Fix uniquerep predicate in codegen (#50295)

Fixes #50293. This code probably predates us being clear on what
the uniquerep predicate is.

Author: Keno

src/codegen.cpp

@@ -1862,6 +1862,7 @@ static inline jl_cgval_t ghostValue(jl_codectx_t &ctx, jl_value_t *typ)
         typ = (jl_value_t*)jl_typeofbottom_type->super;
     }
     if (jl_is_type_type(typ)) {
+        assert(is_uniquerep_Type(typ));
         // replace T::Type{T} with T, by assuming that T must be a leaftype of some sort
         jl_cgval_t constant(NULL, true, typ, NULL, best_tbaa(ctx.tbaa(), typ));
         constant.constant = jl_tparam0(typ);
@@ -1933,16 +1934,14 @@ static inline jl_cgval_t value_to_pointer(jl_codectx_t &ctx, const jl_cgval_t &v
 
 static inline jl_cgval_t mark_julia_type(jl_codectx_t &ctx, Value *v, bool isboxed, jl_value_t *typ)
 {
-    if (jl_is_datatype(typ) && jl_is_datatype_singleton((jl_datatype_t*)typ)) {
-        // no need to explicitly load/store a constant/ghost value
-        return ghostValue(ctx, typ);
-    }
     if (jl_is_type_type(typ)) {
-        jl_value_t *tp0 = jl_tparam0(typ);
-        if (jl_is_concrete_type(tp0) || tp0 == jl_bottom_type) {
+        if (is_uniquerep_Type(typ)) {
             // replace T::Type{T} with T
             return ghostValue(ctx, typ);
         }
+    } else if (jl_is_datatype(typ) && jl_is_datatype_singleton((jl_datatype_t*)typ)) {
+        // no need to explicitly load/store a constant/ghost value
+        return ghostValue(ctx, typ);
     }
     Type *T = julia_type_to_llvm(ctx, typ);
     if (type_is_ghost(T)) {

test/core.jl

@@ -8033,3 +8033,10 @@ bar50250(b, y) = (b ? Bar50250(y, y) : Bar50250(y)).x
 @test_throws UndefRefError foo50250(false, 1)
 @test bar50250(true, 1) === 1
 @test_throws UndefRefError bar50250(false, 1)
+
+# Test that Type{typeof(Union{})} doesn't get codegen'ed as a constant (#50293)
+baz50293(x::Union{Type, Core.Const}) = Base.issingletontype(x)
+bar50293(@nospecialize(u)) = (Base.issingletontype(u.a), baz50293(u.a))
+let u = Union{Type{Union{}}, Type{Any}}, ab = bar50293(u)
+    @test ab[1] == ab[2] == false
+end