Do not fail with an exception when stub is missing.

ada/ast.py

@@ -23495,7 +23495,7 @@ class PackageBody(Body):
         # Since the reference to the package decl is non transitive, we still
         # want to reference the envs that are "used" there.
         reference(Self.cast(AdaNode).singleton,
-                  through=T.PackageBody.package_decl_uses_clauses_envs,
+                  through=T.PackageBody.package_decl_use_clauses_envs,
                   cond=Not(Self.is_compilation_unit_root) | Self.is_subunit)
     )
 
@@ -23511,17 +23511,17 @@ class PackageBody(Body):
     declarative_parts = Property(Entity.decls.singleton)
 
     @langkit_property()
-    def package_decl_uses_clauses_envs():
+    def package_decl_use_clauses_envs():
         """
         Return the environments for the use clauses of the package decl of this
         body. Used because they need to be explicitly referenced.
         """
-        pd = Var(imprecise_fallback.bind(
+        return imprecise_fallback.bind(
             False, Entity.decl_part.cast_or_raise(T.BasePackageDecl)
-        ))
-
-        return Array([pd.public_part.use_clauses_envs,
-                      pd.private_part._.use_clauses_envs]).env_group()
+        ).then(lambda pd: Array([
+            pd.public_part.use_clauses_envs,
+            pd.private_part._.use_clauses_envs
+        ]).env_group())
 
 
 class TaskBody(Body):

testsuite/tests/name_resolution/invalid_subunit/pkg-inner.adb

@@ -0,0 +1,7 @@
+separate (Pkg)
+package body Inner is
+   procedure Not_Used is null;
+begin
+   I := 1;
+   pragma Test_Statement;
+end Inner;

testsuite/tests/name_resolution/invalid_subunit/pkg.adb

@@ -0,0 +1,6 @@
+package body Pkg is
+   --  package body Inner is separate;
+   --  We explicitly make the code illegal by commenting out the stub.
+   --  Libadalang should be able to handle this gracefully without property
+   --  errors.
+end Pkg;

testsuite/tests/name_resolution/invalid_subunit/pkg.ads

@@ -0,0 +1,7 @@
+package Pkg is
+   package Inner is
+      procedure Not_Used;
+
+      I : Integer;
+   end Inner;
+end Pkg;

testsuite/tests/name_resolution/invalid_subunit/test.out

@@ -0,0 +1,17 @@
+Analyzing pkg-inner.adb
+#######################
+
+Resolving xrefs for node <AssignStmt pkg-inner.adb:5:4-5:11>
+************************************************************
+
+Expr: <Id "I" pkg-inner.adb:5:4-5:5>
+  references:    <DefiningName "I" pkg.ads:5:7-5:8>
+  type:          <ConcreteTypeDecl ["Integer"] __standard:4:3-4:54>
+  expected type: None
+Expr: <Int pkg-inner.adb:5:9-5:10>
+  references:    None
+  type:          <ConcreteTypeDecl ["Universal_Int_Type_"] __standard:116:3-116:45>
+  expected type: <ConcreteTypeDecl ["Integer"] __standard:4:3-4:54>
+
+
+Done.

testsuite/tests/name_resolution/invalid_subunit/test.yaml

@@ -0,0 +1,2 @@
+driver: name-resolution
+input_sources: [pkg-inner.adb]