fix: include instance implicits that depend on outParams at outParamsPos

This fixes the fix for #1852

src/Lean/Class.lean

@@ -89,14 +89,19 @@ def hasOutParams (env : Environment) (declName : Name) : Bool :=
 private partial def checkOutParam (i : Nat) (outParamFVarIds : Array FVarId) (outParams : Array Nat) (type : Expr) : Except String (Array Nat) :=
   match type with
   | .forallE _ d b bi =>
-    if d.isOutParam then
+    let addOutParam (_ : Unit) :=
       let fvarId := { name := Name.mkNum `_fvar outParamFVarIds.size }
       let fvar      := mkFVar fvarId
       let b         := b.instantiate1 fvar
       checkOutParam (i+1) (outParamFVarIds.push fvarId) (outParams.push i) b
-    /- See issue #1852 for a motivation for `!bi.isInstImplicit` -/
-    else if !bi.isInstImplicit && d.hasAnyFVar fun fvarId => outParamFVarIds.contains fvarId then
-      Except.error s!"invalid class, parameter #{i+1} depends on `outParam`, but it is not an `outParam`"
+    if d.isOutParam then
+      addOutParam ()
+    else if d.hasAnyFVar fun fvarId => outParamFVarIds.contains fvarId then
+      if bi.isInstImplicit then
+        /- See issue #1852 for a motivation for `bi.isInstImplicit` -/
+        addOutParam ()
+      else
+        Except.error s!"invalid class, parameter #{i+1} depends on `outParam`, but it is not an `outParam`"
     else
       checkOutParam (i+1) outParamFVarIds outParams b
   | _ => return outParams

src/Lean/Meta/SynthInstance.lean

@@ -624,15 +624,19 @@ private def preprocessLevels (us : List Level) : MetaM (List Level × Bool) := d
       r := r.push u
   return (r.toList, modified)
 
-private partial def preprocessArgs (type : Expr) (i : Nat) (args : Array Expr) : MetaM (Array Expr) := do
+private partial def preprocessArgs (type : Expr) (i : Nat) (args : Array Expr) (outParamsPos : Array Nat) : MetaM (Array Expr) := do
   if h : i < args.size then
     let type ← whnf type
     match type with
-    | Expr.forallE _ d b _ => do
+    | .forallE _ d b _ => do
       let arg := args.get ⟨i, h⟩
-      let arg ← if d.isOutParam then mkFreshExprMVar d else pure arg
+      /-
+      We should not simply check `d.isOutParam`. See `checkOutParam` and issue #1852.
+      If an instance implicit argument depends on an `outParam`, it is treated as an `outParam` too.
+      -/
+      let arg ← if outParamsPos.contains i then mkFreshExprMVar d else pure arg
       let args := args.set ⟨i, h⟩ arg
-      preprocessArgs (b.instantiate1 arg) (i+1) args
+      preprocessArgs (b.instantiate1 arg) (i+1) args outParamsPos
     | _ =>
       throwError "type class resolution failed, insufficient number of arguments" -- TODO improve error message
   else
@@ -641,15 +645,16 @@ private partial def preprocessArgs (type : Expr) (i : Nat) (args : Array Expr) :
 private def preprocessOutParam (type : Expr) : MetaM Expr :=
   forallTelescope type fun xs typeBody => do
     match typeBody.getAppFn with
-    | c@(Expr.const constName _) =>
+    | c@(Expr.const declName _) =>
       let env ← getEnv
-      if !hasOutParams env constName then
-        return type
-      else
-        let args := typeBody.getAppArgs
-        let cType ← inferType c
-        let args ← preprocessArgs cType 0 args
-        mkForallFVars xs (mkAppN c args)
+      if let some outParamsPos := getOutParamPositions? env declName then
+        unless outParamsPos.isEmpty do
+          let args := typeBody.getAppArgs
+          let cType ← inferType c
+          trace[Meta.debug] "{declName} : {outParamsPos}"
+          let args ← preprocessArgs cType 0 args outParamsPos
+          return (← mkForallFVars xs (mkAppN c args))
+      return type
     | _ =>
       return type
 

tests/lean/run/1852.lean

@@ -3,3 +3,24 @@ class foo (F : Type) where
 
 class foobar (F : outParam Type) [foo F] where
   bar : F
+
+class C (α : Type) where
+  val : α
+
+class D (α : Type) (β : outParam Type) [C β] where
+  val1 : α
+  val2 : β := C.val
+
+instance : C String where
+  val := "hello"
+
+instance : C Nat where
+  val := 42
+
+instance : D Nat String where
+  val1 := 37
+
+def f (α : Type) {β : Type} {_ : C β} [D α β] : α × β :=
+  (D.val1, D.val2 α)
+
+example : f Nat = (37, "hello") := rfl