fix: don't generate string concatenation without assign to variable

jadx-core/src/main/java/jadx/core/dex/visitors/InitCodeVariables.java

@@ -52,7 +52,7 @@ public class InitCodeVariables extends AbstractVisitor {
 		}
 	}
 
-	private static void initCodeVar(SSAVar ssaVar) {
+	public static void initCodeVar(SSAVar ssaVar) {
 		if (ssaVar.isCodeVarSet()) {
 			return;
 		}

jadx-core/src/main/java/jadx/core/dex/visitors/SimplifyVisitor.java

@@ -401,6 +401,7 @@ public class SimplifyVisitor extends AbstractVisitor {
 			concatInsn.copyAttributesFrom(toStrInsn);
 			concatInsn.remove(AFlag.DONT_GENERATE);
 			concatInsn.remove(AFlag.REMOVE);
+			checkResult(mth, concatInsn);
 			return concatInsn;
 		} catch (Exception e) {
 			LOG.warn("Can't convert string concatenation: {} insn: {}", mth, toStrInsn, e);
@@ -408,6 +409,17 @@ public class SimplifyVisitor extends AbstractVisitor {
 		return null;
 	}
 
+	/* String concat without assign to variable will cause compilation error */
+	private static void checkResult(MethodNode mth, InsnNode concatInsn) {
+		if (concatInsn.getResult() == null) {
+			RegisterArg resArg = InsnArg.reg(0, ArgType.STRING);
+			SSAVar ssaVar = mth.makeNewSVar(resArg);
+			InitCodeVariables.initCodeVar(ssaVar);
+			ssaVar.setType(ArgType.STRING);
+			concatInsn.setResult(resArg);
+		}
+	}
+
 	/**
 	 * Remove and unbind all instructions with StringBuilder
 	 */

jadx-core/src/test/java/jadx/tests/integration/others/TestStringConcatWithoutResult.java

@@ -0,0 +1,32 @@
+package jadx.tests.integration.others;
+
+import org.junit.jupiter.api.Test;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import jadx.tests.api.IntegrationTest;
+
+import static jadx.tests.api.utils.assertj.JadxAssertions.assertThat;
+
+public class TestStringConcatWithoutResult extends IntegrationTest {
+	private static final Logger LOG = LoggerFactory.getLogger(TestStringConcatWithoutResult.class);
+
+	public static class TestCls {
+		public static final boolean LOG_DEBUG = false;
+
+		public void test(int i) {
+			String msg = "Input arg value: " + i;
+			if (LOG_DEBUG) {
+				LOG.debug(msg);
+			}
+		}
+	}
+
+	@Test
+	public void test() {
+		noDebugInfo();
+		assertThat(getClassNode(TestCls.class))
+				.code()
+				.containsOne(" = \"Input arg value: \" + i;");
+	}
+}