fix: Prop instances should be elaborated in the private scope (#10568)

src/Lean/Elab/MutualDef.lean

@@ -1291,13 +1291,27 @@ where
               logWarningAt attr.stx m!"Redundant `[expose]` attribute, it is meaningful on public \
                 definitions only"
 
+    -- Switch to private scope if...
     withoutExporting (when :=
-      headers.all (fun header =>
-        header.modifiers.isMeta && !header.modifiers.attrs.any (·.name == `expose) ||
-        header.modifiers.attrs.any (·.name == `no_expose) ||
-        (!(header.kind == .def && sc.attrs.any (· matches `(attrInstance| expose))) &&
-         !header.modifiers.attrs.any (·.name == `expose) &&
-         !header.kind matches .abbrev | .instance))) do
+      (← headers.allM (fun header => do
+        -- ... there is a `@[no_expose]` attribute
+        if header.modifiers.attrs.any (·.name == `no_expose) then
+          return true
+        -- ... or NONE of the following:
+        -- ... this is a non-`meta` `def` inside a `@[expose] section`
+        if header.kind == .def && !header.modifiers.isMeta && sc.attrs.any (· matches `(attrInstance| expose)) then
+          return false
+        -- ... there is an `@[expose]` attribute directly on the def (of any kind or phase)
+        if header.modifiers.attrs.any (·.name == `expose) then
+          return false
+        -- ... this is an `abbrev`
+        if header.kind == .abbrev then
+          return false
+        -- ... this is a data instance
+        if header.kind == .instance then
+          if !(← isProp header.type) then
+            return false
+        return true))) do
     let headers := headers.map fun header =>
       { header with modifiers.attrs := header.modifiers.attrs.filter (!·.name ∈ [`expose, `no_expose]) }
     let values ← try

tests/pkg/module/Module/Basic.lean

@@ -407,3 +407,34 @@ inst✝
 -/
 #guard_msgs in
 #print instTypeNameFoo
+
+/-! Prop `instance`s should have direct access to the private scope. -/
+
+public class PropClass : Prop where
+  proof : True
+
+theorem privTrue : True := trivial
+public instance : PropClass := ⟨privTrue⟩
+
+/-! Meta defs should only be exposed explicitly. -/
+
+@[expose] section
+public meta def msec := 1
+@[expose] public meta def msecexp := 1
+end
+
+/--
+info: meta def msec : Nat :=
+<not imported>
+-/
+#guard_msgs in
+#with_exporting
+#print msec
+
+/--
+info: @[expose] meta def msecexp : Nat :=
+1
+-/
+#guard_msgs in
+#with_exporting
+#print msecexp