.If(IsTapscript(ms_ctx)) |
             "ndx"_mst; // n, d, x
         case Fragment::WRAP_V: return
             "V"_mst.If(x << "B"_mst) | // V=B_x

src/script/miniscript.h

@@ -249,7 +249,7 @@ constexpr bool IsTapscript(MiniscriptContext ms_ctx)
 namespace internal {
 
 //! Helper function for Node::CalcType.
-Type ComputeType(Fragment fragment, Type x, Type y, Type z, const std::vector<Type>& sub_types, uint32_t k, size_t data_size, size_t n_subs, size_t n_keys);
+Type ComputeType(Fragment fragment, Type x, Type y, Type z, const std::vector<Type>& sub_types, uint32_t k, size_t data_size, size_t n_subs, size_t n_keys, MiniscriptContext ms_ctx);
 
 //! Helper function for Node::CalcScriptLen.
 size_t ComputeScriptLen(Fragment fragment, Type sub0typ, size_t subsize, uint32_t k, size_t n_subs, size_t n_keys);
@@ -576,7 +576,7 @@ struct Node {
         Type y = subs.size() > 1 ? subs[1]->GetType() : ""_mst;
         Type z = subs.size() > 2 ? subs[2]->GetType() : ""_mst;
 
-        return SanitizeType(ComputeType(fragment, x, y, z, sub_types, k, data.size(), subs.size(), keys.size()));
+        return SanitizeType(ComputeType(fragment, x, y, z, sub_types, k, data.size(), subs.size(), keys.size(), m_script_ctx));
     }
 
 public:

src/test/fuzz/miniscript.cpp

@@ -580,7 +580,7 @@ struct SmartInfo
                             if (subs > 0) subt.push_back(x);
                             if (subs > 1) subt.push_back(y);
                             if (subs > 2) subt.push_back(z);
-                            Type res = miniscript::internal::ComputeType(frag, x, y, z, subt, k, data_size, subs, n_keys);
+                            Type res = miniscript::internal::ComputeType(frag, x, y, z, subt, k, data_size, subs, n_keys, MsCtx::P2WSH);
                             // Continue if the result is not a valid node.
                             if ((res << "K"_mst) + (res << "V"_mst) + (res << "B"_mst) + (res << "W"_mst) != 1) continue;