AssertionError while trying to print a diagnostic

From Raluca Sauciuc and Martin Buchholz, at Google

Hi compiler-dev,

Martin and I ran into an unexpected javac7 stacktrace, and I think the following code exposes a bug:

abstract class Formatter<K> {
   abstract String format(K value);

   static <J> Formatter<Object> fromGetter(final Getter<J> g, final Formatter<J> f) {
       return new Formatter<J>() {
           @Override
           public String format(Object obj) {
               return f.format(g.get(obj));
           }
       };
   }

   interface Getter<K> {
       K get(Object obj);
   }
}

The compiler is trying to print this error message:

error: incompatible types
       return new Formatter<J>() {
              ^
 required: Formatter<java.lang.Object>
 found: <anonymous Formatter<J>>

but fails instead with the following stacktrace:

java.lang.AssertionError: Missing type variable in where clause J
       at com.sun.tools.javac.util.RichDiagnosticFormatter.unique(RichDiagnosticFormatter.java:234)
       at com.sun.tools.javac.util.RichDiagnosticFormatter.access$100(RichDiagnosticFormatter.java:67)
       at com.sun.tools.javac.util.RichDiagnosticFormatter$RichPrinter.visitTypeVar(RichDiagnosticFormatter.java:384)
       at com.sun.tools.javac.util.RichDiagnosticFormatter$RichPrinter.visitTypeVar(RichDiagnosticFormatter.java:326)
       at com.sun.tools.javac.code.Type$TypeVar.accept(Type.java:1030)
       at com.sun.tools.javac.code.Printer.visit(Printer.java:131)
       at com.sun.tools.javac.code.Printer.visitTypes(Printer.java:103)
       at com.sun.tools.javac.code.Printer.visitClassType(Printer.java:194)
       at com.sun.tools.javac.util.RichDiagnosticFormatter$RichPrinter.visitClassType(RichDiagnosticFormatter.java:366)
       at com.sun.tools.javac.util.RichDiagnosticFormatter$RichPrinter.visitClassType(RichDiagnosticFormatter.java:326)
       at com.sun.tools.javac.code.Type$ClassType.accept(Type.java:583)
       at com.sun.tools.javac.code.Printer.visit(Printer.java:131)
       at com.sun.tools.javac.code.Printer.className(Printer.java:265)
       at com.sun.tools.javac.util.RichDiagnosticFormatter$RichPrinter.className(RichDiagnosticFormatter.java:374)
       at com.sun.tools.javac.code.Printer.visitClassType(Printer.java:190)
       at com.sun.tools.javac.util.RichDiagnosticFormatter$RichPrinter.visitClassType(RichDiagnosticFormatter.java:366)
       at com.sun.tools.javac.util.RichDiagnosticFormatter$RichPrinter.visitClassType(RichDiagnosticFormatter.java:326)
       at com.sun.tools.javac.code.Type$ClassType.accept(Type.java:583)
       at com.sun.tools.javac.code.Printer.visit(Printer.java:131)
       at com.sun.tools.javac.util.AbstractDiagnosticFormatter.formatArgument(AbstractDiagnosticFormatter.java:187)
       at com.sun.tools.javac.util.AbstractDiagnosticFormatter.formatArguments(AbstractDiagnosticFormatter.java:158)
       at com.sun.tools.javac.util.BasicDiagnosticFormatter.formatMessage(BasicDiagnosticFormatter.java:111)
       at com.sun.tools.javac.util.BasicDiagnosticFormatter.formatMeta(BasicDiagnosticFormatter.java:184)
       at com.sun.tools.javac.util.BasicDiagnosticFormatter.formatDiagnostic(BasicDiagnosticFormatter.java:100)
       at com.sun.tools.javac.util.AbstractDiagnosticFormatter.format(AbstractDiagnosticFormatter.java:114)
       at com.sun.tools.javac.util.RichDiagnosticFormatter.format(RichDiagnosticFormatter.java:112)
       at com.sun.tools.javac.util.RichDiagnosticFormatter.format(RichDiagnosticFormatter.java:67)
       at com.sun.tools.javac.util.Log.writeDiagnostic(Log.java:434)
       at com.sun.tools.javac.util.Log.report(Log.java:416)
       at com.sun.tools.javac.util.AbstractLog.error(AbstractLog.java:94)
       at com.sun.tools.javac.comp.Check.typeError(Check.java:269)
       at com.sun.tools.javac.comp.Check.checkType(Check.java:452)
       at com.sun.tools.javac.comp.Attr.check(Attr.java:203)
       at com.sun.tools.javac.comp.Attr.visitNewClass(Attr.java:1817)
       at com.sun.tools.javac.tree.JCTree$JCNewClass.accept(JCTree.java:1372)
       at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:431)
       at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:418)
       at com.sun.tools.javac.comp.Attr.attribExpr(Attr.java:449)
       at com.sun.tools.javac.comp.Attr.visitReturn(Attr.java:1384)
       at com.sun.tools.javac.tree.JCTree$JCReturn.accept(JCTree.java:1240)
       at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:431)
       at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:418)
       at com.sun.tools.javac.comp.Attr.attribStat(Attr.java:480)
       at com.sun.tools.javac.comp.Attr.attribStats(Attr.java:496)
       at com.sun.tools.javac.comp.Attr.visitBlock(Attr.java:911)
       at com.sun.tools.javac.tree.JCTree$JCBlock.accept(JCTree.java:781)
       at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:431)
       at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:418)
       at com.sun.tools.javac.comp.Attr.attribStat(Attr.java:480)
       at com.sun.tools.javac.comp.Attr.visitMethodDef(Attr.java:829)
       at com.sun.tools.javac.tree.JCTree$JCMethodDecl.accept(JCTree.java:669)
       at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:431)
       at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:418)
       at com.sun.tools.javac.comp.Attr.attribStat(Attr.java:480)
       at com.sun.tools.javac.comp.Attr.attribClassBody(Attr.java:3254)
       at com.sun.tools.javac.comp.Attr.attribClass(Attr.java:3177)
       at com.sun.tools.javac.comp.Attr.attribClass(Attr.java:3133)
       at com.sun.tools.javac.comp.Attr.attribClass(Attr.java:3113)
       at com.sun.tools.javac.comp.Attr.visitClassDef(Attr.java:701)
       at com.sun.tools.javac.tree.JCTree$JCClassDecl.accept(JCTree.java:591)
       at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:431)
       at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:418)
       at com.sun.tools.javac.comp.Attr.attribStat(Attr.java:480)
       at com.sun.tools.javac.comp.Attr.visitNewClass(Attr.java:1781)
       at com.sun.tools.javac.tree.JCTree$JCNewClass.accept(JCTree.java:1372)
       at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:431)
       at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:418)
       at com.sun.tools.javac.comp.Attr.attribArgs(Attr.java:504)
       at com.sun.tools.javac.comp.Attr.visitNewClass(Attr.java:1647)
       at com.sun.tools.javac.tree.JCTree$JCNewClass.accept(JCTree.java:1372)
       at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:431)
       at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:418)
       at com.sun.tools.javac.comp.Attr.attribExpr(Attr.java:449)
       at com.sun.tools.javac.comp.Attr.visitVarDef(Attr.java:880)
       at com.sun.tools.javac.tree.JCTree$JCVariableDecl.accept(JCTree.java:725)
       at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:431)
       at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:418)
       at com.sun.tools.javac.comp.Attr.attribStat(Attr.java:480)
       at com.sun.tools.javac.comp.Attr.attribStats(Attr.java:496)
       at com.sun.tools.javac.comp.Attr.visitBlock(Attr.java:911)
       at com.sun.tools.javac.tree.JCTree$JCBlock.accept(JCTree.java:781)
       at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:431)
       at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:418)
       at com.sun.tools.javac.comp.Attr.attribStat(Attr.java:480)
       at com.sun.tools.javac.comp.Attr.visitMethodDef(Attr.java:829)
       at com.sun.tools.javac.tree.JCTree$JCMethodDecl.accept(JCTree.java:669)
       at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:431)
       at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:418)
       at com.sun.tools.javac.comp.Attr.attribStat(Attr.java:480)
       at com.sun.tools.javac.comp.Attr.attribClassBody(Attr.java:3254)
       at com.sun.tools.javac.comp.Attr.attribClass(Attr.java:3177)
       at com.sun.tools.javac.comp.Attr.attribClass(Attr.java:3113)
       at com.sun.tools.javac.comp.Attr.attrib(Attr.java:3087)
       at com.sun.tools.javac.main.JavaCompiler.attribute(JavaCompiler.java:1184)
       at com.sun.tools.javac.main.JavaCompiler.compile2(JavaCompiler.java:870)
       at com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:829)
       at com.sun.tools.javac.main.Main.compile(Main.java:439)
       at com.sun.tools.javac.main.Main.compile(Main.java:353)
       at com.sun.tools.javac.main.Main.compile(Main.java:342)
       at com.sun.tools.javac.main.Main.compile(Main.java:333)
       at com.sun.tools.javac.Main.compile(Main.java:76)
       at com.sun.tools.javac.Main.main(Main.java:61)