Skip bot type in typecheck to fix false errors on return/next

When a method body ends with a `return` (or a branch contains one),
the implicit return value of the body includes `bot` (bottom type).
Because `bot` is a subtype of all types, typecheck should not reject
it. Previously each typecheck implementation iterated over every type
in the vertex including `bot`, which caused spurious diagnostics like
"expected: bool; actual: bot".

Add `next if ty.is_a?(Type::Bot)` to all leaf typecheck methods in
sig_type.rb so that `bot` is uniformly skipped during type checking.

Fixes #377

Author: mame

lib/typeprof/core/ast/sig_type.rb

@@ -4,6 +4,7 @@ def self.typecheck_for_module(genv, changes, f_mod, f_args, a_vtx, subst)
       changes.add_edge(genv, a_vtx, changes.target)
       found_any = false
       a_vtx.each_type do |ty|
+        next if ty.is_a?(Type::Bot)
         found_any = true
         ty = ty.base_type(genv)
         while ty
@@ -199,6 +200,7 @@ def contravariant_vertex0(genv, changes, vtx, subst)
       def typecheck(genv, changes, vtx, subst)
         changes.add_edge(genv, vtx, changes.target)
         vtx.each_type do |ty|
+          next if ty.is_a?(Type::Bot)
           return false unless ty == genv.true_type || ty == genv.false_type
         end
         true
@@ -221,6 +223,7 @@ def contravariant_vertex0(genv, changes, vtx, subst)
       def typecheck(genv, changes, vtx, subst)
         changes.add_edge(genv, vtx, changes.target)
         vtx.each_type do |ty|
+          next if ty.is_a?(Type::Bot)
           return false unless ty == genv.nil_type
         end
         true
@@ -597,6 +600,7 @@ def typecheck(genv, changes, vtx, subst)
         changes.add_edge(genv, vtx, changes.target)
         found_any = false
         vtx.each_type do |ty|
+          next if ty.is_a?(Type::Bot)
           found_any = true
           case ty
           when Type::Singleton
@@ -735,6 +739,7 @@ def typecheck(genv, changes, vtx, subst)
         changes.add_edge(genv, vtx, changes.target)
         found_any = false
         vtx.each_type do |ty|
+          next if ty.is_a?(Type::Bot)
           found_any = true
           case ty
           when Type::Array
@@ -807,6 +812,7 @@ def typecheck(genv, changes, vtx, subst)
         changes.add_edge(genv, vtx, changes.target)
         found_any = false
         vtx.each_type do |ty|
+          next if ty.is_a?(Type::Bot)
           found_any = true
           case ty
           when Type::Hash
@@ -925,6 +931,7 @@ def typecheck(genv, changes, vtx, subst)
           changes.add_edge(genv, vtx, changes.target)
           found_any = false
           vtx.each_type do |ty|
+            next if ty.is_a?(Type::Bot)
             found_any = true
             case ty
             when Type::Symbol

scenario/rbs/check-block-return-bot.rb

@@ -0,0 +1,32 @@
+## update
+# Instance type (typecheck_for_module): pure bot
+#: { () -> String } -> void
+def yield_instance
+  yield
+end
+
+yield_instance do
+  next "hello"
+end
+
+# Bool type (SigTyBaseBoolNode): pure bot
+#: { () -> bool } -> void
+def yield_bool
+  yield
+end
+
+yield_bool do
+  next false
+end
+
+# Nil type (SigTyBaseNilNode): pure bot
+#: { () -> nil } -> void
+def yield_nil
+  yield
+end
+
+yield_nil do
+  next nil
+end
+
+## diagnostics

scenario/rbs/check-return-type-bot.rb

@@ -0,0 +1,77 @@
+## update
+# Instance type (typecheck_for_module): pure bot
+#: -> String
+def test_instance
+  return "hello"
+end
+
+# Instance type (typecheck_for_module): mixed bot
+#: (bool) -> String
+def test_instance_mixed(a)
+  if a
+    return "hello"
+  end
+  "world"
+end
+
+# Bool type (SigTyBaseBoolNode): pure bot
+#: -> bool
+def test_bool
+  return false
+end
+
+# Bool type (SigTyBaseBoolNode): mixed bot
+#: (bool) -> bool
+def test_bool_mixed(a)
+  if a
+    return true
+  end
+  false
+end
+
+# Nil type (SigTyBaseNilNode): pure bot
+#: -> nil
+def test_nil
+  return nil
+end
+
+# Nil type (SigTyBaseNilNode): mixed bot
+#: (bool) -> nil
+def test_nil_mixed(a)
+  if a
+    return nil
+  end
+  nil
+end
+
+# Tuple type (SigTyTupleNode): pure bot
+#: -> [Integer, String]
+def test_tuple
+  return [1, "hello"]
+end
+
+# Tuple type (SigTyTupleNode): mixed bot
+#: (bool) -> [Integer, String]
+def test_tuple_mixed(a)
+  if a
+    return [1, "hello"]
+  end
+  [2, "world"]
+end
+
+# Literal symbol type (SigTyLiteralNode): pure bot
+#: -> :foo
+def test_symbol
+  return :foo
+end
+
+# Literal symbol type (SigTyLiteralNode): mixed bot
+#: (bool) -> :foo
+def test_symbol_mixed(a)
+  if a
+    return :foo
+  end
+  :foo
+end
+
+## diagnostics

scenario/rbs/check-return-type-bot2.rb

@@ -0,0 +1,20 @@
+## update
+class Foo
+end
+
+# Singleton type (SigTySingletonNode): pure bot
+#: -> singleton(Foo)
+def test_singleton
+  return Foo
+end
+
+# Singleton type (SigTySingletonNode): mixed bot
+#: (bool) -> singleton(Foo)
+def test_singleton_mixed(a)
+  if a
+    return Foo
+  end
+  Foo
+end
+
+## diagnostics