diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index 137fe7198..a185d5785 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -2,6 +2,7 @@
 
 #include "value.hh"
 #include "symbol-table.hh"
+#include "error.hh"
 
 #include <map>
 
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 10e8c6b42..657ce3001 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -1124,7 +1124,7 @@ static void prim_toFile(EvalState & state, const Pos & pos, Value * * args, Valu
                 ErrorInfo { 
                     .hint = hintfmt(
                         "in 'toFile': the file named '%1%' must not contain a reference "
-                        "to a derivation but contains (%2%)"
+                        "to a derivation but contains (%2%)",
                         name,
                         path),
                     .nixCode = NixCode { .errPos = pos }
diff --git a/src/libmain/stack.cc b/src/libmain/stack.cc
index e6224de7d..b0a4a4c5d 100644
--- a/src/libmain/stack.cc
+++ b/src/libmain/stack.cc
@@ -1,4 +1,4 @@
-#include "types.hh"
+#include "error.hh"
 
 #include <cstring>
 #include <cstddef>
diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index d6f3553d9..0359b10ac 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -547,7 +547,7 @@ UserLock::UserLock()
     /* Copy the result of getgrnam. */
     Strings users;
     for (char * * p = gr->gr_mem; *p; ++p) {
-        debug(format("found build user '%1%'") % *p);
+        debug("found build user '%1%'", *p);
         users.push_back(*p);
     }
 
@@ -558,7 +558,7 @@ UserLock::UserLock()
     /* Find a user account that isn't currently in use for another
        build. */
     for (auto & i : users) {
-        debug(format("trying user '%1%'") % i);
+        debug("trying user '%1%'", i);
 
         struct passwd * pw = getpwnam(i.c_str());
         if (!pw)
@@ -1794,7 +1794,7 @@ HookReply DerivationGoal::tryBuildHook()
             }
         }
 
-        debug(format("hook reply is '%1%'") % reply);
+        debug("hook reply is '%1%'", reply);
 
         if (reply == "decline")
             return rpDecline;
@@ -2255,7 +2255,7 @@ void DerivationGoal::startBuilder()
         startDaemon();
 
     /* Run the builder. */
-    printMsg(lvlChatty, format("executing builder '%1%'") % drv->builder);
+    printMsg(lvlChatty, "executing builder '%1%'", drv->builder);
 
     /* Create the log file. */
     Path logFile = openLogFile();
@@ -3195,7 +3195,7 @@ void DerivationGoal::runChild()
                filesystem that we want in the chroot
                environment. */
             auto doBind = [&](const Path & source, const Path & target, bool optional = false) {
-                debug(format("bind mounting '%1%' to '%2%'") % source % target);
+                debug("bind mounting '%1%' to '%2%'", source, target);
                 struct stat st;
                 if (stat(source.c_str(), &st) == -1) {
                     if (optional && errno == ENOENT)
@@ -3572,7 +3572,7 @@ static void moveCheckToStore(const Path & src, const Path & dst)
        directory's parent link ".."). */
     struct stat st;
     if (lstat(src.c_str(), &st) == -1) {
-        throw SysError(format("getting attributes of path '%1%'") % src);
+        throw SysError("getting attributes of path '%1%'", src);
     }
 
     bool changePerm = (geteuid() && S_ISDIR(st.st_mode) && !(st.st_mode & S_IWUSR));
@@ -3581,7 +3581,7 @@ static void moveCheckToStore(const Path & src, const Path & dst)
         chmod_(src, st.st_mode | S_IWUSR);
 
     if (rename(src.c_str(), dst.c_str()))
-        throw SysError(format("renaming '%1%' to '%2%'") % src % dst);
+        throw SysError("renaming '%1%' to '%2%'", src, dst);
 
     if (changePerm)
         chmod_(dst, st.st_mode);
@@ -4911,15 +4911,15 @@ void Worker::waitForInput()
                 // FIXME: is there a cleaner way to handle pt close
                 // than EIO? Is this even standard?
                 if (rd == 0 || (rd == -1 && errno == EIO)) {
-                    debug(format("%1%: got EOF") % goal->getName());
+                    debug("%1%: got EOF", goal->getName());
                     goal->handleEOF(k);
                     j->fds.erase(k);
                 } else if (rd == -1) {
                     if (errno != EINTR)
                         throw SysError("%s: read failed", goal->getName());
                 } else {
-                    printMsg(lvlVomit, format("%1%: read %2% bytes")
-                        % goal->getName() % rd);
+                    printMsg(lvlVomit, "%1%: read %2% bytes",
+                        goal->getName(), rd);
                     string data((char *) buffer.data(), rd);
                     j->lastOutput = after;
                     goal->handleChildOutput(k, data);
diff --git a/src/libstore/filetransfer.hh b/src/libstore/filetransfer.hh
index 517b1a7d3..11dca2fe0 100644
--- a/src/libstore/filetransfer.hh
+++ b/src/libstore/filetransfer.hh
@@ -103,8 +103,9 @@ class FileTransferError : public Error
 {
 public:
     FileTransfer::Error error;
+    template<typename... Args>
     FileTransferError(FileTransfer::Error error, const Args & ... args)
-        : Error(fs), error(error)
+        : Error(args...), error(error)
     { }
 };
 
diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index 1f16a22b1..eb7548543 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -6,6 +6,7 @@
 #include "derivations.hh"
 #include "nar-info.hh"
 #include "references.hh"
+#include "error.hh"
 
 #include <iostream>
 #include <algorithm>
diff --git a/src/libstore/sqlite.hh b/src/libstore/sqlite.hh
index ce27033c9..ccfac48d8 100644
--- a/src/libstore/sqlite.hh
+++ b/src/libstore/sqlite.hh
@@ -3,7 +3,7 @@
 #include <functional>
 #include <string>
 
-#include "types.hh"
+#include "error.hh"
 
 struct sqlite3;
 struct sqlite3_stmt;
diff --git a/src/libutil/args.cc b/src/libutil/args.cc
index 423b99c96..10d6e89bb 100644
--- a/src/libutil/args.cc
+++ b/src/libutil/args.cc
@@ -101,15 +101,8 @@ bool Args::processFlag(Strings::iterator & pos, Strings::iterator end)
         std::vector<std::string> args;
         for (size_t n = 0 ; n < flag.handler.arity; ++n) {
             if (pos == end) {
-<<<<<<< HEAD
-                if (flag.arity == ArityAny) break;
-                throw UsageError("flag '%1%' requires %2% argument(s)",
-                    name,
-                    flag.arity);
-=======
                 if (flag.handler.arity == ArityAny) break;
                 throw UsageError("flag '%s' requires %d argument(s)", name, flag.handler.arity);
->>>>>>> master
             }
             args.push_back(*pos++);
         }
diff --git a/src/libutil/error.hh b/src/libutil/error.hh
index 7e2cca2f9..19c6e35a1 100644
--- a/src/libutil/error.hh
+++ b/src/libutil/error.hh
@@ -2,6 +2,7 @@
 
 
 #include "ref.hh"
+#include "types.hh"
 
 #include <list>
 #include <memory>
@@ -21,9 +22,6 @@
 
 namespace nix {
 
-using std::list;
-using std::vector;
-
 typedef enum {
     lvlError = 0,
     lvlWarn,
diff --git a/src/libutil/logging.hh b/src/libutil/logging.hh
index bb7d356c8..39692a291 100644
--- a/src/libutil/logging.hh
+++ b/src/libutil/logging.hh
@@ -1,6 +1,7 @@
 #pragma once
 
 #include "types.hh"
+#include "error.hh"
 
 namespace nix {
 
diff --git a/src/libutil/types.hh b/src/libutil/types.hh
index a831b9924..4d783b564 100644
--- a/src/libutil/types.hh
+++ b/src/libutil/types.hh
@@ -13,6 +13,7 @@ namespace nix {
 using std::list;
 using std::set;
 using std::vector;
+using std::string;
 
 typedef list<std::string> Strings;
 typedef set<std::string> StringSet;
diff --git a/src/libutil/url.hh b/src/libutil/url.hh
index 1503023a2..a184eadce 100644
--- a/src/libutil/url.hh
+++ b/src/libutil/url.hh
@@ -1,6 +1,6 @@
 #pragma once
 
-#include "types.hh"
+#include "error.hh"
 
 #include <regex>
 
diff --git a/src/libutil/util.cc b/src/libutil/util.cc
index 8207ff701..ac7c2967b 100644
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@@ -276,7 +276,7 @@ DirEntries readDirectory(DIR *dir, const Path & path)
 DirEntries readDirectory(const Path & path)
 {
     AutoCloseDir dir(opendir(path.c_str()));
-    if (!dir) throw SysError(format("opening directory '%1%'") % path);
+    if (!dir) throw SysError("opening directory '%1%'", path);
 
     return readDirectory(dir.get(), path);
 }
@@ -306,7 +306,7 @@ string readFile(const Path & path)
 {
     AutoCloseFD fd = open(path.c_str(), O_RDONLY | O_CLOEXEC);
     if (!fd)
-        throw SysError(format("opening file '%1%'") % path);
+        throw SysError("opening file '%1%'", path);
     return readFile(fd.get());
 }
 
@@ -394,15 +394,15 @@ static void _deletePath(int parentfd, const Path & path, unsigned long long & by
         const auto PERM_MASK = S_IRUSR | S_IWUSR | S_IXUSR;
         if ((st.st_mode & PERM_MASK) != PERM_MASK) {
             if (fchmodat(parentfd, name.c_str(), st.st_mode | PERM_MASK, 0) == -1)
-                throw SysError(format("chmod '%1%'") % path);
+                throw SysError("chmod '%1%'", path);
         }
 
         int fd = openat(parentfd, path.c_str(), O_RDONLY);
         if (!fd)
-            throw SysError(format("opening directory '%1%'") % path);
+            throw SysError("opening directory '%1%'", path);
         AutoCloseDir dir(fdopendir(fd));
         if (!dir)
-            throw SysError(format("opening directory '%1%'") % path);
+            throw SysError("opening directory '%1%'", path);
         for (auto & i : readDirectory(dir.get(), path))
             _deletePath(dirfd(dir.get()), path + "/" + i.name, bytesFreed);
     }
@@ -426,7 +426,7 @@ static void _deletePath(const Path & path, unsigned long long & bytesFreed)
         // for backwards compatibility.
         if (errno == ENOENT) return;
 
-        throw SysError(format("opening directory '%1%'") % path);
+        throw SysError("opening directory '%1%'", path);
     }
 
     _deletePath(dirfd.get(), path, bytesFreed);
@@ -845,7 +845,7 @@ int Pid::kill()
 {
     assert(pid != -1);
 
-    debug(format("killing process %1%") % pid);
+    debug("killing process %1%", pid);
 
     /* Send the requested signal to the child.  If it has its own
        process group, send the signal to every process in the child
@@ -903,7 +903,7 @@ pid_t Pid::release()
 
 void killUser(uid_t uid)
 {
-    debug(format("killing all processes running under uid '%1%'") % uid);
+    debug("killing all processes running under uid '%1%'", uid);
 
     assert(uid != 0); /* just to be safe... */