fix: don't ICE on invalid union in a function arg in a field

spec/lang/declaration/union_spec.lua

@@ -125,6 +125,22 @@ describe("union declaration", function()
       { y = 9, msg = "cannot discriminate a union between multiple table types" },
    }))
 
+   it("cannot declare a union between multiple records in a function in a field (#845)", util.check_type_error([[
+      local record A<T>
+         a: T
+      end
+
+      local record B<T>
+         b: T
+      end
+
+      local record C<T>
+         f: function<T>(A<T> | B<T>)
+      end
+   ]], {
+      { y = 10, msg = "cannot discriminate a union between multiple table types" },
+   }))
+
    it("cannot declare a union between multiple function types", util.check_type_error([[
       local t: function():(number) | function():(string)
    ]], {

tl.lua

@@ -7301,9 +7301,11 @@ do
       end
 
       fresh_typevar_ctr = fresh_typevar_ctr + 1
-      local ok
-      ok, t = typevar_resolver(nil, t, fresh_typevar, fresh_typearg)
-      assert(ok and t, "Internal Compiler Error: error creating fresh type variables")
+      local ok, errs
+      ok, t, errs = typevar_resolver(nil, t, fresh_typevar, fresh_typearg)
+      if not ok then
+         self.errs:collect(errs)
+      end
       return t
    end
 

tl.tl

@@ -7301,9 +7301,11 @@ do
       end
 
       fresh_typevar_ctr = fresh_typevar_ctr + 1
-      local ok: boolean
-      ok, t = typevar_resolver(nil, t, fresh_typevar, fresh_typearg)
-      assert(ok and t, "Internal Compiler Error: error creating fresh type variables")
+      local ok, errs: boolean, {Error}
+      ok, t, errs = typevar_resolver(nil, t, fresh_typevar, fresh_typearg)
+      if not ok then
+         self.errs:collect(errs)
+      end
       return t
    end