fix: type inference with if x == nil or ...

Thanks @svermeulen for the testcase!

Fixes #823.

spec/lang/operator/is_spec.lua

@@ -9,6 +9,7 @@ describe("flow analysis with is", function()
       end
    ]], {
       { y = 1, msg = "unknown type b"},
+      { y = 2, msg = "cannot resolve a type for x here"},
       { y = 2, msg = "unknown variable: x"},
       { y = 2, msg = "cannot resolve a type for x here"},
    }))

spec/lang/operator/or_spec.lua

@@ -201,6 +201,19 @@ describe("or", function()
       end
    ]]))
 
+   it("resolves the negation of 'or' with '== nil' when 'if' returns (regression test for #823)", util.check([[
+      local function foo(bar:string):boolean
+         if bar == nil or #bar == 0 then
+            return true
+         end
+
+         print(bar:upper())
+         return false
+      end
+
+      foo("asdf")
+   ]]))
+
    it("resolves the negation of 'or' when 'if' returns, error case with interface", util.check_type_error([[
       local interface Type
          t: string

tl.lua

@@ -10487,7 +10487,7 @@ a.types[i], b.types[i]), }
          elseif f.fact == "and" and f.f2 and f.f2.fact == "truthy" then
             return eval_not(self, f.f1)
          elseif f.fact == "or" and f.f2 and f.f2.fact == "truthy" then
-            return eval_fact(self, f.f1)
+            return eval_not(self, f.f1)
          elseif f.fact == "and" then
             return or_facts(self, eval_not(self, f.f1), eval_not(self, f.f2))
          elseif f.fact == "or" then

tl.tl

@@ -10487,7 +10487,7 @@ do
          elseif f is AndFact and f.f2 and f.f2.fact == "truthy" then
             return eval_not(self, f.f1)
          elseif f is OrFact and f.f2 and f.f2.fact == "truthy" then
-            return eval_fact(self, f.f1)
+            return eval_not(self, f.f1)
          elseif f is AndFact then
             return or_facts(self, eval_not(self, f.f1), eval_not(self, f.f2))
          elseif f is OrFact then