From 55c5b02df0239b10e0f3f75088a7ddbfe74de236 Mon Sep 17 00:00:00 2001
From: pvcresin <pvcresin0730@gmail.com>
Date: Mon, 17 Aug 2026 17:33:35 +0900
Subject: [PATCH] Fix splat fallback for union types

---
 lib/typeprof/core/ast/misc.rb                 | 16 +++++++++++++---
 lib/typeprof/core/graph/box.rb                | 16 ++++++++++------
 lib/typeprof/core/graph/change_set.rb         | 12 ++++++------
 scenario/array/splat.rb                       | 13 +++++++++++++
 scenario/known-issues/splat-union-fallback.rb | 12 ------------
 5 files changed, 42 insertions(+), 27 deletions(-)
 delete mode 100644 scenario/known-issues/splat-union-fallback.rb

diff --git a/lib/typeprof/core/ast/misc.rb b/lib/typeprof/core/ast/misc.rb
index 52db028d6..04a957d99 100644
--- a/lib/typeprof/core/ast/misc.rb
+++ b/lib/typeprof/core/ast/misc.rb
@@ -256,9 +256,19 @@ def install0(genv)
         vtx = @expr.install(genv)
 
         a_args = ActualArguments.new([], [], nil, nil)
-        to_a_vtx = @changes.add_method_call_box(genv, vtx, :to_a, a_args, false, suppress_errors: true).ret
-
-        @changes.add_splat_box(genv, to_a_vtx, nil, vtx).ret
+        # Keep missing `to_a` receivers separate so Union members can fall back independently.
+        to_a_fallback = Vertex.new(self)
+        to_a_box = @changes.add_method_call_box(
+          genv,
+          vtx,
+          :to_a,
+          a_args,
+          false,
+          suppress_errors: true,
+          fallback: to_a_fallback,
+        )
+
+        @changes.add_splat_box(genv, to_a_box.ret, nil, to_a_fallback).ret
       end
     end
 
diff --git a/lib/typeprof/core/graph/box.rb b/lib/typeprof/core/graph/box.rb
index c0f6268db..d62f31e29 100644
--- a/lib/typeprof/core/graph/box.rb
+++ b/lib/typeprof/core/graph/box.rb
@@ -629,13 +629,13 @@ def wrong_return_type(f_ret_show, changes)
   end
 
   class SplatBox < Box
-    def initialize(node, genv, ary, idx, orig = nil)
+    def initialize(node, genv, ary, idx, fallback = nil)
       super(node)
       @ary = ary
       @idx = idx
-      @orig = orig
+      @fallback = fallback
       @ary.add_edge(genv, self)
-      @orig.add_edge(genv, self) if @orig
+      @fallback.add_edge(genv, self) if @fallback
       @ret = Vertex.new(node)
     end
 
@@ -661,8 +661,8 @@ def run0(genv, changes)
         end
       end
       # For types where to_a is not defined, [*x] wraps x as [x]
-      if @orig && @ary.types.empty?
-        @orig.each_type do |ty|
+      if @fallback
+        @fallback.each_type do |ty|
           changes.add_edge(genv, Source.new(ty), @ret)
         end
       end
@@ -1033,7 +1033,7 @@ def run0(genv, changes)
   end
 
   class MethodCallBox < Box
-    def initialize(node, genv, recv, mid, a_args, subclasses, suppress_errors: false)
+    def initialize(node, genv, recv, mid, a_args, subclasses, suppress_errors: false, fallback: nil)
       raise mid.to_s unless mid
       super(node)
       @recv = recv.new_vertex(genv, node)
@@ -1045,6 +1045,7 @@ def initialize(node, genv, recv, mid, a_args, subclasses, suppress_errors: false
       @ret = Vertex.new(node)
       @subclasses = subclasses
       @suppress_errors = suppress_errors
+      @fallback = fallback
       @generics = {}
     end
 
@@ -1059,6 +1060,9 @@ def run0(genv, changes)
           box = add_symbol_proc_call_box(changes, genv, orig_ty.sym, @a_args.positionals, @a_args.keywords)
           changes.add_edge(genv, box.ret, @ret) if box
         elsif !me
+          if @fallback
+            changes.add_edge(genv, Source.new(orig_ty), @fallback)
+          end
           unless @suppress_errors
             if error_count < 3
               meth = @node.mid_code_range ? :mid_code_range : :code_range
diff --git a/lib/typeprof/core/graph/change_set.rb b/lib/typeprof/core/graph/change_set.rb
index 44f2b54a6..d80a81220 100644
--- a/lib/typeprof/core/graph/change_set.rb
+++ b/lib/typeprof/core/graph/change_set.rb
@@ -71,9 +71,9 @@ def add_edge(genv, src, dst)
 
     # TODO: if an edge is removed during one analysis, we may need to remove sub-boxes?
 
-    def add_method_call_box(genv, recv, mid, a_args, subclasses, suppress_errors: false)
-      key = [:mcall, recv, mid, a_args, subclasses, suppress_errors]
-      new_boxes[key] ||= MethodCallBox.new(@node, genv, recv, mid, a_args, subclasses, suppress_errors: suppress_errors)
+    def add_method_call_box(genv, recv, mid, a_args, subclasses, suppress_errors: false, fallback: nil)
+      key = [:mcall, recv, mid, a_args, subclasses, suppress_errors, fallback]
+      new_boxes[key] ||= MethodCallBox.new(@node, genv, recv, mid, a_args, subclasses, suppress_errors: suppress_errors, fallback: fallback)
     end
 
     def add_escape_box(genv, a_ret)
@@ -81,9 +81,9 @@ def add_escape_box(genv, a_ret)
       new_boxes[key] ||= EscapeBox.new(@node, genv, a_ret)
     end
 
-    def add_splat_box(genv, arg, idx = nil, orig = nil)
-      key = [:splat, arg, idx, orig]
-      new_boxes[key] ||= SplatBox.new(@node, genv, arg, idx, orig)
+    def add_splat_box(genv, arg, idx = nil, fallback = nil)
+      key = [:splat, arg, idx, fallback]
+      new_boxes[key] ||= SplatBox.new(@node, genv, arg, idx, fallback)
     end
 
     def add_hash_splat_box(genv, arg, unified_key, unified_val)
diff --git a/scenario/array/splat.rb b/scenario/array/splat.rb
index 8297b4b9a..5578825f4 100644
--- a/scenario/array/splat.rb
+++ b/scenario/array/splat.rb
@@ -11,3 +11,16 @@ class Object
   def foo: -> Array[Integer]
   def bar: -> Array[String]
 end
+
+## update
+def foo(x)
+  [*x]
+end
+
+foo([:int])
+foo(:sym)
+
+## assert
+class Object
+  def foo: (:sym | [:int]) -> Array[:int | :sym]
+end
diff --git a/scenario/known-issues/splat-union-fallback.rb b/scenario/known-issues/splat-union-fallback.rb
deleted file mode 100644
index a6b6b6252..000000000
--- a/scenario/known-issues/splat-union-fallback.rb
+++ /dev/null
@@ -1,12 +0,0 @@
-## update
-def foo(x)
-  [*x]
-end
-
-foo([:int])
-foo(:sym)
-
-## assert
-class Object
-  def foo: (Array[:int] | :sym) -> Array[:int | :sym]
-end