inference: fix inference error from constructing invalid TypeVar (#56264)

- fixes #56248

Author: aviatesk

base/compiler/tfuncs.jl

@@ -601,8 +601,16 @@ add_tfunc(svec, 0, INT_INF, @nospecs((𝕃::AbstractLattice, args...)->SimpleVec
                 return TypeVar
             end
         end
-        tv = TypeVar(nval, lb, ub)
-        return PartialTypeVar(tv, lb_certain, ub_certain)
+        lb_valid = lb isa Type || lb isa TypeVar
+        ub_valid = ub isa Type || ub isa TypeVar
+        if lb_valid && ub_valid
+            tv = TypeVar(nval, lb, ub)
+            return PartialTypeVar(tv, lb_certain, ub_certain)
+        elseif !lb_valid && lb_certain
+            return Union{}
+        elseif !ub_valid && ub_certain
+            return Union{}
+        end
     end
     return TypeVar
 end

test/compiler/inference.jl

@@ -6055,3 +6055,11 @@ f55916(::Vararg{T,T}) where {T} = "2"
 g55916(x) = f55916(x)
 # this shouldn't error
 @test only(code_typed(g55916, (Any,); optimize=false))[2] == Int
+
+# JuliaLang/julia#56248
+@test Base.infer_return_type() do
+    TypeVar(:Issue56248, 1)
+end === Union{}
+@test Base.infer_return_type() do
+    TypeVar(:Issue56248, Any, 1)
+end === Union{}