fix: boxing may have to correct let binder types (#11426)

This PR closes #11356.

src/Lean/Compiler/IR/Boxing.lean

@@ -267,8 +267,29 @@ def visitVDeclExpr (x : VarId) (ty : IRType) (e : Expr) (b : FnBody) : M FnBody
   | _     =>
     return .vdecl x ty e b
 
+/--
+Up to this point the type system of IR is quite loose so we can for example encounter situations
+such as
+```
+let y : obj := f x
+```
+where `f : obj -> uint8`. It is the job of the boxing pass to enforce a stricter obj/scalar
+separation and as such it needs to correct situations like this.
+-/
+def tryCorrectVDeclType (ty : IRType) (e : Expr) : M IRType :=
+  match e with
+  | .fap f _ => do
+    let decl ← getDecl f
+    return decl.resultType
+  | .pap .. => return .object
+  | .uproj .. => return .usize
+  | .ctor .. | .reuse .. | .ap .. | .lit .. | .sproj .. | .proj .. | .reset .. =>
+    return ty
+  | .unbox .. | .box .. | .isShared .. => unreachable!
+
 partial def visitFnBody : FnBody → M FnBody
   | .vdecl x t v b     => do
+    let t ← tryCorrectVDeclType t v
     let b ← withVDecl x t v (visitFnBody b)
     visitVDeclExpr x t v b
   | .jdecl j xs v b    => do

tests/lean/run/boxing_bug.lean

@@ -0,0 +1,28 @@
+def myCast : NatCast UInt8 where
+  natCast := UInt8.ofNat
+
+class Semiring (α : Type u) where
+  [nsmul : SMul Nat α]
+
+/--
+trace: [Compiler.IR] [result]
+    def instSemiringUInt8._lam_0 (x_1 : @& tobj) (x_2 : u8) : u8 :=
+      let x_3 : u8 := UInt8.ofNat x_1;
+      let x_4 : u8 := UInt8.mul x_3 x_2;
+      ret x_4
+    def instSemiringUInt8 : obj :=
+      let x_1 : obj := pap instSemiringUInt8._lam_0._boxed;
+      ret x_1
+    def instSemiringUInt8._lam_0._boxed (x_1 : tobj) (x_2 : tagged) : tagged :=
+      let x_3 : u8 := unbox x_2;
+      let x_4 : u8 := instSemiringUInt8._lam_0 x_1 x_3;
+      dec x_1;
+      let x_5 : tagged := box x_4;
+      ret x_5
+-/
+#guard_msgs in
+set_option trace.compiler.ir.result true in
+attribute [local instance] myCast UInt8.intCast in
+instance : Semiring UInt8 where
+  nsmul := ⟨(· * ·)⟩
+