Introduce missing tests for immediate SpEL compilation for Elvis operator

spring-expression/src/main/java/org/springframework/expression/spel/ast/Elvis.java

@@ -98,7 +98,11 @@ public class Elvis extends SpelNodeImpl {
 
 	@Override
 	public void generateCode(MethodVisitor mv, CodeFlow cf) {
-		// exit type descriptor can be null if both components are literal expressions
+		// If both elements are literals and the expression was not previously
+		// evaluated in interpreted mode -- or if getValueInternal() did not
+		// properly invoke computeExitTypeDescriptor() -- we may get here without
+		// the exit descriptor having been computed, so we must ensure the exit
+		// descriptor has been computed before proceeding.
 		computeExitTypeDescriptor();
 
 		cf.enterCompilationScope();

spring-expression/src/test/java/org/springframework/expression/spel/SpelCompilationCoverageTests.java

@@ -1470,6 +1470,23 @@ public class SpelCompilationCoverageTests extends AbstractExpressionTests {
 			assertThat(getAst().getExitDescriptor()).isEqualTo("Ljava/lang/String");
 		}
 
+		@Test
+		void elvisWithImmediateCompilation() {
+			expression = parser.parseExpression("'a' ?: 'b'");
+			// Both literals, so we can compile immediately without having previously
+			// evaluated the expression.
+			assertCanCompile(expression);
+			assertThat(expression.getValue(String.class)).isEqualTo("a");
+			assertThat(getAst().getExitDescriptor()).isEqualTo("Ljava/lang/String");
+
+			expression = parser.parseExpression("null ?: 'a'");
+			// Both literals, so we can compile immediately without having previously
+			// evaluated the expression.
+			assertCanCompile(expression);
+			assertThat(expression.getValue(String.class)).isEqualTo("a");
+			assertThat(getAst().getExitDescriptor()).isEqualTo("Ljava/lang/Object");
+		}
+
 		@Test  // gh-19758
 		void elvisMiscellaneous() {
 			SpelParserConfiguration configuration = new SpelParserConfiguration(SpelCompilerMode.IMMEDIATE, null);