2964193af8
This PR fixes a bug in `Sym.introCore.finalize` where the original metavariable was unconditionally assigned via a delayed assignment, even when no binders were introduced. As a result, `Sym.intros` would return `.failed` while the goal metavariable had already been silently assigned, confusing downstream code that relies on `isAssigned` (e.g. VC filters in `mvcgen'`). The test and fix were suggested by Sebastian Graf (@sgraf812). Co-authored-by: Sebastian Graf <sgraf1337@gmail.com> Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
38 lines
1.7 KiB
Text
38 lines
1.7 KiB
Text
/-
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MWE: `Sym.introCore` assigns the original mvar even when no binders are introduced.
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`Sym.intros` returns `.failed` but the mvar is secretly assigned.
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The bug is in `introCore.finalize`: it unconditionally creates an `auxMVar` with a
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delayed assignment and assigns the original mvar to it, even when `fvars` is empty
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(no binders were introduced). Then `intros` checks `fvars.isEmpty` and returns `.failed`,
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but the mvar is already assigned. Downstream code that checks `isAssigned` (e.g. VC filters
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in mvcgen') will wrongly think the goal is solved.
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Fix: guard `finalize` with `if fvars.isEmpty then return (#[], mvarId)`.
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-/
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import Lean
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-- Demonstrate the bug by calling `Sym.intros` on a non-forall goal.
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-- With the fix in `introCore.finalize`, this prints "GOOD".
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-- Without the fix, it prints "BUG".
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/--
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info: before intros: isAssigned=false
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intros returned .failed (as expected for non-forall target)
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after intros: isAssigned=false, isDelayedAssigned=false
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GOOD: mvar is still unassigned
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-/
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#guard_msgs in
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open Lean Meta Sym in
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#eval show MetaM Unit from do
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let goal ← mkFreshExprMVar (mkConst ``False) .syntheticOpaque
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let goalId := goal.mvarId!
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IO.println s!"before intros: isAssigned={← goalId.isAssigned}"
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let result ← Sym.SymM.run (Sym.intros goalId)
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match result with
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| .failed => IO.println "intros returned .failed (as expected for non-forall target)"
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| .goal _ _ => IO.println "intros returned .goal (unexpected)"
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IO.println s!"after intros: isAssigned={← goalId.isAssigned}, isDelayedAssigned={← goalId.isDelayedAssigned}"
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if (← goalId.isAssigned) then
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IO.println "BUG: mvar was assigned despite intros returning .failed"
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else
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IO.println "GOOD: mvar is still unassigned"
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