Update variables after normalizing varargs (#28812)

This code was a bit odd. It started by normalzing the varargs, but
then used `va0` and `va1` from the original, unnormalized type.
Either the unormalized version was intended in which case normalizing
first was wasteful, or the normalized version was intended in which
case the code was incorrct. Talking with Jeff, he believes the latter
was intended, so update the code to do that instead.

src/jltypes.c

@@ -1131,9 +1131,9 @@ static jl_value_t *inst_datatype_inner(jl_datatype_t *dt, jl_svec_t *p, jl_value
  }
 
  jl_datatype_t *ndt = NULL;
- JL_GC_PUSH2(&p, &ndt);
-
  jl_value_t *last = iparams[ntp - 1];
+ JL_GC_PUSH3(&p, &ndt, &last);
+
  int isvatuple = 0;
  if (istuple && ntp > 0 && jl_is_vararg_type(last)) {
  isvatuple = 1;
@@ -1149,17 +1149,13 @@ static jl_value_t *inst_datatype_inner(jl_datatype_t *dt, jl_svec_t *p, jl_value
  JL_GC_POP();
  return (jl_value_t*)jl_anytuple_type;
  }
- {
- JL_GC_PUSH1(&last);
- jl_value_t *last2 = normalize_vararg(last);
- if (last2 != last) {
- last = last2;
- p = jl_alloc_svec(ntp);
- for (size_t i = 0; i < ntp-1; i++)
- jl_svecset(p, i, iparams[i]);
- jl_svecset(p, ntp-1, last);
- }
- JL_GC_POP();
+ int did_normalize = 0;
+ jl_value_t *last2 = normalize_vararg(last);
+ if (last2 != last) {
+ last = last2;
+ did_normalize = 1;
+ va = jl_unwrap_unionall(last);
+ va0 = jl_tparam0(va); va1 = jl_tparam1(va);
  }
  if (jl_is_long(va1)) {
  ssize_t nt = jl_unbox_long(va1);
@@ -1183,6 +1179,12 @@ static jl_value_t *inst_datatype_inner(jl_datatype_t *dt, jl_svec_t *p, jl_value
  return ndt;
  }
  }
+ if (did_normalize) {
+ p = jl_alloc_svec(ntp);
+ for (size_t i = 0; i < ntp-1; i++)
+ jl_svecset(p, i, iparams[i]);
+ jl_svecset(p, ntp-1, last);
+ }
  }
 
  // move array of instantiated parameters to heap; we need to keep it

test/core.jl

@@ -6702,3 +6702,7 @@ end
 @test_throws ErrorException Array{Int, 2}(undef, 0, -10)
 @test_throws ErrorException Array{Int, 2}(undef, -10, 0)
 @test_throws ErrorException Array{Int, 2}(undef, -1, -1)
+
+# issue #28812
+@test Tuple{Vararg{Array{T},3} where T} === Tuple{Array,Array,Array}
+@test Tuple{Vararg{Array{T} where T,3}} === Tuple{Array,Array,Array}