fix: bring elaborator in line with kernel for primitive projections

src/Lean/Compiler/LCNF/InferType.lean

@@ -168,13 +168,12 @@ mutual
       /- TODO: after we erase universe variables, we can just extract a better type using just `structName` and `idx`. -/
       return erasedExpr
     else
-      matchConstStruct structType.getAppFn failed fun structVal structLvls ctorVal =>
-        let n := structVal.numParams
-        let structParams := structType.getAppArgs
-        if n != structParams.size then
+      matchConstStructure structType.getAppFn failed fun structVal structLvls ctorVal =>
+        let structTypeArgs := structType.getAppArgs
+        if structVal.numParams + structVal.numIndices != structTypeArgs.size then
           failed ()
         else do
-          let mut ctorType ← inferAppType (mkAppN (mkConst ctorVal.name structLvls) structParams)
+          let mut ctorType ← inferAppType (mkAppN (mkConst ctorVal.name structLvls) structTypeArgs[:structVal.numParams])
           for _ in [:idx] do
             match ctorType with
             | .forallE _ _ body _ =>

src/Lean/Elab/App.lean

@@ -1188,19 +1188,19 @@ private def resolveLValAux (e : Expr) (eType : Expr) (lval : LVal) : TermElabM L
     if idx == 0 then
       throwError "invalid projection, index must be greater than 0"
     let env ← getEnv
-    unless isStructureLike env structName do
-      throwLValError e eType "invalid projection, structure expected"
-    let numFields := getStructureLikeNumFields env structName
-    if idx - 1 < numFields then
-      if isStructure env structName then
-        let fieldNames := getStructureFields env structName
-        return LValResolution.projFn structName structName fieldNames[idx - 1]!
+    let failK _ := throwLValError e eType "invalid projection, structure expected"
+    matchConstStructure eType.getAppFn failK fun _ _ ctorVal => do
+      let numFields := ctorVal.numFields
+      if idx - 1 < numFields then
+        if isStructure env structName then
+          let fieldNames := getStructureFields env structName
+          return LValResolution.projFn structName structName fieldNames[idx - 1]!
+        else
+          /- `structName` was declared using `inductive` command.
+            So, we don't projection functions for it. Thus, we use `Expr.proj` -/
+          return LValResolution.projIdx structName (idx - 1)
       else
-        /- `structName` was declared using `inductive` command.
-           So, we don't projection functions for it. Thus, we use `Expr.proj` -/
-        return LValResolution.projIdx structName (idx - 1)
-    else
-      throwLValError e eType m!"invalid projection, structure has only {numFields} field(s)"
+        throwLValError e eType m!"invalid projection, structure has only {numFields} field(s)"
   | some structName, LVal.fieldName _ fieldName _ _ =>
     let env ← getEnv
     let searchEnv : Unit → TermElabM LValResolution := fun _ => do

src/Lean/Meta/ExprDefEq.lean

@@ -1975,7 +1975,7 @@ where
       assign `?m`.
       -/
       return false
-    let ctorVal := getStructureCtor (← getEnv) structName
+    let some ctorVal := getStructureLikeCtor? (← getEnv) structName | return false
     if ctorVal.numFields != 1 then
       return false -- It is not a structure with a single field.
     let sType ← whnf (← inferType s)
@@ -2013,7 +2013,7 @@ private def isDefEqApp (t s : Expr) : MetaM Bool := do
 /-- Return `true` if the type of the given expression is an inductive datatype with a single constructor with no fields. -/
 private def isDefEqUnitLike (t : Expr) (s : Expr) : MetaM Bool := do
   let tType ← whnf (← inferType t)
-  matchConstStruct tType.getAppFn (fun _ => return false) fun _ _ ctorVal => do
+  matchConstStructureLike tType.getAppFn (fun _ => return false) fun _ _ ctorVal => do
     if ctorVal.numFields != 0 then
       return false
     else if (← useEtaStruct ctorVal.induct) then

src/Lean/Meta/InferType.lean

@@ -99,13 +99,12 @@ private def inferProjType (structName : Name) (idx : Nat) (e : Expr) : MetaM Exp
   let structType ← whnf structType
   let failed {α} : Unit → MetaM α := fun _ =>
     throwError "invalid projection{indentExpr (mkProj structName idx e)} from type {structType}"
-  matchConstStruct structType.getAppFn failed fun structVal structLvls ctorVal =>
-    let n := structVal.numParams
-    let structParams := structType.getAppArgs
-    if n != structParams.size then
+  matchConstStructure structType.getAppFn failed fun structVal structLvls ctorVal =>
+    let structTypeArgs := structType.getAppArgs
+    if structVal.numParams + structVal.numIndices != structTypeArgs.size then
       failed ()
     else do
-      let mut ctorType ← inferAppType (mkConst ctorVal.name structLvls) structParams
+      let mut ctorType ← inferAppType (mkConst ctorVal.name structLvls) structTypeArgs[:structVal.numParams]
       for i in [:idx] do
         ctorType ← whnf ctorType
         match ctorType with

src/Lean/Meta/Tactic/Constructor.lean

@@ -32,7 +32,7 @@ def _root_.Lean.MVarId.existsIntro (mvarId : MVarId) (w : Expr) : MetaM MVarId :
   mvarId.withContext do
     mvarId.checkNotAssigned `exists
     let target ← mvarId.getType'
-    matchConstStruct target.getAppFn
+    matchConstStructure target.getAppFn
       (fun _ => throwTacticEx `exists mvarId "target is not an inductive datatype with one constructor")
       fun _ us cval => do
         if cval.numFields < 2 then

src/Lean/MonadEnv.lean

@@ -118,7 +118,26 @@ def getConstInfoRec [Monad m] [MonadEnv m] [MonadError m] (constName : Name) : m
   | ConstantInfo.recInfo v => pure v
   | _                      => throwError "'{mkConst constName}' is not a recursor"
 
-@[inline] def matchConstStruct [Monad m] [MonadEnv m] [MonadError m] (e : Expr) (failK : Unit → m α) (k : InductiveVal → List Level → ConstructorVal → m α) : m α :=
+/--
+Matches if `e` is a constant that is an inductive type with one constructor.
+Such types can be used with primitive projections.
+See also `Lean.matchConstStructLike` for a more restrictive version.
+-/
+@[inline] def matchConstStructure [Monad m] [MonadEnv m] [MonadError m] (e : Expr) (failK : Unit → m α) (k : InductiveVal → List Level → ConstructorVal → m α) : m α :=
+  matchConstInduct e failK fun ival us => do
+    match ival.ctors with
+      | [ctor] =>
+        match (← getConstInfo ctor) with
+        | ConstantInfo.ctorInfo cval => k ival us cval
+        | _ => failK ()
+      | _ => failK ()
+
+/--
+Matches if `e` is a constant that is an non-recursive inductive type with no indices and with one constructor.
+Such a type satisfies `Lean.isStructureLike`.
+See also `Lean.matchConstStructure` for a less restrictive version.
+-/
+@[inline] def matchConstStructureLike [Monad m] [MonadEnv m] [MonadError m] (e : Expr) (failK : Unit → m α) (k : InductiveVal → List Level → ConstructorVal → m α) : m α :=
   matchConstInduct e failK fun ival us => do
     if ival.isRec || ival.numIndices != 0 then failK ()
     else match ival.ctors with

src/Lean/Structure.lean

@@ -133,9 +133,17 @@ def getStructureInfo (env : Environment) (structName : Name) : StructureInfo :=
   else
     panic! "structure expected"
 
+/--
+Gets the constructor of an inductive type that has exactly one constructor.
+This is meant to be used with types that have had been registered as a structure by `registerStructure`,
+but this is not checked.
+
+Warning: these do *not* need to be "structure-likes". A structure-like is non-recursive,
+and structure-likes have special kernel support.
+-/
 def getStructureCtor (env : Environment) (constName : Name) : ConstructorVal :=
   match env.find? constName with
-  | some (.inductInfo { isRec := false, ctors := [ctorName], .. }) =>
+  | some (.inductInfo { ctors := [ctorName], .. }) =>
     match env.find? ctorName with
     | some (ConstantInfo.ctorInfo val) => val
     | _ => panic! "ill-formed environment"
@@ -222,9 +230,10 @@ def getStructureFieldsFlattened (env : Environment) (structName : Name) (include
   getStructureFieldsFlattenedAux env structName #[] includeSubobjectFields
 
 /--
-Return true if `constName` is the name of an inductive datatype
+Returns true if `constName` is the name of an inductive datatype
 created using the `structure` or `class` commands.
 
+These are inductive types for which structure information has been registered with `registerStructure`.
 See also `Lean.getStructureInfo?`.
 -/
 def isStructure (env : Environment) (constName : Name) : Bool :=
@@ -269,18 +278,33 @@ partial def getPathToBaseStructureAux (env : Environment) (baseStructName : Name
         | some projFn => getPathToBaseStructureAux env baseStructName parentStructName (projFn :: path)
 
 /--
-If `baseStructName` is an ancestor structure for `structName`, then return a sequence of projection functions
+If `baseStructName` is an ancestor structure for `structName`, then returns a sequence of projection functions
 to go from `structName` to `baseStructName`.
 -/
 def getPathToBaseStructure? (env : Environment) (baseStructName : Name) (structName : Name) : Option (List Name) :=
   getPathToBaseStructureAux env baseStructName structName []
 
-/-- Return true iff `constName` is the a non-recursive inductive datatype that has only one constructor. -/
+/--
+Returns true iff `constName` is a non-recursive inductive datatype that has only one constructor and no indices.
+
+Such types have special kernel support. This must be in sync with `is_structure_like`.
+-/
 def isStructureLike (env : Environment) (constName : Name) : Bool :=
   match env.find? constName with
   | some (.inductInfo { isRec := false, ctors := [_], numIndices := 0, .. }) => true
   | _ => false
 
+/--
+Returns the constructor of the structure named `constName` if it is a non-recursive single-constructor inductive type with no indices.
+-/
+def getStructureLikeCtor? (env : Environment) (constName : Name) : Option ConstructorVal :=
+  match env.find? constName with
+  | some (.inductInfo { isRec := false, ctors := [ctorName], numIndices := 0, .. }) =>
+    match env.find? ctorName with
+    | some (ConstantInfo.ctorInfo val) => val
+    | _ => panic! "ill-formed environment"
+  | _ => none
+
 /-- Return number of fields for a structure-like type -/
 def getStructureLikeNumFields (env : Environment) (constName : Name) : Nat :=
   match env.find? constName with

tests/lean/run/inductive_rec_proj.lean

@@ -0,0 +1,56 @@
+/-!
+# Tests for numeric projections of inductive types
+-/
+
+/-!
+Non-recursive, no indices.
+-/
+inductive I0 where
+  | mk (x : Nat) (xs : List Nat)
+/-- info: fun v => v.1 : I0 → Nat -/
+#guard_msgs in #check fun (v : I0) => v.1
+/-- info: fun v => v.2 : I0 → List Nat -/
+#guard_msgs in #check fun (v : I0) => v.2
+
+/-!
+Recursive, no indices.
+-/
+inductive I1 where
+  | mk (x : Nat) (xs : I1)
+/-- info: fun v => v.1 : I1 → Nat -/
+#guard_msgs in #check fun (v : I1) => v.1
+/-- info: fun v => v.2 : I1 → I1 -/
+#guard_msgs in #check fun (v : I1) => v.2
+
+/-!
+Non-recursive, indices.
+-/
+inductive I2 : Nat → Type where
+  | mk (x : Nat) (xs : List (Fin x)) : I2 (x + 1)
+/-- info: fun v => v.1 : I2 2 → Nat -/
+#guard_msgs in #check fun (v : I2 2) => v.1
+/-- info: fun v => v.2 : (v : I2 2) → List (Fin v.1) -/
+#guard_msgs in #check fun (v : I2 2) => v.2
+
+/-!
+Recursive, indices.
+-/
+inductive I3 : Nat → Type where
+  | mk (x : Nat) (xs : I3 (x + 1)) : I3 x
+/-- info: fun v => v.1 : I3 2 → Nat -/
+#guard_msgs in #check fun (v : I3 2) => v.1
+/-- info: fun v => v.2 : (v : I3 2) → I3 (v.1 + 1) -/
+#guard_msgs in #check fun (v : I3 2) => v.2
+
+
+/-!
+Make sure these can be compiled.
+-/
+def f0_1 (v : I0) : Nat := v.1
+def f0_2 (v : I0) : List Nat := v.2
+def f1_1 (v : I1) : Nat := v.1
+def f1_2 (v : I1) : I1 := v.2
+def f2_1 (v : I2 n) : Nat := v.1
+def f2_2 (v : I2 n) : List (Fin (f2_1 v)) := v.2
+def f3_1 (v : I3 n) : Nat := v.1
+def f3_2 (v : I3 n) : I3 (f3_1 v + 1) := v.2