resolve subprojects of projects with rooted id (#483)

src/build/project.jam

@@ -163,6 +163,7 @@ rule find ( name : current-location )
 {
     name = [ NORMALIZE_PATH $(name) ] ;
     local project-module ;
+    local location ;
 
     # Try interpreting name as project id.
     if [ path.is-rooted $(name) ]
@@ -195,14 +196,37 @@ rule find ( name : current-location )
                     load-used-projects $(caller-module) ;
                 }
             }
+            else
+            {
+                local sub = [ path.basename $(name) ] ;
+                local root = [ path.parent $(name) ] ;
+                while $(root) != /
+                {
+                    local m = $($(root).jamfile-module) ;
+                    if $(m)
+                    {
+                        location = [ path.root $(sub)
+                            [ attribute $(m) location ] ] ;
+                        root = / ;
+                    }
+                    else
+                    {
+                        sub = [ path.join [ path.basename $(root) ] $(sub) ] ;
+                        root = [ path.parent $(root) ] ;
+                    }
+                }
+            }
             project-module = $($(name).jamfile-module) ;
         }
     }
 
     if ! $(project-module)
     {
-        local location = [ path.root [ path.make $(name) ] $(current-location) ]
-            ;
+        if ! $(location)
+        {
+            location = [ path.root [ path.make $(name) ] $(current-location) ]
+                ;
+        }
 
         # If no project is registered for the given location, try to load it.
         # First see if we have a Jamfile. If not, then see if we might have a

test/project_sub_resolution.py

@@ -0,0 +1,26 @@
+#!/usr/bin/env python3
+
+# Copyright 2025 Dmitry Arkhipov <grisumras@yandex.ru>
+# Distributed under the Boost Software License, Version 1.0.
+# (See accompanying file LICENSE.txt or https://www.bfgroup.xyz/b2/LICENSE.txt)
+
+# Test that subprojects of a project with rooted id can be resolved.
+
+import BoostBuild
+
+t = BoostBuild.Tester(use_test_config=False)
+
+t.write("jamroot.jam", """
+project /A ;
+alias x ;
+alias y : /A/B/C//c ;
+alias z : /A/B//b ;
+""")
+
+t.write("B/build.jam", "alias b ;")
+
+t.write("B/C/build.jam", "alias c ;")
+
+t.run_build_system()
+
+t.cleanup()

test/test_all.py

@@ -412,6 +412,7 @@ tests = [
     "project_dependencies",
     "project_glob",
     "project_id",
+    "project_sub_resolution",
     "project_root_constants",
     "project_root_rule",
     "project_test3",