Type narrowing proof of concept

spec/is/is.lua

@@ -0,0 +1,31 @@
+local util = require("spec.util")
+
+describe("Is<T>:", function()
+
+   it("success", util.check [[
+      local record Is<T> end
+
+      local record MyRecord
+         a: number
+      end
+
+      local record OtherRecord
+         a: boolean
+      end
+
+      local r : MyRecord | OtherRecord = { a = 1 }
+
+      local n : number
+
+      local function is_myrecord(x: any): Is<MyRecord>
+         if x is table then
+            local a = x.a
+            return (a is number) as Is<MyRecord>
+         else return false as Is<MyRecord> end
+      end
+      if is_myrecord(r) then
+         n = r.a
+      end
+   ]])
+
+end)

tl.lua

@@ -5585,7 +5585,6 @@ tl.type_check = function(ast, opts)
    local function is_valid_union(typ)
 
 
-      local n_table_types = 0
       local n_function_types = 0
       local n_userdata_types = 0
       local n_string_enum = 0
@@ -5597,11 +5596,6 @@ tl.type_check = function(ast, opts)
             if n_userdata_types > 1 then
                return false, "cannot discriminate a union between multiple userdata types: %s"
             end
-         elseif ut == "table" then
-            n_table_types = n_table_types + 1
-            if n_table_types > 1 then
-               return false, "cannot discriminate a union between multiple table types: %s"
-            end
          elseif ut == "function" then
             n_function_types = n_function_types + 1
             if n_function_types > 1 then
@@ -9114,6 +9108,15 @@ node.exps[3] and node.exps[3].type, }
                   end
                end
             elseif node.op.op == "@funcall" then
+
+               if a.typename == "function" and a.rets[1] and a.rets[1].tk == "Is" and node.e2[1] then
+                  node.known = Fact({
+                     fact = "is",
+                     var = node.e2[1].tk,
+                     typ = a.rets[1].typevals[1],
+                     where = node,
+                  })
+               end
                if lax and is_unknown(a) then
                   if node.e1.op and node.e1.op.op == ":" and node.e1.e1.kind == "variable" then
                      add_unknown_dot(node, node.e1.e1.tk .. "." .. node.e1.e2.tk)

tl.tl

@@ -5585,7 +5585,6 @@ tl.type_check = function(ast: Node, opts: TypeCheckOptions): Result, string
    local function is_valid_union(typ: Type): boolean, string
       -- check for limitations in our union support
       -- due to codegen limitations (we only check with type() so far)
-      local n_table_types = 0
       local n_function_types = 0
       local n_userdata_types = 0
       local n_string_enum = 0
@@ -5597,11 +5596,6 @@ tl.type_check = function(ast: Node, opts: TypeCheckOptions): Result, string
             if n_userdata_types > 1 then
                return false, "cannot discriminate a union between multiple userdata types: %s"
             end
-         elseif ut == "table" then
-            n_table_types = n_table_types + 1
-            if n_table_types > 1 then
-               return false, "cannot discriminate a union between multiple table types: %s"
-            end
          elseif ut == "function" then
             n_function_types = n_function_types + 1
             if n_function_types > 1 then
@@ -9114,6 +9108,15 @@ tl.type_check = function(ast: Node, opts: TypeCheckOptions): Result, string
                   end
                end
             elseif node.op.op == "@funcall" then
+               -- Huge hack
+               if a.typename == "function" and a.rets[1] and a.rets[1].tk == "Is" and node.e2[1] then
+                  node.known = Fact {
+                    fact = "is",
+                    var = node.e2[1].tk, -- first argument of function
+                    typ = a.rets[1].typevals[1], -- type argument of Is<>
+                    where = node,
+                  }
+               end
                if lax and is_unknown(a) then
                   if node.e1.op and node.e1.op.op == ":" and node.e1.e1.kind == "variable" then
                      add_unknown_dot(node, node.e1.e1.tk .. "." .. node.e1.e2.tk)