Support MethodHandle function invocation with varargs array in SpEL

Prior to this commit, the Spring Expression Language (SpEL) could not
invoke a varargs MethodHandle function with an array containing the
variable arguments, although that is supported for a varargs Method
function. Attempting to do so resulted in the array being supplied as a
single argument to the MethodHandle.

This commit addresses this by updating the
executeFunctionViaMethodHandle(...) method in FunctionReference as
follows when the user supplies the varargs already packaged in an array.

- Creates a new array large enough to hold the non-varargs arguments
  and the unpackaged varargs arguments.

- Adds the non-varargs arguments to the beginning of that array and
  adds the unpackaged varargs arguments to the end of that array.

- Invokes the MethodHandle with the new arguments array.

Closes gh-33191

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

@@ -225,10 +225,29 @@ else if (spelParamCount != declaredParamCount) {
 		TypeConverter converter = state.getEvaluationContext().getTypeConverter();
 		ReflectionHelper.convertAllMethodHandleArguments(converter, functionArgs, methodHandle, varArgPosition);
 
-		if (isSuspectedVarargs && declaredParamCount == 1) {
-			// we only repack the varargs if it is the ONLY argument
-			functionArgs = ReflectionHelper.setupArgumentsForVarargsInvocation(
-					methodHandle.type().parameterArray(), functionArgs);
+		if (isSuspectedVarargs) {
+			if (declaredParamCount == 1) {
+				// We only repackage the varargs if it is the ONLY argument -- for example,
+				// when we are dealing with a bound MethodHandle.
+				functionArgs = ReflectionHelper.setupArgumentsForVarargsInvocation(
+						methodHandle.type().parameterArray(), functionArgs);
+			}
+			else if (spelParamCount == declaredParamCount) {
+				// If the varargs were supplied already packaged in an array, we have to create
+				// a new array, add the non-varargs arguments to the beginning of that array,
+				// and add the unpackaged varargs arguments to the end of that array. The reason
+				// is that MethodHandle.invokeWithArguments(Object...) does not expect varargs
+				// to be packaged in an array, in contrast to how method invocation works with
+				// reflection.
+				int actualVarargsIndex = functionArgs.length - 1;
+				if (actualVarargsIndex >= 0 && functionArgs[actualVarargsIndex].getClass().isArray()) {
+					Object[] argsToUnpack = (Object[]) functionArgs[actualVarargsIndex];
+					Object[] newArgs = new Object[actualVarargsIndex + argsToUnpack.length];
+					System.arraycopy(functionArgs, 0, newArgs, 0, actualVarargsIndex);
+					System.arraycopy(argsToUnpack, 0, newArgs, actualVarargsIndex, argsToUnpack.length);
+					functionArgs = newArgs;
+				}
+			}
 		}
 
 		try {

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

@@ -16,7 +16,6 @@
 
 package org.springframework.expression.spel;
 
-import org.junit.jupiter.api.Disabled;
 import org.junit.jupiter.api.Test;
 
 import org.springframework.expression.spel.standard.SpelExpressionParser;
@@ -106,22 +105,6 @@ void functionWithVarargs() {
 		evaluate("#varargsFunction2(9,'a',null,'b')", "9-[a, null, b]", String.class);
 	}
 
-	@Disabled("Disabled until bugs are reported and fixed")
-	@Test
-	void functionWithVarargsViaMethodHandle_CurrentlyFailing() {
-		// Calling 'public static String formatObjectVarargs(String format, Object... args)' -> String.format(format, args)
-
-		// No conversion necessary
-		evaluate("#formatObjectVarargs('x -> %s', new Object[]{''})", "x -> ", String.class);
-		evaluate("#formatObjectVarargs('x -> %s', new String[]{''})", "x -> ", String.class);
-		evaluate("#formatObjectVarargs('x -> %s', new Object[]{' '})", "x ->  ", String.class);
-		evaluate("#formatObjectVarargs('x -> %s', new String[]{' '})", "x ->  ", String.class);
-		evaluate("#formatObjectVarargs('x -> %s', new Object[]{'a'})", "x -> a", String.class);
-		evaluate("#formatObjectVarargs('x -> %s', new String[]{'a'})", "x -> a", String.class);
-		evaluate("#formatObjectVarargs('x -> %s %s %s', new Object[]{'a', 'b', 'c'})", "x -> a b c", String.class);
-		evaluate("#formatObjectVarargs('x -> %s %s %s', new String[]{'a', 'b', 'c'})", "x -> a b c", String.class);
-	}
-
 	@Test  // gh-33013
 	void functionWithVarargsViaMethodHandle() {
 		// Calling 'public static String formatObjectVarargs(String format, Object... args)' -> String.format(format, args)
@@ -138,6 +121,14 @@ void functionWithVarargsViaMethodHandle() {
 		evaluate("#formatObjectVarargs('x -> %s', ' ')", "x ->  ", String.class);
 		evaluate("#formatObjectVarargs('x -> %s', 'a')", "x -> a", String.class);
 		evaluate("#formatObjectVarargs('x -> %s %s %s', 'a', 'b', 'c')", "x -> a b c", String.class);
+		evaluate("#formatObjectVarargs('x -> %s', new Object[]{''})", "x -> ", String.class);
+		evaluate("#formatObjectVarargs('x -> %s', new String[]{''})", "x -> ", String.class);
+		evaluate("#formatObjectVarargs('x -> %s', new Object[]{' '})", "x ->  ", String.class);
+		evaluate("#formatObjectVarargs('x -> %s', new String[]{' '})", "x ->  ", String.class);
+		evaluate("#formatObjectVarargs('x -> %s', new Object[]{'a'})", "x -> a", String.class);
+		evaluate("#formatObjectVarargs('x -> %s', new String[]{'a'})", "x -> a", String.class);
+		evaluate("#formatObjectVarargs('x -> %s %s %s', new Object[]{'a', 'b', 'c'})", "x -> a b c", String.class);
+		evaluate("#formatObjectVarargs('x -> %s %s %s', new String[]{'a', 'b', 'c'})", "x -> a b c", String.class);
 
 		// Conversion necessary
 		evaluate("#add('2', 5.0)", 7, Integer.class);