add some comments about ScopedValue implementation (#52372)

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)

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