Add narrowing for isdefined calls on Unions (#43009)

base/compiler/abstractinterpretation.jl

@@ -1232,6 +1232,28 @@ function abstract_call_builtin(interp::AbstractInterpreter, f::Builtin, (; fargs
                 end
                 return Conditional(aty.var, ifty, elty)
             end
+        elseif f === isdefined
+            uty = argtypes[2]
+            a = ssa_def_slot(fargs[2], sv)
+            if isa(uty, Union) && isa(a, SlotNumber)
+                fld = argtypes[3]
+                vtype = Union{}
+                elsetype = Union{}
+                for ty in uniontypes(uty)
+                    cnd = isdefined_tfunc(ty, fld)
+                    if isa(cnd, Const)
+                        if cnd.val::Bool
+                            vtype = tmerge(vtype, ty)
+                        else
+                            elsetype = tmerge(elsetype, ty)
+                        end
+                    else
+                        vtype = tmerge(vtype, ty)
+                        elsetype = tmerge(elsetype, ty)
+                    end
+                end
+                return Conditional(a, vtype, elsetype)
+            end
         end
     end
     @assert !isa(rt, TypeVar) "unhandled TypeVar"

test/compiler/inference.jl

@@ -3755,3 +3755,63 @@ end |> only == Tuple{Int,Int}
     s2 = Some{Any}(s1)
     s2.value.value
 end |> only == Int
+
+# issue #42986
+@testset "narrow down `Union` using `isdefined` checks" begin
+    # basic functionality
+    @test Base.return_types((Union{Nothing,Core.CodeInstance},)) do x
+        if isdefined(x, :inferred)
+            return x
+        else
+            throw("invalid")
+        end
+    end |> only === Core.CodeInstance
+
+    @test Base.return_types((Union{Nothing,Core.CodeInstance},)) do x
+        if isdefined(x, :not_exist)
+            return x
+        else
+            throw("invalid")
+        end
+    end |> only === Union{}
+
+    # even when isdefined is malformed, we can filter out types with no fields
+    @test Base.return_types((Union{Nothing, Core.CodeInstance},)) do x
+        if isdefined(x, 5)
+            return x
+        else
+            throw("invalid")
+        end
+    end |> only === Core.CodeInstance
+
+    struct UnionNarrowingByIsdefinedA; x; end
+    struct UnionNarrowingByIsdefinedB; x; end
+    struct UnionNarrowingByIsdefinedC; x; end
+
+    # > 2 types in the union
+    @test  Base.return_types((Union{UnionNarrowingByIsdefinedA, UnionNarrowingByIsdefinedB, UnionNarrowingByIsdefinedC},)) do x
+        if isdefined(x, :x)
+            return x
+        else
+            throw("invalid")
+        end
+    end |> only === Union{UnionNarrowingByIsdefinedA, UnionNarrowingByIsdefinedB, UnionNarrowingByIsdefinedC}
+
+    # > 2 types in the union and some aren't defined
+    @test  Base.return_types((Union{UnionNarrowingByIsdefinedA, Core.CodeInstance, UnionNarrowingByIsdefinedC},)) do x
+        if isdefined(x, :x)
+            return x
+        else
+            throw("invalid")
+        end
+    end |> only === Union{UnionNarrowingByIsdefinedA, UnionNarrowingByIsdefinedC}
+
+    # should respect `Const` information still
+    @test Base.return_types((Union{UnionNarrowingByIsdefinedA, UnionNarrowingByIsdefinedB},)) do x
+        if isdefined(x, :x)
+            return x
+        else
+            return nothing # dead branch
+        end
+    end |> only === Union{UnionNarrowingByIsdefinedA, UnionNarrowingByIsdefinedB}
+end