The current makefiles specify a -target value but don't specify a -source value. If the javac being used has a default source level different than the requested -target then compilation fails.

Suggested patch:

diff --git a/make/jtdiff.gmk b/make/jtdiff.gmk
--- a/make/jtdiff.gmk
+++ b/make/jtdiff.gmk
@@ -34,7 +34,7 @@ JAVAFILES.com.sun.javatest.diff := \
                $(JAVAFILES.com.sun.javatest.diff) \
                $(BUILDDIR)/classes.com.sun.javatest.regtest.ok
        CLASSPATH=$(CLASSDIR):$(JAVATEST_JAR):$(JAVAHELP_JAR):$(ANTHOME)/lib/ant.jar \
- $(J2SEJAVAC) -target $(J2SEJAVAC_TARGET) -encoding ISO8859-1 -d $(CLASSDIR) \
+ $(J2SEJAVAC) -source $(J2SEJAVAC_TARGET) -target $(J2SEJAVAC_TARGET) -encoding ISO8859-1 -d $(CLASSDIR) \
            $(JAVAFILES.com.sun.javatest.diff)
        echo "classes built at `date`" > $@
 
@@ -78,5 +78,3 @@ TARGETS.ZIP.jtreg += \
 #----------------------------------------------------------------------
 
 TESTS += $(TESTS.jtdiff)
-
-
diff --git a/make/jtreg.gmk b/make/jtreg.gmk
--- a/make/jtreg.gmk
+++ b/make/jtreg.gmk
@@ -48,7 +48,7 @@ JAVAFILES.com.sun.javatest.regtest-j2se
                $(JAVAFILES.com.sun.javatest.regtest-j2se) \
                $(BUILDDIR)/classes.com.sun.javatest.regtest-jdk11.ok
        CLASSPATH=$(CLASSDIR):$(ANTHOME)/lib/ant.jar:$(JAVATEST_JAR):$(JUNIT_JAR):$(TESTNG_JAR):$(JAVAHELP_JAR) \
- $(J2SEJAVAC) -target $(J2SEJAVAC_TARGET) -encoding ISO8859-1 -d $(CLASSDIR) \
+ $(J2SEJAVAC) -source $(J2SEJAVAC_TARGET) -target $(J2SEJAVAC_TARGET) -encoding ISO8859-1 -d $(CLASSDIR) \
                $(JAVAFILES.com.sun.javatest.regtest-j2se)
        echo "classes built at `date`" > $@