Ensure that ConversionService.canConvert(Enum) no longer throws an exception

spring-core/src/main/java/org/springframework/core/convert/support/ConversionUtils.java

@@ -21,8 +21,9 @@
 import org.springframework.core.convert.ConversionFailedException;
 import org.springframework.core.convert.ConversionService;
 import org.springframework.core.convert.TypeDescriptor;
+import org.springframework.core.convert.converter.ConditionalConverter;
+import org.springframework.core.convert.converter.Converter;
 import org.springframework.core.convert.converter.GenericConverter;
-import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 
 /**
@@ -71,13 +72,30 @@ public static boolean canConvertElements(@Nullable TypeDescriptor sourceElementT
 		return false;
 	}
 
-	public static Class<?> getEnumType(Class<?> targetType) {
+	public static @Nullable Class<?> getEnumType(Class<?> targetType) {
 		Class<?> enumType = targetType;
 		while (enumType != null && !enumType.isEnum()) {
 			enumType = enumType.getSuperclass();
 		}
-		Assert.notNull(enumType, () -> "The target type " + targetType.getName() + " does not refer to an enum");
 		return enumType;
 	}
 
+	static class NonConvertableToEnum<T> implements Converter<String, T>, ConditionalConverter {
+
+		private final Class<T> targetType;
+
+		public NonConvertableToEnum(Class<T> targetType) {
+			this.targetType = targetType;
+		}
+
+		@Override
+		public boolean matches(TypeDescriptor sourceType, TypeDescriptor targetType) {
+			return false;
+		}
+
+		@Override
+		public @Nullable T convert(String source) {
+			throw new IllegalArgumentException("The target type " + targetType.getName() + " does not refer to an enum");
+		}
+	}
 }

spring-core/src/main/java/org/springframework/core/convert/support/IntegerToEnumConverterFactory.java

@@ -31,7 +31,11 @@ final class IntegerToEnumConverterFactory implements ConverterFactory<Integer, E
 
 	@Override
 	public <T extends Enum> Converter<Integer, T> getConverter(Class<T> targetType) {
-		return new IntegerToEnum(ConversionUtils.getEnumType(targetType));
+		Class<?> enumType = ConversionUtils.getEnumType(targetType);
+		if (enumType == null) {
+			return new ConversionUtils.NonConvertableToEnum(targetType);
+		}
+		return new IntegerToEnum(enumType);
 	}
 
 

spring-core/src/main/java/org/springframework/core/convert/support/StringToEnumConverterFactory.java

@@ -33,10 +33,13 @@ final class StringToEnumConverterFactory implements ConverterFactory<String, Enu
 
 	@Override
 	public <T extends Enum> Converter<String, T> getConverter(Class<T> targetType) {
-		return new StringToEnum(ConversionUtils.getEnumType(targetType));
+		Class<?> enumType = ConversionUtils.getEnumType(targetType);
+		if (enumType == null) {
+			return new ConversionUtils.NonConvertableToEnum(targetType);
+		}
+		return new StringToEnum(enumType);
 	}
 
-
 	private static class StringToEnum<T extends Enum> implements Converter<String, T> {
 
 		private final Class<T> enumType;

spring-core/src/test/java/org/springframework/core/convert/support/GenericConversionServiceTests.java

@@ -503,6 +503,16 @@ void stringToEnumWithBaseInterfaceConversion() {
 		assertThat(conversionService.convert("base1", MyEnum.class)).isEqualTo(MyEnum.A);
 	}
 
+	@Test
+	void toEnumCanConvertShouldNotThrowException() {
+		conversionService.addConverterFactory(new IntegerToEnumConverterFactory());
+		conversionService.addConverterFactory(new StringToEnumConverterFactory());
+		assertThat(conversionService.canConvert(Integer.class, Enum.class)).isFalse();
+		assertThat(conversionService.canConvert(Integer.class, MyEnum.class)).isTrue();
+		assertThat(conversionService.canConvert(String.class, Enum.class)).isFalse();
+		assertThat(conversionService.canConvert(String.class, MyEnum.class)).isTrue();
+	}
+
 	@Test
 	void convertNullAnnotatedStringToString() throws Exception {
 		String source = null;