Index: src/select.c
==================================================================
--- src/select.c
+++ src/select.c
@@ -3416,11 +3416,10 @@
   Parse *pParse;            /* The parsing context */
   int iTable;               /* Replace references to this table */
   int iNewTable;            /* New table number */
   int isLeftJoin;           /* Add TK_IF_NULL_ROW opcodes on each replacement */
   ExprList *pEList;         /* Replacement expressions */
-  int bFlattener;           /* True for query-flattener, false otherwise */
 } SubstContext;
 
 /* Forward Declarations */
 static void substExprList(SubstContext*, ExprList*);
 static void substSelect(SubstContext*, Select*, int);
@@ -3478,13 +3477,13 @@
           ExprSetProperty(pNew, EP_FromJoin);
         }
         sqlite3ExprDelete(db, pExpr);
         pExpr = pNew;
 
-        /* If this call is part of query-flattening, ensure that the
-        ** new expression has an implicit collation sequence. */
-        if( pSubst->bFlattener && pExpr ){
+        /* Ensure that the expression now has an implicit collation sequence,
+        ** just as it did when it was a column of a view or sub-query. */
+        if( pExpr ){
           if( pExpr->op!=TK_COLUMN && pExpr->op!=TK_COLLATE ){
             CollSeq *pColl = sqlite3ExprCollSeq(pSubst->pParse, pExpr);
             pExpr = sqlite3ExprAddCollateString(pSubst->pParse, pExpr, 
                 (pColl ? pColl->zName : "BINARY")
             );
@@ -4054,11 +4053,10 @@
       x.pParse = pParse;
       x.iTable = iParent;
       x.iNewTable = iNewParent;
       x.isLeftJoin = isLeftJoin;
       x.pEList = pSub->pEList;
-      x.bFlattener = 1;
       substSelect(&x, pParent, 0);
     }
   
     /* The flattened query is a compound if either the inner or the
     ** outer query is a compound. */
@@ -4380,11 +4378,10 @@
       x.pParse = pParse;
       x.iTable = iCursor;
       x.iNewTable = iCursor;
       x.isLeftJoin = 0;
       x.pEList = pSubq->pEList;
-      x.bFlattener = 0;
       pNew = substExpr(&x, pNew);
       if( pSubq->selFlags & SF_Aggregate ){
         pSubq->pHaving = sqlite3ExprAnd(pParse, pSubq->pHaving, pNew);
       }else{
         pSubq->pWhere = sqlite3ExprAnd(pParse, pSubq->pWhere, pNew);

Index: src/test1.c
==================================================================
--- src/test1.c
+++ src/test1.c
@@ -7198,10 +7198,11 @@
     { "order-by-idx-join",   SQLITE_OrderByIdxJoin },
     { "transitive",          SQLITE_Transitive     },
     { "omit-noop-join",      SQLITE_OmitNoopJoin   },
     { "stat4",               SQLITE_Stat4          },
     { "skip-scan",           SQLITE_SkipScan       },
+    { "push-down",           SQLITE_PushDown       },
   };
 
   if( objc!=4 ){
     Tcl_WrongNumArgs(interp, 1, objv, "DB OPT BOOLEAN");
     return TCL_ERROR;

ADDED   test/tkt-18458b1a.test
Index: test/tkt-18458b1a.test
==================================================================
--- test/tkt-18458b1a.test
+++ test/tkt-18458b1a.test
@@ -0,0 +1,53 @@
+# 2019 September 10
+#
+# The author disclaims copyright to this source code.  In place of
+# a legal notice, here is a blessing:
+#
+#    May you do good and not evil.
+#    May you find forgiveness for yourself and forgive others.
+#    May you share freely, never taking more than you give.
+#
+#***********************************************************************
+# This file implements regression tests for SQLite library. In particular,
+# that problems related to ticket [18458b1a] have been fixed.
+#
+
+set testdir [file dirname $argv0]
+source $testdir/tester.tcl
+set testprefix tkt-18458b1a
+
+foreach tn {1 2} {
+  reset_db
+  if {$tn==1} {
+    # Disable the flattener and push-down optimizations
+    optimization_control db query-flattener 0
+    optimization_control db push-down 0
+  } else {
+    # Enable them
+    optimization_control db query-flattener 1
+    optimization_control db push-down 1
+  }
+
+  db cache size 0
+
+  do_execsql_test $tn.1.1 {
+    CREATE TABLE t0(c0 COLLATE NOCASE);
+    INSERT INTO t0(c0) VALUES ('B');
+    CREATE VIEW v0(c0, c1) AS SELECT DISTINCT t0.c0, 'a' FROM t0;
+  } 
+
+  do_execsql_test $tn.1.2 {
+    SELECT count(*) FROM v0 WHERE c1 >= c0;
+  } 1
+
+  do_execsql_test $tn.1.3 {
+    SELECT count(*) FROM v0 WHERE NOT NOT (c1 >= c0);
+  } 1
+
+  do_execsql_test $tn.1.4 {
+    SELECT count(*) FROM v0 WHERE ((c1 >= c0) OR 0+0);
+  } 1
+}
+
+finish_test
+