Relax type check in FormHttpMessage(Reader|Writer)

Issue: SPR-16055

spring-web/src/main/java/org/springframework/http/codec/FormHttpMessageReader.java

@@ -77,8 +77,9 @@ public class FormHttpMessageReader implements HttpMessageReader<MultiValueMap<St
 
 	@Override
 	public boolean canRead(ResolvableType elementType, @Nullable MediaType mediaType) {
-		return (MULTIVALUE_TYPE.isAssignableFrom(elementType) &&
-				(mediaType == null || MediaType.APPLICATION_FORM_URLENCODED.isCompatibleWith(mediaType)));
+		return (MULTIVALUE_TYPE.isAssignableFrom(elementType)  ||
+				(elementType.hasUnresolvableGenerics() && MultiValueMap.class.isAssignableFrom(elementType.resolve()))) &&
+				(mediaType == null || MediaType.APPLICATION_FORM_URLENCODED.isCompatibleWith(mediaType));
 	}
 
 	@Override

spring-web/src/main/java/org/springframework/http/codec/FormHttpMessageWriter.java

@@ -77,7 +77,8 @@ public class FormHttpMessageWriter implements HttpMessageWriter<MultiValueMap<St
 
 	@Override
 	public boolean canWrite(ResolvableType elementType, @Nullable MediaType mediaType) {
-		return MULTIVALUE_TYPE.isAssignableFrom(elementType) &&
+		return (MULTIVALUE_TYPE.isAssignableFrom(elementType) ||
+				(elementType.hasUnresolvableGenerics() && MultiValueMap.class.isAssignableFrom(elementType.resolve()))) &&
 				(mediaType == null || MediaType.APPLICATION_FORM_URLENCODED.isCompatibleWith(mediaType));
 	}
 

spring-web/src/test/java/org/springframework/http/codec/FormHttpMessageReaderTests.java

@@ -27,6 +27,7 @@ import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpMethod;
 import org.springframework.http.MediaType;
 import org.springframework.mock.http.server.reactive.test.MockServerHttpRequest;
+import org.springframework.util.LinkedMultiValueMap;
 import org.springframework.util.MultiValueMap;
 
 /**
@@ -42,6 +43,10 @@ public class FormHttpMessageReaderTests {
 				ResolvableType.forClassWithGenerics(MultiValueMap.class, String.class, String.class),
 				MediaType.APPLICATION_FORM_URLENCODED));
 
+		assertTrue(this.reader.canRead(
+				ResolvableType.forInstance(new LinkedMultiValueMap<String, String>()),
+				MediaType.APPLICATION_FORM_URLENCODED));
+
 		assertFalse(this.reader.canRead(
 				ResolvableType.forClassWithGenerics(MultiValueMap.class, String.class, Object.class),
 				MediaType.APPLICATION_FORM_URLENCODED));

spring-web/src/test/java/org/springframework/http/codec/FormHttpMessageWriterTests.java

@@ -44,6 +44,10 @@ public class FormHttpMessageWriterTests {
 				ResolvableType.forClassWithGenerics(MultiValueMap.class, String.class, String.class),
 				MediaType.APPLICATION_FORM_URLENCODED));
 
+		assertTrue(this.writer.canWrite(
+				ResolvableType.forInstance(new LinkedMultiValueMap<String, String>()),
+				MediaType.APPLICATION_FORM_URLENCODED));
+
 		assertFalse(this.writer.canWrite(
 				ResolvableType.forClassWithGenerics(MultiValueMap.class, String.class, Object.class),
 				MediaType.APPLICATION_FORM_URLENCODED));