Add support for post hoc prerequisites

Unlike normal and ad hoc prerequisites, a post hoc prerequisite is built
after the target, not before. It may also form a dependency cycle together
with normal/ad hoc prerequisites. In other words, all this form of dependency
guarantees is that a post hoc prerequisite will be built if its dependent
target is built.

See the NEWS file for details and an example.

1 files changed, 17 insertions, 3 deletions

diff --git a/libbuild2/algorithm.ixx b/libbuild2/algorithm.ixx
index 417a10e..7c87a72 100644
--- a/libbuild2/algorithm.ixx
+++ b/libbuild2/algorithm.ixx
@@ -424,6 +424,19 @@ namespace build2
     return r;
   }
 
+  inline target_state
+  match_direct_sync (action a, const target& t, bool fail)
+  {
+    assert (t.ctx.phase == run_phase::match);
+
+    target_state r (match_impl (a, t, 0, nullptr).second);
+
+    if (fail && r == target_state::failed)
+      throw failed ();
+
+    return r;
+  }
+
   inline pair<bool, target_state>
   try_match_sync (action a, const target& t, bool fail)
   {
@@ -787,7 +800,7 @@ namespace build2
   execute_direct_impl (action, const target&, size_t, atomic_count*);
 
   inline target_state
-  execute_direct_sync (action a, const target& t)
+  execute_direct_sync (action a, const target& t, bool fail)
   {
     target_state r (execute_direct_impl (a, t, 0, nullptr));
 
@@ -800,7 +813,7 @@ namespace build2
       r = t.executed_state (a, false);
     }
 
-    if (r == target_state::failed)
+    if (r == target_state::failed && fail)
       throw failed ();
 
     return r;
@@ -960,8 +973,9 @@ namespace build2
       p.first, static_cast<const T&> (p.second));
   }
 
+  template <typename T>
   inline target_state
-  execute_members (action a, const target& t, const target* ts[], size_t n)
+  execute_members (action a, const target& t, T ts[], size_t n)
   {
     return t.ctx.current_mode == execution_mode::first
       ? straight_execute_members (a, t, ts, n, 0)