add test for #18725

Author: stevengj

test/core.jl

@@ -4822,6 +4822,17 @@ end
 @test f14893() == 14893
 @test M14893.f14893() == 14893
 
+# issue #18725
+@test_nowarn begin
+    f18725(x) = 1
+    f18725(x) = 2
+    @test f18725(0) == 2
+end
+@test_warn "WARNING: Method definition f18725(Any) in module Module18725" eval(Main, :(module Module18725
+    f18725(x) = 1
+    f18725(x) = 2
+end))
+
 # issue #19599
 f19599{T}(x::((S)->Vector{S})(T)...) = 1
 @test f19599([1],[1]) == 1