Fixed #5731 (False positive with opposite conditions)

lib/checkother.cpp

@@ -3321,6 +3321,9 @@ void CheckOther::oppositeInnerCondition()
                 nonlocal |= (var && (!var->isLocal() || var->isStatic()));
                 // TODO: if var is pointer check what it points at
                 nonlocal |= (var && var->isPointer());
+            } else if (cond->isName()) {
+                // varid is 0. this is possibly a nonlocal variable..
+                nonlocal |= (cond->astParent() && cond->astParent()->isConstOp());
             }
         }
 

test/testother.cpp

@@ -392,6 +392,16 @@ private:
               "    }\n"
               "}");
         ASSERT_EQUALS("", errout.str());
+
+        // #5731 - fp when undeclared variable is used
+        check("void f() {\n"
+              "   if (x == -1){\n"
+              "       x = do_something();\n"
+              "       if (x != -1) {}\n"
+              "    }\n"
+              "}");
+        ASSERT_EQUALS("", errout.str());
+
     }
 
     void emptyBrackets() {