From 19949811ceab167862c4bbcbcfa413be11f754c8 Mon Sep 17 00:00:00 2001
From: Jeff Bezanson <jeff.bezanson@gmail.com>
Date: Fri, 23 Oct 2020 17:03:26 -0400
Subject: [PATCH] avoid forming circular lower bound in type intersection
 (#38114)

fixes #37685, fixes #38081
---
 src/subtype.c   |  5 ++++-
 test/subtype.jl | 14 ++++++++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/src/subtype.c b/src/subtype.c
index c9a6180e4a27c..34590492c11e2 100644
--- a/src/subtype.c
+++ b/src/subtype.c
@@ -2931,7 +2931,10 @@ static jl_value_t *intersect(jl_value_t *x, jl_value_t *y, jl_stenv_t *e, int pa
                 jl_value_t *ub=NULL, *lb=NULL;
                 JL_GC_PUSH2(&lb, &ub);
                 ub = intersect_aside(xub, yub, e, 0, xx ? xx->depth0 : 0);
-                lb = simple_join(xlb, ylb);
+                if (xlb == y)
+                    lb = ylb;
+                else
+                    lb = simple_join(xlb, ylb);
                 if (yy) {
                     if (lb != y)
                         yy->lb = lb;
diff --git a/test/subtype.jl b/test/subtype.jl
index be324df04e2f5..c987167a57712 100644
--- a/test/subtype.jl
+++ b/test/subtype.jl
@@ -1772,3 +1772,17 @@ end
 
 # issue #37255
 @test Type{Union{}} == Type{T} where {Union{}<:T<:Union{}}
+
+# issue #38081
+struct AlmostLU{T, S<:AbstractMatrix{T}}
+end
+let X1 = Tuple{AlmostLU, Vector{T}} where T,
+    X2 = Tuple{AlmostLU{S, X} where X<:Matrix, Vector{S}} where S<:Union{Float32, Float64},
+    I = typeintersect(X1, X2)
+    # TODO: the quality of this intersection is not great; for now just test that it
+    # doesn't stack overflow
+    @test I<:X1 || I<:X2
+    actual = Tuple{AlmostLU{S, X} where X<:Matrix{S}, Vector{S}} where S<:Union{Float32, Float64}
+    @test I >: actual
+    @test_broken I == actual
+end