Build test code using JDK 17

This enables usage of text blocks, which in combination with IntelliJ
IDEA's _language injection_ functionality greatly simplify writing
`BugChecker` tests.

error-prone-contrib/README.md

@@ -33,16 +33,11 @@ Two other goals that one may find relevant:
   `target/pit-reports/index.html` files. For more information check the [PIT
   Maven plugin][pitest-maven].
 
-When running the project's tests in IntelliJ IDEA, you might see the following
-error:
-```
-java: exporting a package from system module jdk.compiler is not allowed with --release
-```
-
-If this happens, go to _Settings -> Build, Execution, Deployment -> Compiler ->
-Java Compiler_ and deselect the option _Use '--release' option for
-cross-compilation (Java 9 and later)_. See [IDEA-288052][idea-288052] for
-details.
+The `BugChecker` implementations provided by this project are tested using
+Error Prone's `CompilationTestHelper` and `BugCheckerRefactoringTestHelper`
+classes. These utilities accept text blocks containing inline Java source code.
+To ease modification of this inline source code, consider using IntelliJ IDEA's
+[language injection][idea-language-injection] feature.
 
 ### Contribution guidelines
 
@@ -342,7 +337,7 @@ Refaster's expressiveness:
 [forbidden-apis]: https://github.com/policeman-tools/forbidden-apis
 [fossa]: https://fossa.io
 [google-java-format]: https://github.com/google/google-java-format
-[idea-288052]: https://youtrack.jetbrains.com/issue/IDEA-288052
+[idea-language-injection]: https://www.jetbrains.com/help/idea/using-language-injections.html
 [maven]: https://maven.apache.org
 [modernizer-maven-plugin]: https://github.com/gaul/modernizer-maven-plugin
 [pitest]: https://pitest.org

pom.xml

@@ -150,6 +150,7 @@
         <version.findbugs-format-string>3.0.0</version.findbugs-format-string>
         <version.guava-beta-checker>1.0</version.guava-beta-checker>
         <version.jdk>11</version.jdk>
+        <version.jdk.test>17</version.jdk.test>
         <version.maven>3.6.3</version.maven>
         <version.mockito>4.6.1</version.mockito>
         <version.nopen-checker>1.0.1</version.nopen-checker>
@@ -854,6 +855,8 @@
                         <parameters>true</parameters>
                         <source>${version.jdk}</source>
                         <target>${version.jdk}</target>
+                        <testSource>${version.jdk.test}</testSource>
+                        <testTarget>${version.jdk.test}</testTarget>
                         <!-- Erroneously inverted logic... for details, see
                         https://issues.apache.org/jira/browse/MCOMPILER-209. -->
                         <useIncrementalCompilation>false</useIncrementalCompilation>
@@ -897,7 +900,7 @@
                                 <includes>src/main/resources/**/*.properties,src/test/resources/**/*.properties</includes>
                             </requireEncoding>
                             <requireJavaVersion>
-                                <version>${version.jdk}</version>
+                                <version>${version.jdk.test}</version>
                             </requireJavaVersion>
                             <requireMavenVersion>
                                 <version>${version.maven}</version>
@@ -1740,5 +1743,32 @@
                 </plugins>
             </build>
         </profile>
+        <profile>
+            <!-- This profile is active when the project is loaded into
+            IntelliJ IDEA. Drop it once IDEA properly supports the `testSource`
+            and `testTarget` settings of the `maven-compiler-plugin`, such that
+            specifying different language levels for main and test code works
+            out of the box. See
+            https://youtrack.jetbrains.com/issue/IDEA-85478. -->
+            <id>idea</id>
+            <activation>
+                <property>
+                    <name>idea.maven.embedder.version</name>
+                </property>
+            </activation>
+            <build>
+                <pluginManagement>
+                    <plugins>
+                        <plugin>
+                            <groupId>org.apache.maven.plugins</groupId>
+                            <artifactId>maven-compiler-plugin</artifactId>
+                            <configuration>
+                                <release>${version.jdk.test}</release>
+                            </configuration>
+                        </plugin>
+                    </plugins>
+                </pluginManagement>
+            </build>
+        </profile>
     </profiles>
 </project>