Improve diagnostics for negative repeated text count in SpEL

Due to the changes in gh-31341, if the repeat count in a SpEL
expression (using the repeat operator '*') is negative, we throw a
SpelEvaluationException with the MAX_REPEATED_TEXT_SIZE_EXCEEDED
message which is incorrect and misleading.

Prior to gh-31341, a negative repeat count resulted in an
IllegalArgumentException being thrown by String#repeat(), which was
acceptable in terms of diagnostics, but that did not make it
immediately clear to the user what the underlying cause was.

In light of the above, this commit improves diagnostics for a negative
repeated text count in SpEL expressions by throwing a
SpelEvaluationException with a new NEGATIVE_REPEATED_TEXT_COUNT error
message.

Closes gh-31342

spring-expression/src/main/java/org/springframework/expression/spel/SpelMessage.java

@@ -284,7 +284,11 @@ public enum SpelMessage {
 
 	/** @since 5.2.24 */
 	VARIABLE_ASSIGNMENT_NOT_SUPPORTED(Kind.ERROR, 1080,
-			"Assignment to variable ''{0}'' is not supported");
+			"Assignment to variable ''{0}'' is not supported"),
+
+	/** @since 6.0.13 */
+	NEGATIVE_REPEATED_TEXT_COUNT(Kind.ERROR, 1081,
+			"Repeat count ''{0}'' must not be negative");
 
 
 	private final Kind kind;

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

@@ -123,6 +123,10 @@ public class OpMultiply extends Operator {
 	}
 
 	private void checkRepeatedTextSize(String text, int count) {
+		if (count < 0) {
+			throw new SpelEvaluationException(getStartPosition(),
+					SpelMessage.NEGATIVE_REPEATED_TEXT_COUNT, count);
+		}
 		int result = text.length() * count;
 		if (result < 0 || result > MAX_REPEATED_TEXT_SIZE) {
 			throw new SpelEvaluationException(getStartPosition(),

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

@@ -28,6 +28,7 @@ import org.springframework.expression.spel.standard.SpelExpression;
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.springframework.expression.spel.SpelMessage.MAX_CONCATENATED_STRING_LENGTH_EXCEEDED;
 import static org.springframework.expression.spel.SpelMessage.MAX_REPEATED_TEXT_SIZE_EXCEEDED;
+import static org.springframework.expression.spel.SpelMessage.NEGATIVE_REPEATED_TEXT_COUNT;
 
 /**
  * Tests the evaluation of expressions using various operators.
@@ -587,6 +588,13 @@ class OperatorTests extends AbstractExpressionTests {
 		evaluateAndCheckError("'ab' * " + repeatCount, String.class, MAX_REPEATED_TEXT_SIZE_EXCEEDED, 5);
 	}
 
+	@Test
+	void stringRepeatWithNegativeRepeatCount() {
+		// 4 is the position of the '*' (repeat operator)
+		// -1 is the negative repeat count
+		evaluateAndCheckError("'a' * -1", String.class, NEGATIVE_REPEATED_TEXT_COUNT, 4, -1);
+	}
+
 	@Test
 	void stringConcatenation() {
 		evaluate("'' + ''", "", String.class);