Error message if constant not found.

src/main/java/core/ru/ispras/microtesk/translator/nml/antlrex/LocationFactory.java

@@ -35,6 +35,7 @@
 import ru.ispras.microtesk.translator.nml.ir.primitive.Instance;
 import ru.ispras.microtesk.translator.nml.ir.primitive.Primitive;
 import ru.ispras.microtesk.translator.nml.ir.primitive.PrimitiveAnd;
+import ru.ispras.microtesk.translator.nml.ir.shared.LetConstant;
 import ru.ispras.microtesk.translator.nml.ir.shared.MemoryResource;
 import ru.ispras.microtesk.translator.nml.ir.shared.Struct;
 import ru.ispras.microtesk.translator.nml.ir.shared.Type;
@@ -64,7 +65,14 @@ public Expr location(
 
     // Hack to deal with internal variables described by string constants.
     if (NmlSymbolKind.LET_CONST == kind) {
-      final Expr expr = getIr().getConstants().get(name).getExpr();
+      final LetConstant constant = getIr().getConstants().get(name);
+      if (null == constant) {
+        // Forward definition.
+        raiseError(where, String.format(
+            "Constant %s is not found. Forward definitions are not allowed.", name));
+      }
+
+      final Expr expr = constant.getExpr();
       if (expr.isInternalVariable()) {
         return new Expr(expr);
       }

src/test/java/core/ru/ispras/microtesk/translator/nml/LetForwardTestCase.java

@@ -0,0 +1,28 @@
+/*
+ * Copyright 2020 ISP RAS (http://www.ispras.ru)
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
+ * in compliance with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under the License
+ * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
+ * or implied. See the License for the specific language governing permissions and limitations under
+ * the License.
+ */
+
+package ru.ispras.microtesk.translator.nml;
+
+import org.junit.Test;
+import ru.ispras.microtesk.translator.TranslatorTest;
+import ru.ispras.microtesk.translator.nml.ir.Ir;
+
+public class LetForwardTestCase extends TranslatorTest<Ir> {
+
+  @Test
+  public void test() {
+    final NmlTranslator translator = new NmlTranslator();
+    translate(translator, "./src/test/nml/let_forward.nml");
+  }
+}

src/test/nml/let_forward.nml

@@ -0,0 +1,2 @@
+let CONST_1 = CONST_2 // ERROR: Forward definition.
+let CONST_2 = 0x1