Support lists for varargs invocations in SpEL

The changes made in conjunction with gh-33013 resulted in a regression
for varargs support in SpEL expressions. Specifically, before gh-33013
one could supply a list -- for example, an "inline list" -- as the
varargs array when invoking a varargs constructor, method, or function
within a SpEL expression. However, after gh-33013 an inline list (or
collection in general) is no longer converted to an array for varargs
invocations. Instead, the list is supplied as a single argument of the
resulting varargs array which breaks applications that depend on the
previous behavior.

Although it was never intended that one could supply a collection as
the set of varargs, we concede that this is a regression in existing
behavior, and this commit therefore restores support for supplying a
java.util.List as the varargs "array".

In addition, this commit introduces the same "list to array" conversion
support for MethodHandle-based functions that accept varargs.

Note, however, that this commit does not restore support for converting
arbitrary single objects to an array for varargs invocations if the
single object is already an instance of the varargs array's component
type.

See gh-33013
Closes gh-33315

spring-expression/src/main/java/org/springframework/expression/spel/support/ReflectionHelper.java

@@ -314,11 +314,14 @@ public abstract class ReflectionHelper {
 				// convert it or wrap it in an array. For example, using StringToArrayConverter to convert
 				// a String containing a comma would result in the String being split and repackaged in an
 				// array when it should be used as-is. Similarly, if the argument is an array that is
-				// assignable to the varargs array type, there is no need to convert it.
+				// assignable to the varargs array type, there is no need to convert it. However, if the
+				// argument is a java.util.List, we let the TypeConverter convert the list to an array.
 				else if (!sourceType.isAssignableTo(componentTypeDesc) ||
-						(sourceType.isArray() && !sourceType.isAssignableTo(targetType))) {
+						(sourceType.isArray() && !sourceType.isAssignableTo(targetType)) ||
+						(argument instanceof List)) {
 
-					TypeDescriptor targetTypeToUse = (sourceType.isArray() ? targetType : componentTypeDesc);
+					TypeDescriptor targetTypeToUse =
+							(sourceType.isArray() || argument instanceof List ? targetType : componentTypeDesc);
 					arguments[varargsPosition] = converter.convertValue(argument, sourceType, targetTypeToUse);
 				}
 				// Possible outcomes of the above if-else block:
@@ -413,11 +416,14 @@ public abstract class ReflectionHelper {
 				// convert it. For example, using StringToArrayConverter to convert a String containing a
 				// comma would result in the String being split and repackaged in an array when it should
 				// be used as-is. Similarly, if the argument is an array that is assignable to the varargs
-				// array type, there is no need to convert it.
+				// array type, there is no need to convert it. However, if the argument is a java.util.List,
+				// we let the TypeConverter convert the list to an array.
 				else if (!sourceType.isAssignableTo(varargsComponentType) ||
-						(sourceType.isArray() && !sourceType.isAssignableTo(varargsArrayType))) {
+						(sourceType.isArray() && !sourceType.isAssignableTo(varargsArrayType)) ||
+						(argument instanceof List)) {
 
-					TypeDescriptor targetTypeToUse = (sourceType.isArray() ? varargsArrayType : varargsComponentType);
+					TypeDescriptor targetTypeToUse =
+							(sourceType.isArray() || argument instanceof List ? varargsArrayType : varargsComponentType);
 					arguments[varargsPosition] = converter.convertValue(argument, sourceType, targetTypeToUse);
 				}
 				// Possible outcomes of the above if-else block:

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

@@ -32,6 +32,7 @@ import org.springframework.expression.MethodResolver;
 import org.springframework.expression.spel.standard.SpelExpression;
 import org.springframework.expression.spel.standard.SpelExpressionParser;
 import org.springframework.expression.spel.support.StandardEvaluationContext;
+import org.springframework.expression.spel.support.StandardTypeLocator;
 import org.springframework.expression.spel.testresources.Inventor;
 import org.springframework.expression.spel.testresources.PlaceOfBirth;
 
@@ -364,6 +365,21 @@ class MethodInvocationTests extends AbstractExpressionTests {
 		evaluate("formatObjectVarargs('x -> %s %s %s', new int[]{1, 2, 3})", "x -> 1 2 3", String.class); // int[] to Object[]
 	}
 
+	@Test  // gh-33315
+	void varargsWithListConvertedToVarargsArray() {
+		((StandardTypeLocator) context.getTypeLocator()).registerImport("java.util");
+
+		// Calling 'public String aVarargsMethod(String... strings)' -> Arrays.toString(strings)
+		String expected = "[a, b, c]";
+		evaluate("aVarargsMethod(T(List).of('a', 'b', 'c'))", expected, String.class);
+		evaluate("aVarargsMethod({'a', 'b', 'c'})", expected, String.class);
+
+		// Calling 'public String formatObjectVarargs(String format, Object... args)' -> String.format(format, args)
+		expected = "x -> a b c";
+		evaluate("formatObjectVarargs('x -> %s %s %s', T(List).of('a', 'b', 'c'))", expected, String.class);
+		evaluate("formatObjectVarargs('x -> %s %s %s', {'a', 'b', 'c'})", expected, String.class);
+	}
+
 	@Test
 	void varargsOptionalInvocation() {
 		// Calling 'public String optionalVarargsMethod(Optional<String>... values)'

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

@@ -20,6 +20,7 @@ import org.junit.jupiter.api.Test;
 
 import org.springframework.expression.spel.standard.SpelExpressionParser;
 import org.springframework.expression.spel.support.StandardEvaluationContext;
+import org.springframework.expression.spel.support.StandardTypeLocator;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
@@ -211,6 +212,33 @@ class VariableAndFunctionTests extends AbstractExpressionTests {
 		evaluate("#formatObjectVarargs('x -> %s %s %s', new int[]{1, 2, 3})", "x -> 1 2 3", String.class); // int[] to Object[]
 	}
 
+	@Test  // gh-33315
+	void functionFromMethodWithListConvertedToVarargsArray() {
+		((StandardTypeLocator) context.getTypeLocator()).registerImport("java.util");
+		String expected = "[a, b, c]";
+
+		evaluate("#varargsFunction(T(List).of('a', 'b', 'c'))", expected, String.class);
+		evaluate("#varargsFunction({'a', 'b', 'c'})", expected, String.class);
+
+		// Calling 'public String formatObjectVarargs(String format, Object... args)' -> String.format(format, args)
+		evaluate("#varargsObjectFunction(T(List).of('a', 'b', 'c'))", expected, String.class);
+		evaluate("#varargsObjectFunction({'a', 'b', 'c'})", expected, String.class);
+	}
+
+	@Test  // gh-33315
+	void functionFromMethodHandleWithListConvertedToVarargsArray() {
+		((StandardTypeLocator) context.getTypeLocator()).registerImport("java.util");
+		String expected = "x -> a b c";
+
+		// Calling 'public static String message(String template, String... args)' -> template.formatted((Object[]) args)
+		evaluate("#message('x -> %s %s %s', T(List).of('a', 'b', 'c'))", expected, String.class);
+		evaluate("#message('x -> %s %s %s', {'a', 'b', 'c'})", expected, String.class);
+
+		// Calling 'public static String formatObjectVarargs(String format, Object... args)' -> String.format(format, args)
+		evaluate("#formatObjectVarargs('x -> %s %s %s', T(List).of('a', 'b', 'c'))", expected, String.class);
+		evaluate("#formatObjectVarargs('x -> %s %s %s', {'a', 'b', 'c'})", expected, String.class);
+	}
+
 	@Test
 	void functionMethodMustBeStatic() throws Exception {
 		SpelExpressionParser parser = new SpelExpressionParser();