Add lazy function with auto inferred type

This allow to write `lazy(f)` and get the type of the object
automatically inferred from the type of `f`.

Author: romainbrenguier

src/util/lazy.h

@@ -43,4 +43,12 @@ class lazyt
   }
 };
 
+/// Delay the computation of \p fun to the next time the \c force method
+/// is called.
+template<typename funt>
+auto lazy(funt fun) -> lazyt<decltype(fun())>
+{
+  return lazyt<decltype(fun())>::from_fun(std::move(fun));
+}
+
 #endif // CPROVER_UTIL_LAZY_H

unit/util/lazy.cpp

@@ -16,8 +16,8 @@ SCENARIO("lazy test", "[core][util][lazy]")
     ++call_counter;
     return s.length();
   };
-  auto lazy_length =
-    lazyt<int>::from_fun([&]() { return length_with_counter("foo"); });
+  lazyt<std::size_t> lazy_length =
+    lazy([&] { return length_with_counter("foo"); });
 
   REQUIRE(call_counter == 0);
   auto result = lazy_length.force();