From fe7d095647756c7d0e5060965efaaa96505470eb Mon Sep 17 00:00:00 2001
From: ridiculousfish <rf@fishshell.com>
Date: Sun, 16 Oct 2022 15:40:33 -0700
Subject: [PATCH] Add maybe_t::value_or

This enables getting the value or returning the passed-in value.
This is helpful for "default if none."
---
 src/fish_tests.cpp | 9 +++++++++
 src/maybe.h        | 8 ++++++++
 2 files changed, 17 insertions(+)

diff --git a/src/fish_tests.cpp b/src/fish_tests.cpp
index f29bca3c7..28c38bd37 100644
--- a/src/fish_tests.cpp
+++ b/src/fish_tests.cpp
@@ -6435,6 +6435,10 @@ void test_maybe() {
     do_test(std::is_copy_assignable<maybe_t<noncopyable>>::value == false);
     do_test(std::is_copy_constructible<maybe_t<noncopyable>>::value == false);
 
+    // We can construct a maybe_t from noncopyable things.
+    maybe_t<noncopyable> nmt{make_unique<int>(42)};
+    do_test(**nmt == 42);
+
     maybe_t<std::string> c1{"abc"};
     maybe_t<std::string> c2 = c1;
     do_test(c1.value() == "abc");
@@ -6442,6 +6446,11 @@ void test_maybe() {
     c2 = c1;
     do_test(c1.value() == "abc");
     do_test(c2.value() == "abc");
+
+    do_test(c2.value_or("derp") == "abc");
+    do_test(c2.value_or("derp") == "abc");
+    c2.reset();
+    do_test(c2.value_or("derp") == "derp");
 }
 
 void test_layout_cache() {
diff --git a/src/maybe.h b/src/maybe.h
index bfcb4c83f..e48a07972 100644
--- a/src/maybe.h
+++ b/src/maybe.h
@@ -209,6 +209,14 @@ class maybe_t : private maybe_detail::conditionally_copyable_t<T> {
     // Transfer the value to the caller.
     T acquire() { return impl_.acquire(); }
 
+    // Return (a copy of) our value, or the given value if we are empty.
+    T value_or(T v) const {
+        if (this->has_value()) {
+            return this->value();
+        }
+        return v;
+    }
+
     // Clear the value.
     void reset() { impl_.reset(); }