Investigate JIT assert 'block->HasFlag(BBF_RECURSIVE_TAILCALL)' during Morph - Inlining under PGO stress (no fix applied)

[Copilot]

CI reported a Checked-JIT assert (block->HasFlag(BBF_RECURSIVE_TAILCALL)) firing during "Morph - Inlining" under PGO stress, across multiple libraries-pgo work items with differing inlinee methods.

Investigation

• Reproduced/analyzed the assert and traced both flag-setting call sites (impTransformDevirtualizedCall and the direct-call recursive-tailcall check in impImportCall) plus the IndirectCallTransformer GDV-expansion code.
• Implemented an initial hypothesis (stale compCurBB/block after multi-block inlining splits in fgInline/fgInsertInlineeBlocks), then disproved it via a full Checked CoreCLR build + crossgen2-corelib compile: it introduced new, unrelated asserts. Fully reverted.
• Confirmed JIT: Fix Crossgen2 assertion for non-inlineable tail-call GDV #133014 (merged before this issue's CI run) already fixed one variant of this bug, yet the failing build already includes that fix — there's a distinct, still-unfixed gap for the inlineable GDV-candidate case that PR's author flagged as needing further work.
• Built and ran several targeted repros (self-recursive virtual calls, polymorphic base/derived recursion, interface-based devirtualization-to-recursive-target scenarios) against a real Checked JIT with DOTNET_JitRandomGuardedDevirtualization stress and full tiered PGO — none reproduced the assert against the unmodified baseline JIT.

Status

Root cause not conclusively identified within this session. No code change is included — the tree matches upstream main — since landing an unverified fix risks repeating the regression already caught and reverted during this investigation. Recommend obtaining a SuperPMI MC/mch replay of one of the actual failing inlinee methods (e.g. MethodSymbol:get_HasSetsRequiredMembers()) to get a deterministic, minimal repro before attempting another fix.

• Fixes [ci-scan] Test failure: JIT assert 'block->HasFlag(BBF_RECURSIVE_TAILCALL)' during Morph - Inlining under PGO stress #133352

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