[release/10.0] Fix LoadExactInterfaceMap for sub-interfaces with complex type arguments under special marker parents (#124835)

Backport of #124684 to release/10.0

/cc @davidwrighton

## Customer Impact

- [x] Customer reported
- [ ] Found internally

The earlier fix for #123254 did not fix all possible cases where the
incorrect type would be loaded. This fix fills out the special case
matrix some more, and I believe should fix all the issues found.

## Regression

- [x] Yes
- [ ] No

This issue was introduced with PR #120712, and mitigated somewhat with
PR #123520. This fix builds on that fix to fix additional issues found
by @reduckted.

## Testing

The fix includes a fairly large test suite covering possible ways type
loading could fail around this fix. It was missed previously in the
previous small fix as the importance of comparing the fully loaded, and
partially loaded interface maps on the generic type definition was
missed during test development. This fix has also been tested by
delivering a hotfixed build to @reduckted which which he was able to
verify fixed the issues he had seen in production as well as in testing.

## Risk

Low. Its filling out more special cases in the new codepaths added in PR
#123520.

**IMPORTANT**: If this backport is for a servicing release, please
verify that:

- For .NET 8 and .NET 9: The PR target branch is `release/X.0-staging`,
not `release/X.0`.
- For .NET 10+: The PR target branch is `release/X.0` (no `-staging`
suffix).

## Package authoring no longer needed in .NET 9

**IMPORTANT**: Starting with .NET 9, you no longer need to edit a NuGet
package's csproj to enable building and bump the version.
Keep in mind that we still need package authoring in .NET 8 and older
versions.

---------

Co-authored-by: David Wrighton <davidwr@microsoft.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: 4 people

src/coreclr/vm/methodtablebuilder.cpp

@@ -9669,13 +9669,22 @@ MethodTableBuilder::LoadExactInterfaceMap(MethodTable *pMT)
                     {
                         MethodTable *pItfPossiblyApprox = intIt.GetInterfaceApprox();
 
-                        // pItfPossiblyApprox can be in 4 situations
+                        // pItfPossiblyApprox can be in 6 situations
                         // 1. It has no instantiation
-                        // 2. It is a special marker type, AND pNewItfMT is a special marker type. Compute the exact instantiation as containing entirely a list of types corresponding to calling GetSpecialInstantiationType on pMT (This rule works based on the current behavior of GetSpecialInstantiationType where it treats all interfaces the same)
-                        // 3. It is a special marker type, but pNewItfMT is NOT a special marker type. Compute the exact instantiation as containing entirely a list of types corresponding to calling GetSpecialInstantiationType on pNewItfMT
-                        // 4. It is an exact instantiation
+                        // 2. It is a special marker type, AND pNewIntfMT is a special marker type. Compute the exact instantiation as containing entirely a list of
+                        //    types corresponding to calling GetSpecialInstantiationType on pMT (This rule works based on the current behavior of
+                        //    GetSpecialInstantiationType where it treats all interfaces the same)
+                        // 3. It is a special marker type, but pNewIntfMT is NOT a special marker type. Compute the exact instantiation as containing entirely a list
+                        //    of types corresponding to calling GetSpecialInstantiationType on pNewIntfMT
+                        // 4. It is an exact instantiation, but pNewIntfMT was a special marker type, and the exact instantiation type found here could have been a
+                        //    special marker type. This should produce a result equivalent to case 2 (the special marker type)
+                        // 5. It is an exact instantiation, but pNewIntfMT was a special marker type, and the exact instantiation type is NOT one which would have
+                        //    been on the exact instantiation of pNewIntfMT if it were not a special marker type. In theory we could reconstruct this, but this is a
+                        //    rare scenario, so we just fallback to the retry with exact interfaces pathway
+                        // 6. It is an exact instantiation, and pNewIntfMT is NOT a special marker type, compute the result and insert either a special marker or
+                        //    the exact instantiation already found.
                         //
-                        // NOTE: pItfPossiblyApprox must not be considered a special marker type if pNewItfMT has the MayHaveOpenInterfacesInInterfaceMap flag set
+                        // NOTE: pItfPossiblyApprox must not be considered a special marker type if pNewIntfMT has the MayHaveOpenInterfacesInInterfaceMap flag set
                         //
                         // Then determine if all of the following conditions hold true.
                         // 1. All generic arguments are the same (always true for cases 2 and 3 above)
@@ -9725,17 +9734,40 @@ MethodTableBuilder::LoadExactInterfaceMap(MethodTable *pMT)
                         }
                         else
                         {
-                            // case 4 (We have an exact interface)
-                            if (ClassLoader::EligibleForSpecialMarkerTypeUsage(pItfPossiblyApprox->GetInstantiation(), pMT))
+                            // case 4, 5, or 6 (We have an exact interface)
+                            bool pNewIntfMTIsSpecialMarkerType = pNewIntfMT->IsSpecialMarkerTypeForGenericCasting() && !pMT->GetAuxiliaryData()->MayHaveOpenInterfacesInInterfaceMap();
+                            if (pNewIntfMTIsSpecialMarkerType)
                             {
-                                // Validated that all generic arguments are the same, and that the first generic argument in the instantiation is exactly the value of calling GetSpecialInstantiationType on pMT
-                                // Then use the special marker type here
-                                pItfToInsert = ClassLoader::LoadTypeDefThrowing(pItfPossiblyApprox->GetModule(), pItfPossiblyApprox->GetCl(), ClassLoader::ThrowIfNotFound, ClassLoader::PermitUninstDefOrRef, 0, CLASS_LOAD_EXACTPARENTS).AsMethodTable();
+                                if (ClassLoader::EligibleForSpecialMarkerTypeUsage(pItfPossiblyApprox->GetInstantiation(), pNewIntfMT))
+                                {
+                                    // Case 4 - we have an exact instantiation, but pNewIntfMT was a special marker type, and the exact instantiation type found here could have been a special marker type. We need to check if the exact instantiation we found
+                                    // here could have been a special marker type. If it is, we need to insert the special marker type here.
+                                    pItfToInsert = ClassLoader::LoadTypeDefThrowing(pItfPossiblyApprox->GetModule(), pItfPossiblyApprox->GetCl(), ClassLoader::ThrowIfNotFound, ClassLoader::PermitUninstDefOrRef, 0, CLASS_LOAD_EXACTPARENTS).AsMethodTable();
+                                }
+                                else if (pItfPossiblyApprox->ContainsGenericVariables())
+                                {
+                                    // Case 5
+                                    // If the instantiation contains generic variables and can't be converted to the special marker type, then we would need to fully re-instantiate the type with a deep substitution to get the exact instantiation, which is complex and expensive, and we expect this to be a rare case, so we can just fallback to the retry with exact interfaces pathway in this case.
+                                    retry = true;
+                                    break;
+                                }
+                                // If we reach here, we've already found the correct pItfToInsert (case 4), OR we can proceed to case 6 since pItfPossiblyApprox was an exact type defined on the generic type.
                             }
-                            else
+
+                            if (pItfToInsert == NULL)
                             {
-                                pItfToInsert = pItfPossiblyApprox;
-                                intendedExactMatch = true;
+                                // Case 6, this is an exact instantiation of exactly the right type. Insert it here.
+                                if (ClassLoader::EligibleForSpecialMarkerTypeUsage(pItfPossiblyApprox->GetInstantiation(), pMT))
+                                {
+                                    // Validated that all generic arguments are the same, and that the first generic argument in the instantiation is exactly the value of calling GetSpecialInstantiationType on pMT
+                                    // Then use the special marker type here
+                                    pItfToInsert = ClassLoader::LoadTypeDefThrowing(pItfPossiblyApprox->GetModule(), pItfPossiblyApprox->GetCl(), ClassLoader::ThrowIfNotFound, ClassLoader::PermitUninstDefOrRef, 0, CLASS_LOAD_EXACTPARENTS).AsMethodTable();
+                                }
+                                else
+                                {
+                                    pItfToInsert = pItfPossiblyApprox;
+                                    intendedExactMatch = true;
+                                }
                             }
                         }