Make 2 flavors of parametrized cache: nullable and notnull

python/src/com/jetbrains/python/psi/PyUtil.java

@@ -843,7 +843,17 @@ public class PyUtil {
     });
   }
 
-  public static <T, P> T getParameterizedCachedValue(@NotNull PsiElement element, @Nullable P param, @NotNull NullableFunction<P, T> f) {
+  @NotNull
+  public static <T, P> T getParameterizedCachedValue(@NotNull PsiElement element, @Nullable P param, @NotNull NotNullFunction<P, T> f) {
+    final T result = getNullableParameterizedCachedValue(element, param, f);
+    assert result != null;
+    return result;
+  }
+
+  @Nullable
+  public static <T, P> T getNullableParameterizedCachedValue(@NotNull PsiElement element,
+                                                             @Nullable P param,
+                                                             @NotNull NullableFunction<P, T> f) {
     final CachedValuesManager manager = CachedValuesManager.getManager(element.getProject());
     final Map<Optional<P>, Optional<T>> cache = CachedValuesManager.getCachedValue(element, manager.getKeyForClass(f.getClass()), () -> {
       // concurrent hash map is a null-hostile collection
@@ -899,7 +909,7 @@ public class PyUtil {
 
   /**
    * Returns the line comment that immediately precedes statement list of the given compound statement. Python parser ensures
-   * that it follows the statement header, i.e. it's directly after the colon, not on its own line. 
+   * that it follows the statement header, i.e. it's directly after the colon, not on its own line.
    */
   @Nullable
   public static PsiComment getCommentOnHeaderLine(@NotNull PyStatementListContainer container) {

python/src/com/jetbrains/python/psi/impl/PyClassImpl.java

@@ -79,7 +79,7 @@ public class PyClassImpl extends PyBaseElementImpl<PyClassStub> implements PyCla
   /**
    * Engine to create list of ancestors based on context
    */
-  private final class CachedAncestorsProvider implements NullableFunction<TypeEvalContext, List<PyClassLikeType>> {
+  private final class CachedAncestorsProvider implements NotNullFunction<TypeEvalContext, List<PyClassLikeType>> {
     @NotNull
     @Override
     public List<PyClassLikeType> fun(@NotNull TypeEvalContext context) {

python/src/com/jetbrains/python/psi/resolve/QualifiedNameFinder.java

@@ -115,7 +115,7 @@ public class QualifiedNameFinder {
    */
   @Nullable
   public static QualifiedName findCanonicalImportPath(@NotNull PsiElement symbol, @Nullable PsiElement foothold) {
-    return PyUtil.getParameterizedCachedValue(symbol, Couple.of(symbol, foothold), QualifiedNameFinder::doFindCanonicalImportPath);
+    return PyUtil.getNullableParameterizedCachedValue(symbol, Couple.of(symbol, foothold), QualifiedNameFinder::doFindCanonicalImportPath);
   }
 
   @Nullable