method hierarchy: don't merge methods with return types which are not substitutable (IDEA-57393)

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/genericsHighlighting/ReturnTypeSubstitutableForSameOverrideEquivalentMethods.java

@@ -0,0 +1,8 @@
+interface I<T, S extends Throwable> {
+  int foo(T x);
+  void foo(S x);
+}
+
+class A implements I<Throwable, Throwable>{
+  public <error descr="'foo(Throwable)' in 'A' clashes with 'foo(T)' in 'I'; attempting to use incompatible return type">void</error> foo(Throwable x) { }
+}