Deducing return type of map generator helper (#1576)

* Deduce map return type implicitly

Giving the first template argument to map generator function to deduce
return type is now optional even if the return type is different from
the type generated by mapped generator.

projects/SelfTest/UsageTests/Generators.tests.cpp

@@ -144,6 +144,11 @@ TEST_CASE("Generators -- adapters", "[generators][generic]") {
             auto i = GENERATE(map<std::string>([] (int val) { return std::to_string(val); }, values({ 1, 2, 3 })));
             REQUIRE(i.size() == 1);
         }
+        SECTION("Different deduced type") {
+            // This takes a generator that returns ints and maps them into strings
+            auto i = GENERATE(map([] (int val) { return std::to_string(val); }, values({ 1, 2, 3 })));
+            REQUIRE(i.size() == 1);
+        }
     }
     SECTION("Repeating a generator") {
         // This will return values [1, 2, 3, 1, 2, 3]