add test for behavior of conditional flow with global (#57414)

tests #57315

Author: adienes

test/core.jl

@@ -8448,3 +8448,18 @@ myfun57023a(::Type{T}) where {T} = (x = @ccall mycfun()::Ptr{T}; x)
 @test only(code_lowered(myfun57023a)).has_fcall
 myfun57023b(::Type{T}) where {T} = (x = @cfunction myfun57023a Ptr{T} (Ref{T},); x)
 @test only(code_lowered(myfun57023b)).has_fcall
+
+# issue #57315
+global flag57315=false
+function f57315()
+    global flag57315
+    if flag57315
+        flag_2=true
+    else
+        if flag_2
+            return 2
+        end
+    end
+    return 1
+end
+@test_throws UndefVarError(:flag_2, :local) f57315()