Fix message interpolation when code is used as default message

When `setUseCodeAsDefaultMessage(true)` was set on a message source,
attempting to interpolate the default message returned from the message
source would result in the code being unusable by upstream message
resolvers.

Fixes gh-28930

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/validation/MessageSourceMessageInterpolator.java

@@ -30,6 +30,7 @@
  * message using the underlying {@link MessageInterpolator}.
  *
  * @author Dmytro Nosan
+ * @author Scott Frederick
  */
 class MessageSourceMessageInterpolator implements MessageInterpolator {
 
@@ -115,7 +116,12 @@ else if (buf.charAt(i) == SUFFIX) {
 	private String replaceParameter(String parameter, Locale locale, Set<String> visitedParameters) {
 		parameter = replaceParameters(parameter, locale, visitedParameters);
 		String value = this.messageSource.getMessage(parameter, null, null, locale);
-		return (value != null) ? replaceParameters(value, locale, visitedParameters) : null;
+		return (value != null && !isUsingCodeAsDefaultMessage(value, parameter))
+				? replaceParameters(value, locale, visitedParameters) : null;
+	}
+
+	private boolean isUsingCodeAsDefaultMessage(String value, String parameter) {
+		return value.equals(parameter);
 	}
 
 }

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/validation/MessageSourceMessageInterpolatorTests.java

@@ -34,6 +34,7 @@
  *
  * @author Dmytro Nosan
  * @author Andy Wilkinson
+ * @author Scott Frederick
  */
 class MessageSourceMessageInterpolatorTests {
 
@@ -58,6 +59,14 @@ void interpolateWhenParametersAreUnknownShouldLeaveThemUnchanged() {
 				.isEqualTo("{foo}{child}+{child}{bar}");
 	}
 
+	@Test
+	void interpolateWhenParametersAreUnknownUsingCodeAsDefaultShouldLeaveThemUnchanged() {
+		this.messageSource.setUseCodeAsDefaultMessage(true);
+		this.messageSource.addMessage("top", Locale.getDefault(), "{child}+{child}");
+		assertThat(this.interpolator.interpolate("{foo}{top}{bar}", this.context))
+				.isEqualTo("{foo}{child}+{child}{bar}");
+	}
+
 	@Test
 	void interpolateWhenParametersAreNestedShouldFullyReplaceAllParameters() {
 		this.messageSource.addMessage("top", Locale.getDefault(), "{child}+{child}");