fix: compiler autoselection on 1st run alr get --build

src/alire/alire-roots-optional.adb

@@ -137,6 +137,15 @@ package body Alire.Roots.Optional is
         Reference'(Ptr => This.Data.Value'Unrestricted_Access);
    end Value;
 
+   -------------
+   -- Discard --
+   -------------
+
+   procedure Discard (This : in out Root) is
+   begin
+      This.Data := (Status => Outside);
+   end Discard;
+
    ---------------------
    -- Outcome_Failure --
    ---------------------

src/alire/alire-roots-optional.ads

@@ -42,6 +42,9 @@ package Alire.Roots.Optional is
    function Value (This : in out Root) return Reference with
      Pre => This.Is_Valid;
 
+   procedure Discard (This : in out Root);
+   --  Sets the root to Outside
+
    function Brokenness (This : Root) return String with
      Pre => This.Is_Broken;
 

src/alr/alr-commands-get.adb

@@ -187,14 +187,24 @@ package body Alr.Commands.Get is
                Build_OK := True;
 
             else
+               --  Require the workspace to ensure the same checks as in a
+               --  manual `get` followed by `cd` and `build` are performed.
+
+               Cmd.Optional_Root.Discard;
+               --  It will be reloaded next. This is needed or otherwise the
+               --  following call would do nothing (it would see a proper root
+               --  already available so it would just return).
+
+               Cmd.Requires_Workspace;
+
                --  Build in release mode for a `get --build`
                Cmd.Root.Set_Build_Profile
-                 (Crate   =>  Cmd.Root.Name,
-                  Profile =>  Alire.Utils.Switches.Release);
+                 (Crate   => Cmd.Root.Name,
+                  Profile => Alire.Utils.Switches.Release);
 
                Build_OK := Cmd.Root.Build
-                 (Cmd_Args         =>  AAA.Strings.Empty_Vector,
-                  Saved_Profiles   => False);
+                 (Cmd_Args       => AAA.Strings.Empty_Vector,
+                  Saved_Profiles => False);
             end if;
          else
             Build_OK := True;

testsuite/drivers/alr.py

@@ -617,3 +617,11 @@ def unselect_compiler():
     """
     run_alr("settings", "--global", "--unset", "toolchain.use.gnat")
     run_alr("settings", "--global", "--unset", "toolchain.external.gnat")
+
+
+def set_default_user_settings():
+    """
+    Set the default alr settings that are undone by the testsuite defaults
+    """
+    run_alr("settings", "--global", "--set", "index.auto_community", "true")
+    run_alr("settings", "--global", "--set", "toolchain.assistant", "true")

testsuite/tests/dockerized/get/build/test.py

@@ -0,0 +1,26 @@
+"""
+Check the `alr get --build` combo on 1st run with no available compiler. We
+test under a pristine environment to avoid re-occurrence of issue #1671, which
+only happens when no compiler has been selected yet, and none is available in
+the environment.
+"""
+
+import subprocess
+from drivers.alr import run_alr, set_default_user_settings
+
+# First undo any testsuite compiler and index setup
+set_default_user_settings()
+
+# Remove gnat from path so no compiler is available
+subprocess.run(['sudo', 'mv', '/usr/bin/gnat', '/usr/bin/gnat.bak']).check_returncode()
+try:
+    assert subprocess.run(['gnat', '--version']).returncode == 0, "Unexpected GNAT found"
+except FileNotFoundError:
+    pass
+
+# Should succeed, issue in #1671 was that no compiler was available nor
+# selected automatically as should have been the case.
+p = run_alr('get', '--build', 'hello')
+
+
+print('SUCCESS')

testsuite/tests/dockerized/get/build/test.yaml

@@ -0,0 +1,5 @@
+driver: docker-wrapper
+control:
+    - [SKIP, "skip_docker",  "Docker disabled"]
+    - [SKIP, "skip_network", "Network disabled"]
+# To exactly reproduce the issue we use the community index here