fix: do not try to decide validity of unions too early

Looks like this moves some errors around, but overall it looks like
a better behavior.

Fixes #903.

spec/lang/arguments/union_spec.lua

@@ -13,6 +13,7 @@ describe("union argument", function()
       R.f = function(a: boolean | E)
       end
    ]], {
-      { y = 9, msg = "unknown type E" }
+      { y = 9, msg = "argument 1: types are incompatible" },
+      { y = 9, msg = "unknown type E" },
    }))
 end)

spec/lang/declaration/union_spec.lua

@@ -163,4 +163,25 @@ describe("union declaration", function()
          end
       end
    ]]))
+
+   it("do not try to resolve validity of unions too early (regression test for #903)", util.check([[
+      local interface A
+         where true
+      end
+
+      local record D
+         v: A | C
+      end
+
+      local interface C
+         where true
+      end
+
+      local d: D
+
+      local v = d.v
+      if v is A then
+      end
+   ]]))
+
 end)

spec/lang/macroexp/is_spec.lua

@@ -57,15 +57,15 @@ describe("__is with macroexp", function()
 
    it("can expand self in an expression", util.gen([[
       local record R1
-         metamethod __is: function(self: R1|R2): boolean = macroexp(self: R1|R2): boolean
+         metamethod __is: function(self: R1): boolean = macroexp(self: R1): boolean
             return self.kind == "r1"
          end
 
          kind: string
       end
 
       local record R2
-         metamethod __is: function(self: R1|R2): boolean = macroexp(self: R1|R2): boolean
+         metamethod __is: function(self: R2): boolean = macroexp(self: R2): boolean
             return self.kind == "r2"
          end
 

tl.lua

@@ -13753,9 +13753,9 @@ self:expand_type(node, values, elements) })
          },
          ["union"] = {
             after = function(self, typ, _children)
-               local ok, err = is_valid_union(typ)
-               if not ok then
-                  return err and self.errs:invalid_at(typ, err, typ) or a_type(typ, "invalid", {})
+               local _, err = is_valid_union(typ)
+               if err then
+                  return self.errs:invalid_at(typ, err, typ)
                end
                return typ
             end,

tl.tl

@@ -13753,9 +13753,9 @@ do
          },
          ["union"] = {
             after = function(self: TypeChecker, typ: UnionType, _children: {Type}): Type
-               local ok, err = is_valid_union(typ)
-               if not ok then
-                  return err and self.errs:invalid_at(typ, err, typ) or an_invalid(typ)
+               local _, err = is_valid_union(typ)
+               if err then
+                  return self.errs:invalid_at(typ, err, typ)
                end
                return typ
             end