When running the command

    make JAVAC_WARNINGS="-XDfind=diamond -Xmaxwarns 1000" java.base-only

on a JDK 9 forest, while many diamond warnings are printed, in the end javac exits with a stack trace:

...
536 warnings
An exception has occurred in the compiler (1.9.0-internal). Please file a bug at the Java Developer Connection (http://java.sun.com/webapps/bugreport) after checking the Bug Parade for duplicates. Include your program and the following diagnostic in your report. Thank you.
java.lang.NullPointerException
at com.sun.tools.javac.comp.Check$Validator.visitSelect(Check.java:1365)
at com.sun.tools.javac.tree.JCTree$JCFieldAccess.accept(JCTree.java:2069)
at com.sun.tools.javac.comp.Check$Validator.validateTree(Check.java:1414)
at com.sun.tools.javac.comp.Check$Validator.visitWildcard(Check.java:1360)
at com.sun.tools.javac.tree.JCTree$JCWildcard.accept(JCTree.java:2448)
at com.sun.tools.javac.comp.Check$Validator.validateTree(Check.java:1414)
at com.sun.tools.javac.comp.Check$Validator.visitTypeApply(Check.java:1335)
at com.sun.tools.javac.tree.JCTree$JCTypeApply.accept(JCTree.java:2320)
at com.sun.tools.javac.comp.Check$Validator.validateTree(Check.java:1414)
at com.sun.tools.javac.comp.Check.validate(Check.java:1285)
at com.sun.tools.javac.comp.Attr.visitVarDef(Attr.java:1046)
at com.sun.tools.javac.tree.JCTree$JCVariableDecl.accept(JCTree.java:920)
at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:570)
at com.sun.tools.javac.comp.Attr.attribStat(Attr.java:639)
at com.sun.tools.javac.comp.Attr.visitMethodDef(Attr.java:911)
at com.sun.tools.javac.tree.JCTree$JCMethodDecl.accept(JCTree.java:836)
at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:570)
at com.sun.tools.javac.comp.Attr.attribStat(Attr.java:639)
at com.sun.tools.javac.comp.Attr.attribClassBody(Attr.java:4325)
at com.sun.tools.javac.comp.Attr.attribClass(Attr.java:4232)
at com.sun.tools.javac.comp.Attr.attribClass(Attr.java:4161)
at com.sun.tools.javac.comp.Attr.visitClassDef(Attr.java:862)
at com.sun.tools.javac.tree.JCTree$JCClassDecl.accept(JCTree.java:744)
at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:570)
at com.sun.tools.javac.comp.Attr.attribStat(Attr.java:639)
at com.sun.tools.javac.comp.Attr.visitAnonymousClassDefinition(Attr.java:2197)
at com.sun.tools.javac.comp.Attr.visitNewClass(Attr.java:2097)
at com.sun.tools.javac.tree.JCTree$JCNewClass.accept(JCTree.java:1653)
at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:570)
at com.sun.tools.javac.comp.Attr.visitReturn(Attr.java:1677)
at com.sun.tools.javac.tree.JCTree$JCReturn.accept(JCTree.java:1510)
at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:570)
at com.sun.tools.javac.comp.Attr.attribStat(Attr.java:639)
at com.sun.tools.javac.comp.Attr.attribStats(Attr.java:658)
at com.sun.tools.javac.comp.Attr.visitBlock(Attr.java:1114)
at com.sun.tools.javac.tree.JCTree$JCBlock.accept(JCTree.java:984)
at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:570)
at com.sun.tools.javac.comp.DeferredAttr.attribSpeculative(DeferredAttr.java:410)
at com.sun.tools.javac.comp.Analyzer.analyze(Analyzer.java:356)
at com.sun.tools.javac.comp.Analyzer.analyzeIfNeeded(Analyzer.java:336)
at com.sun.tools.javac.comp.Attr.attribStat(Attr.java:641)
at com.sun.tools.javac.comp.Attr.attribStats(Attr.java:658)
at com.sun.tools.javac.comp.Attr.visitBlock(Attr.java:1114)
at com.sun.tools.javac.tree.JCTree$JCBlock.accept(JCTree.java:984)
at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:570)
at com.sun.tools.javac.comp.Attr.attribStat(Attr.java:639)
at com.sun.tools.javac.comp.Attr.visitMethodDef(Attr.java:1005)
at com.sun.tools.javac.tree.JCTree$JCMethodDecl.accept(JCTree.java:836)
at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:570)
at com.sun.tools.javac.comp.Attr.attribStat(Attr.java:639)
at com.sun.tools.javac.comp.Attr.attribClassBody(Attr.java:4325)
at com.sun.tools.javac.comp.Attr.attribClass(Attr.java:4232)
at com.sun.tools.javac.comp.Attr.attribClass(Attr.java:4161)
at com.sun.tools.javac.comp.Attr.attrib(Attr.java:4136)
at com.sun.tools.javac.main.JavaCompiler.attribute(JavaCompiler.java:1223)
at com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:865)
at com.sun.tools.javac.main.Main.compile(Main.java:253)
at com.sun.tools.javac.main.Main.compile(Main.java:141)
at com.sun.tools.javac.Main.compile(Main.java:56)
at com.sun.tools.javac.Main.main(Main.java:42)