inference: handle Union with TypeVar properly within isdefined_tfunc (#46534)

Improves the robustness of `isdefined_tfunc` when it splits `Union`
with `TypeVar`s.
Originally reported at <aviatesk/JET.jl#379>.

Author: aviatesk

base/compiler/tfuncs.jl

@@ -257,14 +257,14 @@ end
 isdefined_tfunc(arg1, sym, order) = (@nospecialize; isdefined_tfunc(arg1, sym))
 function isdefined_tfunc(@nospecialize(arg1), @nospecialize(sym))
     if isa(arg1, Const)
-        a1 = typeof(arg1.val)
+        arg1t = typeof(arg1.val)
     else
-        a1 = widenconst(arg1)
+        arg1t = widenconst(arg1)
     end
-    if isType(a1)
+    if isType(arg1t)
         return Bool
     end
-    a1 = unwrap_unionall(a1)
+    a1 = unwrap_unionall(arg1t)
     if isa(a1, DataType) && !isabstracttype(a1)
         if a1 === Module
             hasintersect(widenconst(sym), Symbol) || return Bottom
@@ -307,8 +307,8 @@ function isdefined_tfunc(@nospecialize(arg1), @nospecialize(sym))
             end
         end
     elseif isa(a1, Union)
-        return tmerge(isdefined_tfunc(a1.a, sym),
-                      isdefined_tfunc(a1.b, sym))
+        return tmerge(isdefined_tfunc(rewrap_unionall(a1.a, arg1t), sym),
+                      isdefined_tfunc(rewrap_unionall(a1.b, arg1t), sym))
     end
     return Bool
 end

test/compiler/inference.jl

@@ -1160,6 +1160,9 @@ struct UnionIsdefinedB; x; end
 @test isdefined_tfunc(Union{UnionIsdefinedA,UnionIsdefinedB}, Const(:x)) === Const(true)
 @test isdefined_tfunc(Union{UnionIsdefinedA,UnionIsdefinedB}, Const(:y)) === Const(false)
 @test isdefined_tfunc(Union{UnionIsdefinedA,Nothing}, Const(:x)) === Bool
+# https://github.com/aviatesk/JET.jl/issues/379
+fJET379(x::Union{Complex{T}, T}) where T = isdefined(x, :im)
+@test only(Base.return_types(fJET379)) === Bool
 
 @noinline map3_22347(f, t::Tuple{}) = ()
 @noinline map3_22347(f, t::Tuple) = (f(t[1]), map3_22347(f, Base.tail(t))...)