See https://github.com/openjdk/jdk/blob/master/src/jdk.compiler/share/classes/com/sun/tools/javac/code/Types.java#L1410-L1413:

                        if (tMap.containsKey(ti)) {
                            throw new AssertionError("Malformed intersection");
                        }
                        tMap.put(ti.tsym, ti);

Map<Symbol,Type> may not contain keys of type 'Type'

Obvious fix (changing to containsKey(ti.tsym) ) causes the AssertionError to be thrown in the following tests:
tools/javac/lambda/intersection/IntersectionTargetTypeTest.java
tools/javac/cast/intersection/IntersectionTypeCastTest.java

Snippet from the IntersectionTargetTypeTest log:

java.lang.AssertionError: Error thrown when compiling the following code:
..
interface C { void m(); }
..
class Test {
   void m() { }
   void test() {
      Object o = (C & C)()->{};
   } }

The code in question was introduced in JDK-8143852