Issue 14459 - String literal merge bug causes incorrect runtime program behavior

src/expression.c

@@ -13571,6 +13571,18 @@ Expression *EqualExp::semantic(Scope *sc)
         return e;
     }
 
+    if (t1->ty == Tpointer || t2->ty == Tpointer)
+    {
+        /* Rewrite:
+         *      ptr1 == ptr2
+         * as:
+         *      ptr1 is ptr2
+         */
+        e = new IdentityExp(op == TOKequal ? TOKidentity : TOKnotidentity, loc, e1, e2);
+        e = e->semantic(sc);
+        return e;
+    }
+
     if (t1->ty == Tstruct && t2->ty == Tstruct)
     {
         StructDeclaration *sd = ((TypeStruct *)t1)->sym;

src/optimize.c

@@ -74,7 +74,18 @@ Expression *expandVar(int result, VarDeclaration *v)
                 {
                     AssignExp *ae = (AssignExp *)ei;
                     ei = ae->e2;
-                    if (ei->isConst() != 1 && ei->op != TOKstring)
+                    if (ei->isConst() == 1)
+                    {
+                    }
+                    else if (ei->op == TOKstring)
+                    {
+                        // Bugzilla 14459: We should not constfold the string literal
+                        // if it's typed as a C string, because the value expansion
+                        // will drop the pointer identity.
+                        if (!(result & WANTexpand) && ei->type->toBasetype()->ty == Tpointer)
+                            goto L1;
+                    }
+                    else
                         goto L1;
 
                     if (ei->type == v->type)

test/runnable/constfold.d

@@ -743,6 +743,37 @@ void test3697or()
     auto z = y || 1 / x;
 }
 
+/************************************/
+// 14459
+
+void test14459()
+{
+    const char* s0 = "hi0";
+    const(char)* p0 = s0;
+    assert(p0 == s0);
+
+    const char* s1 = "hi1";
+    const char* s2 = "hi2";
+    const char* s3 = "hi3";
+    const char* s4 = "hi4";
+    const char* s5 = "hi5";
+    const char* s6 = "hi6";
+    const char* s7 = "hi7";
+    const char* s8 = "hi8";
+    const char* s9 = "hi9";
+    const char* s10 = "hi10";
+    const char* s11 = "hi11";
+    const char* s12 = "hi12";
+    const char* s13 = "hi13";
+    const char* s14 = "hi14";
+    const char* s15 = "hi15";
+    assert(p0 == s0);           // ok
+    const char* s16 = "hi16";
+    assert(p0 == s0);           // ok <- fails
+}
+
+/************************************/
+
 int main()
 {
     test1();
@@ -757,6 +788,7 @@ int main()
     test11159();
     test13977();
     test13978();
+    test14459();
 
     printf("Success\n");
     return 0;