Support varargs-only MethodHandle as SpEL function

Prior to this commit, if a MethodHandle was registered as a custom
function in the Spring Expression Language (SpEL) for a static method
that accepted only a variable argument list (for example,
`static String func(String... args)`), attempting to invoke the
registered function within a SpEL expression resulted in a
ClassCastException because the varargs array was unnecessarily wrapped
in an Object[].

This commit modifies the logic in FunctionReference's internal
executeFunctionViaMethodHandle() method to address that.

Closes spring-projectsgh-34109

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

@@ -229,8 +229,9 @@ else if (spelParamCount != declaredParamCount) {
 		ReflectionHelper.convertAllMethodHandleArguments(converter, functionArgs, methodHandle, varArgPosition);
 
 		if (isSuspectedVarargs) {
-			if (declaredParamCount == 1) {
-				// We only repackage the varargs if it is the ONLY argument -- for example,
+			if (declaredParamCount == 1 && !methodHandle.isVarargsCollector()) {
+				// We only repackage the arguments if the MethodHandle accepts a single
+				// argument AND the MethodHandle is not a "varargs collector" -- for example,
 				// when we are dealing with a bound MethodHandle.
 				functionArgs = ReflectionHelper.setupArgumentsForVarargsInvocation(
 						methodHandle.type().parameterArray(), functionArgs);

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

@@ -108,11 +108,16 @@ private static void populateMethodHandles(StandardEvaluationContext testContext)
 				"formatObjectVarargs", MethodType.methodType(String.class, String.class, Object[].class));
 		testContext.registerFunction("formatObjectVarargs", formatObjectVarargs);
 
-		// #formatObjectVarargs(format, args...)
+		// #formatPrimitiveVarargs(format, args...)
 		MethodHandle formatPrimitiveVarargs = MethodHandles.lookup().findStatic(TestScenarioCreator.class,
 				"formatPrimitiveVarargs", MethodType.methodType(String.class, String.class, int[].class));
 		testContext.registerFunction("formatPrimitiveVarargs", formatPrimitiveVarargs);
 
+		// #varargsFunctionHandle(args...)
+		MethodHandle varargsFunctionHandle = MethodHandles.lookup().findStatic(TestScenarioCreator.class,
+				"varargsFunction", MethodType.methodType(String.class, String[].class));
+		testContext.registerFunction("varargsFunctionHandle", varargsFunctionHandle);
+
 		// #add(int, int)
 		MethodHandle add = MethodHandles.lookup().findStatic(TestScenarioCreator.class,
 				"add", MethodType.methodType(int.class, int.class, int.class));

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

@@ -79,6 +79,8 @@ void functionInvocationWithStringArgument() {
 
 	@Test
 	void functionWithVarargs() {
+		// static String varargsFunction(String... strings) -> Arrays.toString(strings)
+
 		evaluate("#varargsFunction()", "[]", String.class);
 		evaluate("#varargsFunction(new String[0])", "[]", String.class);
 		evaluate("#varargsFunction('a')", "[a]", String.class);
@@ -241,6 +243,27 @@ void functionFromMethodHandleWithListConvertedToVarargsArray() {
 		evaluate("#formatObjectVarargs('x -> %s %s %s', {'a', 'b', 'c'})", expected, String.class);
 	}
 
+	@Test  // gh-34109
+	void functionViaMethodHandleForStaticMethodThatAcceptsOnlyVarargs() {
+		// #varargsFunctionHandle: static String varargsFunction(String... strings) -> Arrays.toString(strings)
+
+		evaluate("#varargsFunctionHandle()", "[]", String.class);
+		evaluate("#varargsFunctionHandle(new String[0])", "[]", String.class);
+		evaluate("#varargsFunctionHandle('a')", "[a]", String.class);
+		evaluate("#varargsFunctionHandle('a','b','c')", "[a, b, c]", String.class);
+		evaluate("#varargsFunctionHandle(new String[]{'a','b','c'})", "[a, b, c]", String.class);
+		// Conversion from int to String
+		evaluate("#varargsFunctionHandle(25)", "[25]", String.class);
+		evaluate("#varargsFunctionHandle('b',25)", "[b, 25]", String.class);
+		evaluate("#varargsFunctionHandle(new int[]{1, 2, 3})", "[1, 2, 3]", String.class);
+		// Strings that contain a comma
+		evaluate("#varargsFunctionHandle('a,b')", "[a,b]", String.class);
+		evaluate("#varargsFunctionHandle('a', 'x,y', 'd')", "[a, x,y, d]", String.class);
+		// null values
+		evaluate("#varargsFunctionHandle(null)", "[null]", String.class);
+		evaluate("#varargsFunctionHandle('a',null,'b')", "[a, null, b]", String.class);
+	}
+
 	@Test
 	void functionMethodMustBeStatic() throws Exception {
 		SpelExpressionParser parser = new SpelExpressionParser();