correct fold region length assertions, allow single character folding

platform/lang-api/src/com/intellij/lang/folding/FoldingDescriptor.java

@@ -94,12 +94,12 @@ public class FoldingDescriptor {
                            @Nullable FoldingGroup group,
                            Set<Object> dependencies,
                            boolean neverExpands) {
-    assert range.getStartOffset() + 1 < range.getEndOffset() : range + ", text: " + node.getText() + ", language = " + node.getPsi().getLanguage();
+    assert range.getStartOffset() + 1 <= range.getEndOffset() : range + ", text: " + node.getText() + ", language = " + node.getPsi().getLanguage();
     myElement = node;
     ProperTextRange.assertProperRange(range);
     myRange = range;
     myGroup = group;
-    assert getRange().getLength() >= 2 : "range:" + getRange();
+    assert getRange().getLength() >= 1 : "range:" + getRange();
     myDependencies = dependencies;
     myNeverExpands = neverExpands;
   }

platform/platform-impl/src/com/intellij/openapi/editor/impl/FoldRegionImpl.java

@@ -91,7 +91,7 @@ public class FoldRegionImpl extends RangeMarkerImpl implements FoldRegion {
 
   @Override
   public boolean isValid() {
-    return super.isValid() && intervalStart() + 1 < intervalEnd();
+    return super.isValid() && intervalStart() + 1 <= intervalEnd();
   }
 
   public void setExpandedInternal(boolean toExpand) {

platform/platform-impl/src/com/intellij/openapi/editor/impl/FoldingModelImpl.java

@@ -503,7 +503,7 @@ public class FoldingModelImpl implements FoldingModelEx, PrioritizedDocumentList
   public FoldRegion createFoldRegion(int startOffset, int endOffset, @NotNull String placeholder, @Nullable FoldingGroup group,
                                      boolean neverExpands)
   {
-    if (startOffset + 1 >= endOffset) {
+    if (startOffset + 1 > endOffset) {
       LOG.error("Invalid offsets: ("+startOffset+", "+endOffset+")");
     }
     FoldRegionImpl region = new FoldRegionImpl(myEditor, startOffset, endOffset, placeholder, group, neverExpands);