diff --git a/src/Lean/Meta/Basic.lean b/src/Lean/Meta/Basic.lean
index 2b4f5a4da2..6bc9ee7e0f 100644
--- a/src/Lean/Meta/Basic.lean
+++ b/src/Lean/Meta/Basic.lean
@@ -415,6 +415,10 @@ structure PostponedEntry where
 structure Diagnostics where
   /-- Number of times each declaration has been unfolded -/
   unfoldCounter : PHashMap Name Nat := {}
+  /--
+  Number of times each axiom was tried to be unfolded, which may point to an inaccessible def value.
+  -/
+  unfoldAxiomCounter : PHashMap Name Nat := {}
   /-- Number of times `f a =?= f b` heuristic has been used per function `f`. -/
   heuristicCounter : PHashMap Name Nat := {}
   /-- Number of times a TC instance is used. -/
@@ -670,21 +674,30 @@ def mkInfoCacheKey (expr : Expr) (nargs? : Option Nat) : MetaM InfoCacheKey :=
 
 /-- If diagnostics are enabled, record that `declName` has been unfolded. -/
 def recordUnfold (declName : Name) : MetaM Unit := do
-  modifyDiag fun { unfoldCounter, heuristicCounter, instanceCounter, synthPendingFailures } =>
+  modifyDiag fun { unfoldCounter, unfoldAxiomCounter, heuristicCounter, instanceCounter, synthPendingFailures } =>
     let newC := if let some c := unfoldCounter.find? declName then c + 1 else 1
-    { unfoldCounter := unfoldCounter.insert declName newC, heuristicCounter, instanceCounter, synthPendingFailures }
+    { unfoldCounter := unfoldCounter.insert declName newC, unfoldAxiomCounter, heuristicCounter, instanceCounter, synthPendingFailures }
+
+def recordUnfoldAxiom (declName : Name) : MetaM Unit := do
+  modifyDiag fun { unfoldCounter, unfoldAxiomCounter, heuristicCounter, instanceCounter, synthPendingFailures } =>
+    let newC := if let some c := unfoldAxiomCounter.find? declName then c + 1 else 1
+    { unfoldCounter, unfoldAxiomCounter := unfoldAxiomCounter.insert declName newC, heuristicCounter, instanceCounter, synthPendingFailures }
+
+def resetUnfoldAxiom : MetaM Unit := do
+  modifyDiag fun { unfoldCounter, heuristicCounter, instanceCounter, synthPendingFailures, .. } =>
+    { unfoldCounter, unfoldAxiomCounter := {}, heuristicCounter, instanceCounter, synthPendingFailures }
 
 /-- If diagnostics are enabled, record that heuristic for solving `f a =?= f b` has been used. -/
 def recordDefEqHeuristic (declName : Name) : MetaM Unit := do
-  modifyDiag fun { unfoldCounter, heuristicCounter, instanceCounter, synthPendingFailures } =>
+  modifyDiag fun { unfoldCounter, unfoldAxiomCounter, heuristicCounter, instanceCounter, synthPendingFailures } =>
     let newC := if let some c := heuristicCounter.find? declName then c + 1 else 1
-    { unfoldCounter, heuristicCounter := heuristicCounter.insert declName newC, instanceCounter, synthPendingFailures }
+    { unfoldCounter, unfoldAxiomCounter, heuristicCounter := heuristicCounter.insert declName newC, instanceCounter, synthPendingFailures }
 
 /-- If diagnostics are enabled, record that instance `declName` was used during TC resolution. -/
 def recordInstance (declName : Name) : MetaM Unit := do
-  modifyDiag fun { unfoldCounter, heuristicCounter, instanceCounter, synthPendingFailures } =>
+  modifyDiag fun { unfoldCounter, unfoldAxiomCounter, heuristicCounter, instanceCounter, synthPendingFailures } =>
     let newC := if let some c := instanceCounter.find? declName then c + 1 else 1
-    { unfoldCounter, heuristicCounter, instanceCounter := instanceCounter.insert declName newC, synthPendingFailures }
+    { unfoldCounter, unfoldAxiomCounter, heuristicCounter, instanceCounter := instanceCounter.insert declName newC, synthPendingFailures }
 
 /-- If diagnostics are enabled, record that synth pending failures. -/
 def recordSynthPendingFailure (type : Expr) : MetaM Unit := do
@@ -692,8 +705,8 @@ def recordSynthPendingFailure (type : Expr) : MetaM Unit := do
     unless (← get).diag.synthPendingFailures.contains type do
       -- We need to save the full context since type class resolution uses multiple metavar contexts and different local contexts
       let msg ← addMessageContextFull m!"{type}"
-      modifyDiag fun { unfoldCounter, heuristicCounter, instanceCounter, synthPendingFailures } =>
-        { unfoldCounter, heuristicCounter, instanceCounter, synthPendingFailures := synthPendingFailures.insert type msg }
+      modifyDiag fun { unfoldCounter, unfoldAxiomCounter, heuristicCounter, instanceCounter, synthPendingFailures } =>
+        { unfoldCounter, unfoldAxiomCounter, heuristicCounter, instanceCounter, synthPendingFailures := synthPendingFailures.insert type msg }
 
 def getLocalInstances : MetaM LocalInstances :=
   return (← read).localInstances
diff --git a/src/Lean/Meta/Check.lean b/src/Lean/Meta/Check.lean
index 2e5b1c41b9..a4156653c4 100644
--- a/src/Lean/Meta/Check.lean
+++ b/src/Lean/Meta/Check.lean
@@ -8,6 +8,7 @@ module
 prelude
 public import Lean.Meta.InferType
 public import Lean.Meta.Sorry
+import Lean.AddDecl
 
 public section
 
@@ -206,13 +207,13 @@ def mkHasTypeButIsExpectedMsg (givenType expectedType : Expr)
     (trailing? : Option MessageData := none) (trailingExprs : Array Expr := #[])
     : MetaM MessageData := do
   return MessageData.ofLazyM (es := #[givenType, expectedType] ++ trailingExprs) do
-    try
+    let mut msg ← (try
       let givenTypeType ← inferType givenType
       let expectedTypeType ← inferType expectedType
       if (← isDefEqGuarded givenTypeType expectedTypeType) then
         let (givenType, expectedType) ← addPPExplicitToExposeDiff givenType expectedType
         let trailing := trailing?.map (m!"\n" ++ ·) |>.getD .nil
-        return m!"has type{indentExpr givenType}\n\
+        pure m!"has type{indentExpr givenType}\n\
           but is expected to have type{indentExpr expectedType}{trailing}"
       else
         let (givenType, expectedType) ← addPPExplicitToExposeDiff givenType expectedType
@@ -220,12 +221,25 @@ def mkHasTypeButIsExpectedMsg (givenType expectedType : Expr)
         let trailing := match trailing? with
           | none => inlineExprTrailing expectedTypeType
           | some trailing => inlineExpr expectedTypeType ++ trailing
-        return m!"has type{indentExpr givenType}\nof sort{inlineExpr givenTypeType}\
+        pure m!"has type{indentExpr givenType}\nof sort{inlineExpr givenTypeType}\
           but is expected to have type{indentExpr expectedType}\nof sort{trailing}"
     catch _ =>
       let (givenType, expectedType) ← addPPExplicitToExposeDiff givenType expectedType
       let trailing := trailing?.map (m!"\n" ++ ·) |>.getD .nil
-      return m!"has type{indentExpr givenType}\nbut is expected to have type{indentExpr expectedType}{trailing}"
+      pure m!"has type{indentExpr givenType}\nbut is expected to have type{indentExpr expectedType}{trailing}")
+    let env ← getEnv
+    if env.header.isModule then
+      let origDiag := (← get).diag
+      let _ ← observing <| withOptions (diagnostics.set · true)  <| isDefEq givenType expectedType
+      let blocked := (← get).diag.unfoldAxiomCounter.toList.filterMap fun (n, count) => do
+        let count := count - origDiag.unfoldAxiomCounter.findD n 0
+        guard <| count > 0 && getOriginalConstKind? env n matches some .defn
+        return m!"{.ofConstName n} ↦ {count}"
+      if !blocked.isEmpty then
+        msg := msg ++ MessageData.note m!"The following definitions were not unfolded because \
+          their definition is not exposed:{indentD <| .joinSep blocked Format.line}"
+      modify ({ · with diag := origDiag })
+    return msg
 
 def throwAppTypeMismatch (f a : Expr) : MetaM α := do
   -- Clarify that `a` is "last" only if it may be confused with some preceding argument; otherwise,
diff --git a/src/Lean/Meta/GetUnfoldableConst.lean b/src/Lean/Meta/GetUnfoldableConst.lean
index 7d5b1a1a33..ddd3f13809 100644
--- a/src/Lean/Meta/GetUnfoldableConst.lean
+++ b/src/Lean/Meta/GetUnfoldableConst.lean
@@ -58,6 +58,7 @@ def getUnfoldableConstNoEx? (constName : Name) : MetaM (Option ConstantInfo) :=
   match (← getEnv).find? constName with
   | some (info@(.thmInfo _))  => getTheoremInfo info
   | some (info@(.defnInfo _)) => if (← canUnfold info) then return info else return none
+  | some (.axiomInfo _)       => recordUnfoldAxiom constName; return none
   | _                         => return none
 
 end Meta
diff --git a/src/Lean/Meta/WHNF.lean b/src/Lean/Meta/WHNF.lean
index 8cb60b50d3..6ab878d83a 100644
--- a/src/Lean/Meta/WHNF.lean
+++ b/src/Lean/Meta/WHNF.lean
@@ -691,6 +691,7 @@ where
                 deltaBetaDefinition c lvls e.getAppRevArgs (fun _ => return e) go
               else
                 return e
+            | .axiomInfo val => recordUnfoldAxiom val.name; return e
             | _ => return e
       | .proj _ i c =>
         let k (c : Expr) := do
@@ -813,6 +814,8 @@ mutual
               recordUnfold fInfo.name
               deltaBetaDefinition fInfo fLvls e.getAppRevArgs (fun _ => pure none) (fun e => pure (some e))
             else
+              if fInfo.isAxiom then
+                recordUnfoldAxiom fInfo.name
               return none
           if smartUnfolding.get (← getOptions) then
             match ((← getEnv).find? (skipRealize := true) (mkSmartUnfoldingNameFor fInfo.name)) with
@@ -882,7 +885,10 @@ mutual
       if smartUnfolding.get (← getOptions) && (← getEnv).contains (mkSmartUnfoldingNameFor declName) then
         return none
       else
-        unless cinfo.hasValue do return none
+        unless cinfo.hasValue do
+          if cinfo.isAxiom then
+            recordUnfoldAxiom cinfo.name
+          return none
         deltaDefinition cinfo lvls
           (fun _ => pure none)
           (fun e => do recordUnfold declName; pure (some e))
diff --git a/tests/pkg/module/Module/Basic.lean b/tests/pkg/module/Module/Basic.lean
index e6a0ff0a25..4a92ec5580 100644
--- a/tests/pkg/module/Module/Basic.lean
+++ b/tests/pkg/module/Module/Basic.lean
@@ -96,6 +96,9 @@ has type
   Vector Unit 1
 but is expected to have type
   Vector Unit f
+
+Note: The following definitions were not unfolded because their definition is not exposed:
+  f ↦ 1
 -/
 #guard_msgs in
 public theorem v (x : Vector Unit f) (y : Vector Unit 1) : x = y := testSorry
diff --git a/tests/pkg/module/Module/Imported.lean b/tests/pkg/module/Module/Imported.lean
index d6a40c804b..63e17f13e9 100644
--- a/tests/pkg/module/Module/Imported.lean
+++ b/tests/pkg/module/Module/Imported.lean
@@ -12,6 +12,20 @@ info: def f : Nat :=
 #guard_msgs in
 #print f
 
+/--
+error: Type mismatch
+  rfl
+has type
+  ?m.5 = ?m.5
+but is expected to have type
+  f = 1
+
+Note: The following definitions were not unfolded because their definition is not exposed:
+  f ↦ 1
+-/
+#guard_msgs in
+example : f = 1 := rfl
+
 /-! Theorems should be exported without their bodies -/
 
 /--
diff --git a/tests/pkg/module/Module/ImportedAll.lean b/tests/pkg/module/Module/ImportedAll.lean
index f2ef22a419..a2b1b22e25 100644
--- a/tests/pkg/module/Module/ImportedAll.lean
+++ b/tests/pkg/module/Module/ImportedAll.lean
@@ -24,6 +24,9 @@ has type
   Vector Unit 1
 but is expected to have type
   Vector Unit f
+
+Note: The following definitions were not unfolded because their definition is not exposed:
+  f ↦ 1
 -/
 #guard_msgs in
 public theorem v (x : Vector Unit f) (y : Vector Unit 1) : x = y := sorry