From 3e4b38684e38a015446253f5752ee9cf840f50cc Mon Sep 17 00:00:00 2001
From: Shuhei Kadowaki <40514306+aviatesk@users.noreply.github.com>
Date: Sun, 3 Dec 2023 13:15:57 +0900
Subject: [PATCH] add some comments about `ScopedValue` implementation (#52372)

---
 base/scopedvalues.jl |  4 +++-
 test/scopedvalues.jl | 15 ++++++++++-----
 2 files changed, 13 insertions(+), 6 deletions(-)

diff --git a/base/scopedvalues.jl b/base/scopedvalues.jl
index 94491294c798a..6e1cc4755d618 100644
--- a/base/scopedvalues.jl
+++ b/base/scopedvalues.jl
@@ -38,7 +38,9 @@ julia> sval[]
     implementation is available from the package ScopedValues.jl.
 """
 mutable struct ScopedValue{T}
-    const has_default::Bool
+    # NOTE this struct must be defined as mutable one since it's used as a key of
+    #      `ScopeStorage` dictionary and thus needs object identity
+    const has_default::Bool # this field is necessary since isbitstype `default` field may be initialized with undefined value
     const default::T
     ScopedValue{T}() where T = new(false)
     ScopedValue{T}(val) where T = new{T}(true, val)
diff --git a/test/scopedvalues.jl b/test/scopedvalues.jl
index c9d376ab05cbd..b2a0e574fe7aa 100644
--- a/test/scopedvalues.jl
+++ b/test/scopedvalues.jl
@@ -4,13 +4,18 @@ import Base: ScopedValues
 @testset "errors" begin
     @test ScopedValue{Float64}(1)[] == 1.0
     @test_throws InexactError ScopedValue{Int}(1.5)
-    val = ScopedValue(1)
-    @test_throws MethodError val[] = 2
-    with() do
+    let val = ScopedValue(1)
         @test_throws MethodError val[] = 2
+        with() do
+            @test_throws MethodError val[] = 2
+        end
+    end
+    let val = ScopedValue{String}()
+        @test_throws KeyError val[]
+    end
+    let val = ScopedValue{Int}()
+        @test_throws KeyError val[]
     end
-    val = ScopedValue{Int}()
-    @test_throws KeyError val[]
     @test_throws MethodError ScopedValue()
 end