experiment: make type system more nominal

Instead of treating nominal records nominally and all other nominal
types structurally, with this commit we treat all nominal types
nominally except for unions, which are treated structurally.

Give this branch a try in your codebase and let me know your impressions!

spec/declaration/record_spec.lua

@@ -702,7 +702,7 @@ for i, name in ipairs({"records", "arrayrecords"}) do
          end
 
          function Foo.new(): Foo
-            return setmetatable({}, Foo) -- typing of arguments is being very permissive here, may change in the future and require a cast
+            return setmetatable({}, Foo as metatable<Foo>)
          end
 
          local foo = Foo.new()

tl.lua

@@ -1418,7 +1418,7 @@ end
 
 local function new_node(tokens, i, kind)
    local t = tokens[i]
-   return { y = t.y, x = t.x, tk = t.tk, kind = kind or t.kind }
+   return { y = t.y, x = t.x, tk = t.tk, kind = kind or (t.kind) }
 end
 
 local function a_type(t)
@@ -7075,17 +7075,13 @@ tl.type_check = function(ast, opts)
          end
          return false, terr(t1, "cannot match against any alternatives of the polymorphic type")
       elseif t1.typename == "nominal" and t2.typename == "nominal" then
-         local same, err = are_same_nominals(t1, t2)
-         if same then
-            return true
-         end
          local t1r = resolve_tuple_and_nominal(t1)
          local t2r = resolve_tuple_and_nominal(t2)
-         if is_record_type(t1r) and is_record_type(t2r) then
-            return same, err
-         else
+         if t1r.typename == "union" or t2r.typename == "union" then
             return is_a(t1r, t2r, for_equality)
          end
+
+         return are_same_nominals(t1, t2)
       elseif t1.typename == "enum" and t2.typename == "string" then
          local ok
          if for_equality then
@@ -9086,7 +9082,7 @@ tl.type_check = function(ast, opts)
                   local infertype = infertypes[i]
 
                   local rt = resolve_tuple_and_nominal(t)
-                  if rt.typename ~= "enum" and not same_type(t, infertype) then
+                  if rt.typename ~= "enum" and (t.typename ~= "nominal" or rt.typename == "union") and not same_type(t, infertype) then
                      add_var(where, var.tk, infer_at(where, infertype), "const", "narrowed_declaration")
                   end
                end
@@ -10062,6 +10058,14 @@ tl.type_check = function(ast, opts)
                   end
                end
 
+               if orig_a.typename == "nominal" and orig_b.typename == "nominal" and not meta_on_operator then
+                  if is_a(orig_a, orig_b) then
+                     node.type = resolve_tuple(orig_a)
+                  else
+                     node_error(node, "cannot use operator '" .. node.op.op:gsub("%%", "%%%%") .. "' for distinct nominal types %s and %s", resolve_tuple(orig_a), resolve_tuple(orig_b))
+                  end
+               end
+
                if types_op == numeric_binop or node.op.op == ".." then
                   node.known = FACT_TRUTHY
                end

tl.tl

@@ -1418,7 +1418,7 @@ end
 
 local function new_node(tokens: {Token}, i: integer, kind: NodeKind): Node
    local t = tokens[i]
-   return { y = t.y, x = t.x, tk = t.tk, kind = kind or t.kind }
+   return { y = t.y, x = t.x, tk = t.tk, kind = kind or (t.kind as NodeKind) }
 end
 
 local function a_type(t: Type): Type
@@ -7075,17 +7075,13 @@ tl.type_check = function(ast: Node, opts: TypeCheckOptions): Result, string
          end
          return false, terr(t1, "cannot match against any alternatives of the polymorphic type")
       elseif t1.typename == "nominal" and t2.typename == "nominal" then
-         local same, err = are_same_nominals(t1, t2)
-         if same then
-            return true
-         end
          local t1r = resolve_tuple_and_nominal(t1)
          local t2r = resolve_tuple_and_nominal(t2)
-         if is_record_type(t1r) and is_record_type(t2r) then
-            return same, err
-         else
+         if t1r.typename == "union" or t2r.typename == "union" then
             return is_a(t1r, t2r, for_equality)
          end
+
+         return are_same_nominals(t1, t2)
       elseif t1.typename == "enum" and t2.typename == "string" then
          local ok: boolean
          if for_equality then
@@ -9086,7 +9082,7 @@ tl.type_check = function(ast: Node, opts: TypeCheckOptions): Result, string
                   local infertype = infertypes[i]
 
                   local rt = resolve_tuple_and_nominal(t)
-                  if rt.typename ~= "enum" and not same_type(t, infertype) then
+                  if rt.typename ~= "enum" and (t.typename ~= "nominal" or rt.typename == "union") and not same_type(t, infertype) then
                      add_var(where, var.tk, infer_at(where, infertype), "const", "narrowed_declaration")
                   end
                end
@@ -10062,6 +10058,14 @@ tl.type_check = function(ast: Node, opts: TypeCheckOptions): Result, string
                   end
                end
 
+               if orig_a.typename == "nominal" and orig_b.typename == "nominal" and not meta_on_operator then
+                  if is_a(orig_a, orig_b) then
+                     node.type = resolve_tuple(orig_a)
+                  else
+                     node_error(node, "cannot use operator '" .. node.op.op:gsub("%%", "%%%%") .. "' for distinct nominal types %s and %s", resolve_tuple(orig_a), resolve_tuple(orig_b))
+                  end
+               end
+
                if types_op == numeric_binop or node.op.op == ".." then
                   node.known = FACT_TRUTHY
                end