[release/10.0] Fix LoadExactInterfaceMap for sub-interfaces with complex type arguments under special marker parents#124835
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JulieLeeMSFT merged 4 commits intorelease/10.0from Mar 4, 2026
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…nts under special marker parents When pNewIntfMT is a special marker type and its sub-interface has an exact instantiation containing complex type arguments (e.g. IList(Of T) rather than bare T), the previous code would fall through to case 4 which checked EligibleForSpecialMarkerTypeUsage against pMT. This could incorrectly treat the sub-interface as an exact match when it still contains unresolved generic variables. Split old case 4 into three new cases: - Case 4: pNewIntfMT is a special marker type and the sub-interface's instantiation is eligible for special marker usage relative to pNewIntfMT (e.g. ILayer1(Of T) where T matches). Insert the marker. - Case 5: pNewIntfMT is a special marker type but the sub-interface has generic variables that don't match (e.g. ILayer1(Of IList(Of T))). Trigger retry with exact interfaces since full substitution is needed. - Case 6: The sub-interface is fully concrete or pNewIntfMT is not a special marker. Fall through to the existing pMT-based eligibility check. Add regression tests covering all 8 combinations of pre-loading the 3 interface layers. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
jkotas
approved these changes
Feb 25, 2026
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Backport of #124684 to release/10.0
/cc @davidwrighton
Customer Impact
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
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:
release/X.0-staging, notrelease/X.0.release/X.0(no-stagingsuffix).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.