fix: allow arbitrary sorts in structural recursion over reflexive inductive types (#7639)

cppio · web-flow · commit 5d50433e6ac6 · 2025-06-13T21:51:09.000Z
This PR changes the generated `below` and `brecOn` implementations for reflexive inductive types to support motives in `Sort u` rather than `Type u`. Closes #7638
diff --git a/src/Lean/Elab/PreDefinition/Structural/BRecOn.lean b/src/Lean/Elab/PreDefinition/Structural/BRecOn.lean
@@ -240,14 +240,7 @@ def mkBRecOnConst (recArgInfos : Array RecArgInfo) (positions : Positions)
   let indGroup := recArgInfos[0]!.indGroupInst
   let motive := motives[0]!
   let brecOnUniv ← lambdaTelescope motive fun _ type => getLevel type
-  let indInfo ← getConstInfoInduct indGroup.all[0]!
-  let useBInductionOn := indInfo.isReflexive && brecOnUniv == levelZero
-  let brecOnUniv ←
-    if indInfo.isReflexive && brecOnUniv != levelZero then
-      decLevel brecOnUniv
-    else
-      pure brecOnUniv
-  let brecOnCons := fun idx => indGroup.brecOn useBInductionOn brecOnUniv idx
+  let brecOnCons := fun idx => indGroup.brecOn false brecOnUniv idx
   -- Pick one as a prototype
   let brecOnAux := brecOnCons 0
   -- Infer the type of the packed motive arguments
diff --git a/src/Lean/Elab/PreDefinition/Structural/FindRecArg.lean b/src/Lean/Elab/PreDefinition/Structural/FindRecArg.lean
@@ -70,41 +70,38 @@ def getRecArgInfo (fnName : Name) (fixedParamPerm : FixedParamPerm) (xs : Array
       throwError "it is a let-binding"
     let xType ← whnfD localDecl.type
     matchConstInduct xType.getAppFn (fun _ => throwError "its type is not an inductive") fun indInfo us => do
-    if indInfo.isReflexive && !(← hasConst (mkBInductionOnName indInfo.name)) && !(← isInductivePredicate indInfo.name) then
-      throwError "its type {indInfo.name} is a reflexive inductive, but {mkBInductionOnName indInfo.name} does not exist and it is not an inductive predicate"
+    let indArgs    : Array Expr := xType.getAppArgs
+    let indParams  : Array Expr := indArgs[0:indInfo.numParams]
+    let indIndices : Array Expr := indArgs[indInfo.numParams:]
+    if !indIndices.all Expr.isFVar then
+      throwError "its type {indInfo.name} is an inductive family and indices are not variables{indentExpr xType}"
+    else if !indIndices.allDiff then
+      throwError "its type {indInfo.name} is an inductive family and indices are not pairwise distinct{indentExpr xType}"
     else
-      let indArgs    : Array Expr := xType.getAppArgs
-      let indParams  : Array Expr := indArgs[0:indInfo.numParams]
-      let indIndices : Array Expr := indArgs[indInfo.numParams:]
-      if !indIndices.all Expr.isFVar then
-        throwError "its type {indInfo.name} is an inductive family and indices are not variables{indentExpr xType}"
-      else if !indIndices.allDiff then
-        throwError "its type {indInfo.name} is an inductive family and indices are not pairwise distinct{indentExpr xType}"
-      else
-        let ys := fixedParamPerm.pickVarying xs
-        match (← hasBadIndexDep? ys indIndices) with
-        | some (index, y) =>
-          throwError "its type {indInfo.name} is an inductive family{indentExpr xType}\nand index{indentExpr index}\ndepends on the non index{indentExpr y}"
+      let ys := fixedParamPerm.pickVarying xs
+      match (← hasBadIndexDep? ys indIndices) with
+      | some (index, y) =>
+        throwError "its type {indInfo.name} is an inductive family{indentExpr xType}\nand index{indentExpr index}\ndepends on the non index{indentExpr y}"
+      | none =>
+        match (← hasBadParamDep? ys indParams) with
+        | some (indParam, y) =>
+          throwError "its type is an inductive datatype{indentExpr xType}\nand the datatype parameter{indentExpr indParam}\ndepends on the function parameter{indentExpr y}\nwhich is not fixed."
         | none =>
-          match (← hasBadParamDep? ys indParams) with
-          | some (indParam, y) =>
-            throwError "its type is an inductive datatype{indentExpr xType}\nand the datatype parameter{indentExpr indParam}\ndepends on the function parameter{indentExpr y}\nwhich is not fixed."
-          | none =>
-            let indAll := indInfo.all.toArray
-            let .some indIdx := indAll.idxOf? indInfo.name | panic! "{indInfo.name} not in {indInfo.all}"
-            let indicesPos := indIndices.map fun index => match xs.idxOf? index with | some i => i | none => unreachable!
-            let indGroupInst := {
-              IndGroupInfo.ofInductiveVal indInfo with
-              levels := us
-              params := indParams }
-            return { fnName       := fnName
-                     fixedParamPerm := fixedParamPerm
-                     recArgPos    := i
-                     indicesPos   := indicesPos
-                     indGroupInst := indGroupInst
-                     indIdx       := indIdx }
-    else
-      throwError "the index #{i+1} exceeds {xs.size}, the number of parameters"
+          let indAll := indInfo.all.toArray
+          let .some indIdx := indAll.idxOf? indInfo.name | panic! "{indInfo.name} not in {indInfo.all}"
+          let indicesPos := indIndices.map fun index => match xs.idxOf? index with | some i => i | none => unreachable!
+          let indGroupInst := {
+            IndGroupInfo.ofInductiveVal indInfo with
+            levels := us
+            params := indParams }
+          return { fnName       := fnName
+                   fixedParamPerm := fixedParamPerm
+                   recArgPos    := i
+                   indicesPos   := indicesPos
+                   indGroupInst := indGroupInst
+                   indIdx       := indIdx }
+  else
+    throwError "the index #{i+1} exceeds {xs.size}, the number of parameters"
 
 /--
 Collects the `RecArgInfos` for one function, and returns a report for why the others were not
diff --git a/src/Lean/Meta/Constructions/BRecOn.lean b/src/Lean/Meta/Constructions/BRecOn.lean
@@ -13,14 +13,6 @@ import Lean.Meta.PProdN
 namespace Lean
 open Meta
 
-/-- Transforms `e : xᵢ → (t₁ ×' t₂)` into `(xᵢ → t₁) ×' (xᵢ → t₂) -/
-private def etaPProd (xs : Array Expr) (e : Expr) : MetaM Expr := do
-  if xs.isEmpty then return e
-  let r := mkAppN e xs
-  let r₁ ← mkLambdaFVars xs (← mkPProdFstM r)
-  let r₂ ← mkLambdaFVars xs (← mkPProdSndM r)
-  mkPProdMk r₁ r₂
-
 /--
 If `minorType` is the type of a minor premies of a recursor, such as
 ```
@@ -40,7 +32,6 @@ of type
 private def buildBelowMinorPremise (rlvl : Level) (motives : Array Expr) (minorType : Expr) : MetaM Expr :=
   forallTelescope minorType fun minor_args _ => do go #[] minor_args.toList
 where
-  ibelow := rlvl matches .zero
   go (prods : Array Expr) : List Expr → MetaM Expr
   | [] => PProdN.pack rlvl prods
   | arg::args => do
@@ -50,8 +41,7 @@ where
         let name ← arg.fvarId!.getUserName
         let type' ← forallTelescope argType fun args _ => mkForallFVars args (.sort rlvl)
         withLocalDeclD name type' fun arg' => do
-          let snd ← mkForallFVars arg_args (mkAppN arg' arg_args)
-          let e' ← mkPProd argType snd
+          let e' ← mkForallFVars arg_args <| ← mkPProd arg_type (mkAppN arg' arg_args)
           mkLambdaFVars #[arg'] (← go (prods.push e') args)
       else
         mkLambdaFVars #[arg] (← go prods args)
@@ -86,8 +76,6 @@ private def mkBelowFromRec (recName : Name) (ibelow reflexive : Bool) (nParams :
   let refType :=
     if ibelow then
       recVal.type.instantiateLevelParams [lvlParam] [0]
-    else if reflexive then
-      recVal.type.instantiateLevelParams [lvlParam] [lvl.succ]
     else
       recVal.type
 
@@ -116,12 +104,9 @@ private def mkBelowFromRec (recName : Name) (ibelow reflexive : Bool) (nParams :
       if ibelow then
         0
       else if reflexive then
-        if let .max 1 ilvl' := ilvl then
-          mkLevelMax' (.succ lvl) ilvl'
-        else
-          mkLevelMax' (.succ lvl) ilvl
+        mkLevelMax ilvl lvl
       else
-        mkLevelMax' 1 lvl
+        mkLevelMax 1 lvl
 
     let mut val := .const recName (rlvl.succ :: lvls)
     -- add parameters
@@ -168,8 +153,8 @@ private def mkBelowOrIBelow (indName : Name) (ibelow : Bool) : MetaM Unit := do
       let belowName := belowName.appendIndexAfter (i + 1)
       mkBelowFromRec recName ibelow indVal.isReflexive indVal.numParams belowName
 
-def mkBelow (declName : Name) : MetaM Unit := mkBelowOrIBelow declName true
-def mkIBelow (declName : Name) : MetaM Unit := mkBelowOrIBelow declName false
+def mkBelow (declName : Name) : MetaM Unit := mkBelowOrIBelow declName false
+def mkIBelow (declName : Name) : MetaM Unit := mkBelowOrIBelow declName true
 
 /--
 If `minorType` is the type of a minor premies of a recursor, such as
@@ -207,8 +192,7 @@ private def buildBRecOnMinorPremise (rlvl : Level) (motives : Array Expr)
               let type' ← mkForallFVars arg_args
                 (← mkPProd arg_type (mkAppN belows[idx]! arg_type_args) )
               withLocalDeclD name type' fun arg' => do
-                let r ← etaPProd arg_args arg'
-                mkLambdaFVars #[arg'] (← go (prods.push r) args)
+                mkLambdaFVars #[arg'] (← go (prods.push arg') args)
             else
               mkLambdaFVars #[arg] (← go prods args)
     go #[] minor_args.toList
@@ -251,8 +235,6 @@ private def mkBRecOnFromRec (recName : Name) (ind reflexive : Bool) (nParams : N
   let refType :=
     if ind then
       recVal.type.instantiateLevelParams [lvlParam] [0]
-    else if reflexive then
-      recVal.type.instantiateLevelParams [lvlParam] [lvl.succ]
     else
       recVal.type
 
@@ -279,12 +261,9 @@ private def mkBRecOnFromRec (recName : Name) (ind reflexive : Bool) (nParams : N
       if ind then
         0
       else if reflexive then
-        if let .max 1 ilvl' := ilvl then
-          mkLevelMax' (.succ lvl) ilvl'
-        else
-          mkLevelMax' (.succ lvl) ilvl
+        mkLevelMax ilvl lvl
       else
-        mkLevelMax' 1 lvl
+        mkLevelMax 1 lvl
 
     -- One `below` for each motive, with the same motive parameters
     let blvls := if ind then lvls else lvl::lvls
diff --git a/src/library/suffixes.h b/src/library/suffixes.h
@@ -8,8 +8,6 @@ Author: Leonardo de Moura
 
 namespace lean {
 constexpr char const * g_rec               = "rec";
-constexpr char const * g_brec_on           = "brecOn";
-constexpr char const * g_binduction_on     = "binductionOn";
 constexpr char const * g_cases_on          = "casesOn";
 constexpr char const * g_no_confusion      = "noConfusion";
 constexpr char const * g_no_confusion_type = "noConfusionType";
diff --git a/tests/lean/run/7638.lean b/tests/lean/run/7638.lean
@@ -0,0 +1,24 @@
+inductive Foo : Type
+  | mk : Foo → Foo
+
+inductive Bar : Type
+  | mk : (Unit → Bar) → Bar
+
+def Foo.elim {α : Sort u} : Foo → α
+  | ⟨foo⟩ => elim foo
+  termination_by structural foo => foo
+
+def Bar.elim {α : Sort u} : Bar → α
+  | ⟨bar⟩ => elim (bar ())
+  termination_by structural bar => bar
+
+inductive StressTest : Type 5
+  | f (x : Type 4 → StressTest)
+  | g (x : Type 3 → StressTest)
+  | h (x : Type 4 → StressTest) (y : Type 3 → StressTest)
+
+def StressTest.elim {α : Sort u} : StressTest → α
+  | f x => elim (x (Type 3))
+  | g x => elim (x (Type 2))
+  | h x _y => elim (x (Type 3))
+  termination_by structural t => t
diff --git a/tests/lean/run/issue4650.lean b/tests/lean/run/issue4650.lean
@@ -4,7 +4,7 @@ inductive Foo1 : Sort (max 1 u) where
   | intro: (h : Nat → Foo1) → Foo1
 
 /--
-info: Foo1.below.{u_1, u} {motive : Foo1.{u} → Type u_1} (t : Foo1.{u}) : Sort (max (u_1 + 1) u)
+info: Foo1.below.{u_1, u} {motive : Foo1.{u} → Sort u_1} (t : Foo1.{u}) : Sort (max (max 1 u) u_1)
 -/
 #guard_msgs in
 #check Foo1.below
@@ -13,15 +13,15 @@ inductive Foo2 : Sort (max u 1) where
   | intro: (h : Nat → Foo2) → Foo2
 
 /--
-info: Foo2.below.{u_1, u} {motive : Foo2.{u} → Type u_1} (t : Foo2.{u}) : Sort (max (u_1 + 1) u 1)
+info: Foo2.below.{u_1, u} {motive : Foo2.{u} → Sort u_1} (t : Foo2.{u}) : Sort (max (max u 1) u_1)
 -/
 #guard_msgs in
 #check Foo2.below
 
 inductive Foo3 : Sort (u+1) where
   | intro: (h : Nat → Foo3) → Foo3
 
-/-- info: Foo3.below.{u_1, u} {motive : Foo3.{u} → Type u_1} (t : Foo3.{u}) : Type (max u_1 u) -/
+/-- info: Foo3.below.{u_1, u} {motive : Foo3.{u} → Sort u_1} (t : Foo3.{u}) : Sort (max (u + 1) u_1) -/
 #guard_msgs in
 #check Foo3.below
 
