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

- fixes #56248

(cherry picked from commit 08d11d0)

Author: aviatesk

base/compiler/tfuncs.jl

@@ -587,8 +587,16 @@ 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

@@ -5685,3 +5685,11 @@ t255751 = Array{Float32, 3}
 
 issue55882_nfields(x::Union{T,Nothing}) where T<:Number = nfields(x)
 @test Base.infer_return_type(issue55882_nfields) <: 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{}