Index: src/expr.c
==================================================================
--- src/expr.c
+++ src/expr.c
@@ -3365,11 +3365,30 @@
       /* Otherwise, fall thru into the TK_COLUMN case */
     }
     case TK_COLUMN: {
       int iTab = pExpr->iTable;
       if( ExprHasProperty(pExpr, EP_FixedCol) ){
-        return sqlite3ExprCodeTarget(pParse, pExpr->pLeft,target);
+        /* This COLUMN expression is really a constant due to WHERE clause
+        ** constraints, and that constant is coded by the pExpr->pLeft
+        ** expresssion.  However, make sure the constant has the correct
+        ** datatype by applying the Affinity of the table column to the
+        ** constant.
+        */
+        int iReg = sqlite3ExprCodeTarget(pParse, pExpr->pLeft,target);
+        int aff = sqlite3TableColumnAffinity(pExpr->pTab, pExpr->iColumn);
+        if( aff!=SQLITE_AFF_BLOB ){
+          static const char zAff[] = "B\000C\000D\000E";
+          assert( SQLITE_AFF_BLOB=='A' );
+          assert( SQLITE_AFF_TEXT=='B' );
+          if( iReg!=target ){
+            sqlite3VdbeAddOp2(v, OP_SCopy, iReg, target);
+            iReg = target;
+          }
+          sqlite3VdbeAddOp4(v, OP_Affinity, iReg, 1, 0,
+                            &zAff[(aff-'B')*2], P4_STATIC);
+        }
+        return iReg;
       }
       if( iTab<0 ){
         if( pParse->iSelfTab<0 ){
           /* Generating CHECK constraints or inserting into partial index */
           return pExpr->iColumn - pParse->iSelfTab;

Index: test/whereL.test
==================================================================
--- test/whereL.test
+++ test/whereL.test
@@ -64,7 +64,51 @@
 # query, the wrong answer results:
 #
 do_execsql_test 201 {
   SELECT * FROM c3 WHERE x='abc' AND y='abc' AND z='abc';
 } {}
+
+# Constant propagation caused an incorrect answer in the following
+# query.  (Reported by Bentley system on 2018-08-09.)
+#
+do_execsql_test 300 {
+  CREATE TABLE A(id INTEGER PRIMARY KEY, label TEXT);
+  CREATE TABLE B(id INTEGER PRIMARY KEY, label TEXT, Aid INTEGER);
+  CREATE TABLE C(
+    id INTEGER PRIMARY KEY,
+    xx INTEGER NOT NULL,
+    yy INTEGER,
+    zz INTEGER
+  );
+  CREATE UNIQUE INDEX x2 ON C(yy);
+  CREATE UNIQUE INDEX x4 ON C(yy, zz);
+  INSERT INTO A(id) VALUES(1);
+  INSERT INTO B(id) VALUES(2);
+  INSERT INTO C(id,xx,yy,zz) VALUES(99,50,1,2);
+  SELECT 1
+    FROM A,
+         (SELECT id,xx,yy,zz FROM C) subq,
+         B
+   WHERE A.id='1'
+     AND A.id=subq.yy
+     AND B.id=subq.zz;
+} {1}  
+do_execsql_test 301 {
+  SELECT 1
+    FROM A,
+         (SELECT id,xx,yy,zz FROM C) subq,
+         B
+   WHERE A.id=1
+     AND A.id=subq.yy
+     AND B.id=subq.zz;
+} {1}  
+do_execsql_test 302 {
+  SELECT 1
+    FROM A,
+         (SELECT id,yy,zz FROM C) subq,
+         B
+   WHERE A.id='1'
+     AND A.id=subq.yy
+     AND B.id=subq.zz;
+} {1}  
 
 finish_test