fix #41908, inference error in subst_trivial_bounds (#41976)

Co-authored-by: Martin Holters <martin.holters@hsu-hh.de>
(cherry picked from commit e2aeefb)

base/compiler/utilities.jl

@@ -154,7 +154,15 @@ function subst_trivial_bounds(@nospecialize(atypes))
     end
     v = atypes.var
     if isconcretetype(v.ub) || v.lb === v.ub
-        return subst_trivial_bounds(atypes{v.ub})
+        subst = try
+            atypes{v.ub}
+        catch
+            # Note in rare cases a var bound might not be valid to substitute.
+            nothing
+        end
+        if subst !== nothing
+            return subst_trivial_bounds(subst)
+        end
     end
     return UnionAll(v, subst_trivial_bounds(atypes.body))
 end

test/compiler/inference.jl

@@ -3085,3 +3085,8 @@ end
         end
         return x
     end) === Union{Int, Float64, Char}
+
+# issue #41908
+f41908(x::Complex{T}) where {String<:T<:String} = 1
+g41908() = f41908(Any[1][1])
+@test only(Base.return_types(g41908, ())) <: Int