Add test for WeakKeyDict soundness

The underlying issues was addressed with the change in WeakRef
semantics in #38180. However, we still want the test.
Closes #38727

test/dict.jl

@@ -1162,3 +1162,42 @@ end
         @test testdict[:b] == 1
     end
 end
+
+# WeakKeyDict soundness (#38727)
+mutable struct ComparesWithGC38727
+	i::Int
+end
+const armed = Ref{Bool}(true)
+@noinline fwdab38727(a, b) = invoke(Base.isequal, Tuple{Any, WeakRef}, a, b)
+function Base.isequal(a::ComparesWithGC38727, b::WeakRef)
+	# This GC.gc() here simulates a GC during compilation in the original issue
+	armed[] && GC.gc()
+        armed[] = false
+        fwdab38727(a, b)
+end
+Base.isequal(a::WeakRef, b::ComparesWithGC38727) = isequal(b, a)
+Base.:(==)(a::ComparesWithGC38727, b::ComparesWithGC38727) = a.i == b.i
+Base.hash(a::ComparesWithGC38727, u::UInt) = Base.hash(a.i, u)
+function make_cwgc38727(wkd, i)
+	f = ComparesWithGC38727(i)
+	function fin(f)
+		f.i = -1
+	end
+	finalizer(fin, f)
+	f
+end
+@noinline mk38727(wkd) = wkd[make_cwgc38727(wkd, 1)] = nothing
+function bar()
+	wkd = WeakKeyDict{Any, Nothing}()
+	mk38727(wkd)
+	armed[] = true
+	z = getkey(wkd, ComparesWithGC38727(1), missing)
+end
+# Run this twice, in case compilation the first time around
+# masks something.
+let c = bar()
+    @test c === missing || c == ComparesWithGC38727(1)
+end
+let c = bar()
+    @test c === missing || c == ComparesWithGC38727(1)
+end