fix is inference taking into account that unions imply nil

spec/inference/if_spec.lua

@@ -131,4 +131,17 @@ describe("flow typing in 'if' statements", function()
    ]], {
       { msg = "cannot use operator '+' for types integer | boolean | string and integer" },
    }))
+
+   -- see also pending test "detects empty unions" in spec/operators/is_spec.lua
+   it("detects a union value to be nil if all types are exhausted (regression test for #695)", util.check_warnings([[
+      global function f2(val: string|number)
+         if val is string then
+            print(val)
+         elseif val is number then
+            print(val)
+         else
+            error("string or number expected")
+         end
+      end
+   ]], {}, {}))
 end)

spec/operator/is_spec.lua

@@ -233,7 +233,8 @@ describe("flow analysis with is", function()
          { y = 5, msg = [[cannot use operator '+' for types string (inferred at foo.tl:4:10) and integer]] },
       }))
 
-      it("detects empty unions", util.check_type_error([[
+      -- this is not an empty union because `number | string` implies nil.
+      pending("detects empty unions", util.check_type_error([[
          local t: number | string
          if t is number then
             t = t + 1

tl.lua

@@ -8238,7 +8238,7 @@ tl.type_check = function(ast, opts)
             elseif is_a(t2, t1) then
                return t2
             else
-               return INVALID
+               return NIL
             end
          end
       end
@@ -8279,7 +8279,7 @@ tl.type_check = function(ast, opts)
          end
 
          if #types == 0 then
-            return INVALID
+            return NIL
          end
 
          return unite(types)
@@ -8403,7 +8403,8 @@ tl.type_check = function(ast, opts)
             end
             if typ.typename ~= "typevar" then
                if is_a(typ, f.typ) then
-                  node_warning("branch", f.where, f.var .. " (of type %s) is always a %s", show_type(typ), show_type(f.typ))
+
+
                   return { [f.var] = f }
                elseif not is_a(f.typ, typ) then
                   node_error(f.where, f.var .. " (of type %s) can never be a %s", typ, f.typ)

tl.tl

@@ -8238,7 +8238,7 @@ tl.type_check = function(ast: Node, opts: TypeCheckOptions): Result, string
             elseif is_a(t2, t1) then
                return t2
             else
-               return INVALID
+               return NIL -- because of implicit nil in all unions
             end
          end
       end
@@ -8279,7 +8279,7 @@ tl.type_check = function(ast: Node, opts: TypeCheckOptions): Result, string
          end
 
          if #types == 0 then
-            return INVALID
+            return NIL -- because of implicit nil in all unions
          end
 
          return unite(types)
@@ -8403,7 +8403,8 @@ tl.type_check = function(ast: Node, opts: TypeCheckOptions): Result, string
             end
             if typ.typename ~= "typevar" then
                if is_a(typ, f.typ) then
-                  node_warning("branch", f.where, f.var .. " (of type %s) is always a %s", show_type(typ), show_type(f.typ))
+                  -- drop this warning because of implicit nil in all unions
+                  -- node_warning("branch", f.where, f.var .. " (of type %s) is always a %s", show_type(typ), show_type(f.typ))
                   return { [f.var] = f }
                elseif not is_a(f.typ, typ) then
                   node_error(f.where, f.var .. " (of type %s) can never be a %s", typ, f.typ)