fix: resolve self correctly in methods

Fixes #812.

spec/lang/operator/colon_spec.lua

@@ -0,0 +1,22 @@
+local util = require("spec.util")
+
+describe(":", function()
+   describe("on self", function()
+      it("can resolve methods (regression test for #812)", util.check([[
+         local interface A
+            get_type: function(A): string
+         end
+
+         local interface B is A where self:get_type() == "b"
+         end
+
+         local b: A = {
+            get_type = function(): string return "b" end
+         }
+
+         if b is B then
+            print("woaw")
+         end
+      ]]))
+   end)
+end)

tl.lua

@@ -8481,12 +8481,8 @@ do
       if not t then
          return a_type(w, "invalid", {})
       end
-
-      if t.typename == "typedecl" then
-         t = t.def
-      end
-
-      return t
+      assert(t.typename == "typedecl")
+      return t.def
    end
 
 
@@ -9431,7 +9427,11 @@ a.types[i], b.types[i]), }
          check_call = function(self, w, wargs, f, args, expected_rets, cm, argdelta)
             local arg1 = args.tuple[1]
             if cm == "method" and arg1 then
-               self:add_var(nil, "@self", a_type(w, "typedecl", { def = arg1 }))
+               local selftype = arg1
+               if selftype.typename == "self" then
+                  selftype = self:type_of_self(selftype)
+               end
+               self:add_var(nil, "@self", a_type(w, "typedecl", { def = selftype }))
             end
 
             local fargs = f.args.tuple
@@ -9661,6 +9661,10 @@ a.types[i], b.types[i]), }
 
       tbl = self:to_structural(tbl)
 
+      if tbl.typename == "self" then
+         tbl = self:type_of_self(tbl)
+      end
+
       if tbl.typename == "string" or tbl.typename == "enum" then
          tbl = self:find_var_type("string")
       end

tl.tl

@@ -8481,12 +8481,8 @@ do
       if not t then
          return an_invalid(w)
       end
-
-      if t is TypeDeclType then
-         t = t.def
-      end
-
-      return t
+      assert(t is TypeDeclType)
+      return t.def
    end
 
    -- ∃ x ∈ xs. t <: x
@@ -9431,7 +9427,11 @@ do
          check_call = function(self: TypeChecker, w: Where, wargs: {Where}, f: FunctionType, args: TupleType, expected_rets: TupleType, cm: CallMode, argdelta: integer): boolean, {Error}
             local arg1 = args.tuple[1]
             if cm == "method" and arg1 then
-               self:add_var(nil, "@self", a_typedecl(w, arg1))
+               local selftype = arg1
+               if selftype is SelfType then
+                  selftype = self:type_of_self(selftype)
+               end
+               self:add_var(nil, "@self", a_typedecl(w, selftype))
             end
 
             local fargs = f.args.tuple
@@ -9661,6 +9661,10 @@ do
 
       tbl = self:to_structural(tbl)
 
+      if tbl is SelfType then
+         tbl = self:type_of_self(tbl)
+      end
+
       if tbl is StringType or tbl is EnumType then
          tbl = self:find_var_type("string") -- simulate string metatable
       end