more precise inference of splatnew (#35976)

This allows constant-folding NamedTuple constructors, in turn
allowing constant prop through keyword arguments.

(cherry picked from commit 7124472)

base/compiler/abstractinterpretation.jl

@@ -1026,6 +1026,17 @@ function abstract_eval(@nospecialize(e), vtypes::VarTable, sv::InferenceState)
         end
     elseif e.head === :splatnew
         t = instanceof_tfunc(abstract_eval(e.args[1], vtypes, sv))[1]
+        if length(e.args) == 2 && isconcretetype(t) && !t.mutable
+            at = abstract_eval(e.args[2], vtypes, sv)
+            n = fieldcount(t)
+            if isa(at, Const) && isa(at.val, Tuple) && n == length(at.val) &&
+                    _all(i->at.val[i] isa fieldtype(t, i), 1:n)
+                t = Const(ccall(:jl_new_structt, Any, (Any, Any), t, at.val))
+            elseif isa(at, PartialStruct) && at ⊑ Tuple && n == length(at.fields) &&
+                    _all(i->at.fields[i] ⊑ fieldtype(t, i), 1:n)
+                t = PartialStruct(t, at.fields)
+            end
+        end
     elseif e.head === :&
         abstract_eval(e.args[1], vtypes, sv)
         t = Any

test/compiler/inference.jl

@@ -2586,3 +2586,16 @@ f() = _foldl_iter(step, (Missing[],), [0.0], 1)
 end
 @test Core.Compiler.typesubtract(Tuple{Union{Int,Char}}, Tuple{Char}) == Tuple{Int}
 @test Base.return_types(Issue35566.f) == [Val{:expected}]
+
+# constant prop through keyword arguments
+_unstable_kw(;x=1,y=2) = x == 1 ? 0 : ""
+_use_unstable_kw_1() = _unstable_kw(x = 2)
+_use_unstable_kw_2() = _unstable_kw(x = 2, y = rand())
+@test Base.return_types(_use_unstable_kw_1) == Any[String]
+@test Base.return_types(_use_unstable_kw_2) == Any[String]
+@eval struct StructWithSplatNew
+    x::Int
+    StructWithSplatNew(t) = $(Expr(:splatnew, :StructWithSplatNew, :t))
+end
+_construct_structwithsplatnew() = StructWithSplatNew(("",))
+@test Base.return_types(_construct_structwithsplatnew) == Any[StructWithSplatNew]