Actually, solve this in a lighter-weight way

The templating is very superficial

src/libstore/local-store.cc

@@ -10,7 +10,6 @@
 #include "topo-sort.hh"
 #include "finally.hh"
 #include "compression.hh"
-#include "sqlite-impl.hh"
 
 #include <iostream>
 #include <algorithm>

src/libstore/sqlite-impl.hh

@@ -1,42 +0,0 @@
-#include "sqlite.hh"
-#include "globals.hh"
-#include "util.hh"
-
-#include <sqlite3.h>
-
-#include <atomic>
-
-namespace nix {
-
-template<typename... Args>
-SQLiteError::SQLiteError(const char *path, int errNo, int extendedErrNo, const Args & ... args)
-  : Error(""), path(path), errNo(errNo), extendedErrNo(extendedErrNo)
-{
-    auto hf = hintfmt(args...);
-    err.msg = hintfmt("%s: %s (in '%s')",
-        normaltxt(hf.str()),
-        sqlite3_errstr(extendedErrNo),
-        path ? path : "(in-memory)");
-}
-
-template<typename... Args>
-[[noreturn]] void SQLiteError::throw_(sqlite3 * db, const std::string & fs, const Args & ... args)
-{
-    int err = sqlite3_errcode(db);
-    int exterr = sqlite3_extended_errcode(db);
-
-    auto path = sqlite3_db_filename(db, nullptr);
-
-    if (err == SQLITE_BUSY || err == SQLITE_PROTOCOL) {
-        auto exp = SQLiteBusy(path, err, exterr, fs, args...);
-        exp.err.msg = hintfmt(
-            err == SQLITE_PROTOCOL
-                ? "SQLite database '%s' is busy (SQLITE_PROTOCOL)"
-                : "SQLite database '%s' is busy",
-            path ? path : "(in-memory)");
-        throw exp;
-    } else
-        throw SQLiteError(path, err, exterr, fs, args...);
-}
-
-}

src/libstore/sqlite.cc

@@ -1,5 +1,4 @@
 #include "sqlite.hh"
-#include "sqlite-impl.hh"
 #include "globals.hh"
 #include "util.hh"
 
@@ -9,6 +8,34 @@
 
 namespace nix {
 
+SQLiteError::SQLiteError(const char *path, int errNo, int extendedErrNo, hintformat && hf)
+  : Error(""), path(path), errNo(errNo), extendedErrNo(extendedErrNo)
+{
+    err.msg = hintfmt("%s: %s (in '%s')",
+        normaltxt(hf.str()),
+        sqlite3_errstr(extendedErrNo),
+        path ? path : "(in-memory)");
+}
+
+[[noreturn]] void SQLiteError::throw_(sqlite3 * db, hintformat && hf)
+{
+    int err = sqlite3_errcode(db);
+    int exterr = sqlite3_extended_errcode(db);
+
+    auto path = sqlite3_db_filename(db, nullptr);
+
+    if (err == SQLITE_BUSY || err == SQLITE_PROTOCOL) {
+        auto exp = SQLiteBusy(path, err, exterr, std::move(hf));
+        exp.err.msg = hintfmt(
+            err == SQLITE_PROTOCOL
+                ? "SQLite database '%s' is busy (SQLITE_PROTOCOL)"
+                : "SQLite database '%s' is busy",
+            path ? path : "(in-memory)");
+        throw exp;
+    } else
+        throw SQLiteError(path, err, exterr, std::move(hf));
+}
+
 SQLite::SQLite(const Path & path, bool create)
 {
     // useSQLiteWAL also indicates what virtual file system we need.  Using

src/libstore/sqlite.hh

@@ -102,11 +102,21 @@ struct SQLiteError : Error
     int errNo, extendedErrNo;
 
     template<typename... Args>
-    [[noreturn]] static void throw_(sqlite3 * db, const std::string & fs, const Args & ... args);
+    [[noreturn]] static void throw_(sqlite3 * db, const std::string & fs, const Args & ... args) {
+        throw_(db, hintfmt(fs, args...));
+    }
 
 protected:
+
+    SQLiteError(const char *path, int errNo, int extendedErrNo, hintformat && hf);
+
     template<typename... Args>
-    SQLiteError(const char *path, int errNo, int extendedErrNo, const Args & ... args);
+    SQLiteError(const char *path, int errNo, int extendedErrNo, const std::string & fs, const Args & ... args)
+      : SQLiteError(path, errNo, extendedErrNo, hintfmt(fs, args...))
+    { }
+
+    [[noreturn]] static void throw_(sqlite3 * db, hintformat && hf);
+
 };
 
 MakeError(SQLiteBusy, SQLiteError);