Fix `if not disabler()` case

Closes: #6010

mesonbuild/interpreterbase.py

@@ -500,6 +500,8 @@ class InterpreterBase:
 
     def evaluate_notstatement(self, cur):
         v = self.evaluate_statement(cur.value)
+        if is_disabler(v):
+            return v
         if not isinstance(v, bool):
             raise InterpreterException('Argument to "not" is not a boolean.')
         return not v

test cases/common/163 disabler/meson.build

@@ -76,3 +76,25 @@ else
     has_not_changed = true
 endif
 assert(has_not_changed, 'App has changed.')
+
+if_is_disabled = true
+if disabler()
+  if_is_disabled = false
+else
+  if_is_disabled = false
+endif
+assert(if_is_disabled, 'Disabler in "if condition" must skip both blocks')
+
+if not disabler()
+  if_is_disabled = false
+else
+  if_is_disabled = false
+endif
+assert(if_is_disabled, 'Disabler in "if not condition" must skip both blocks')
+
+if disabler() == 1
+  if_is_disabled = false
+else
+  if_is_disabled = false
+endif
+assert(if_is_disabled, 'Disabler in "if a==b" must skip both blocks')