compiler: Fix lifetime tracking for function call return values

GitHub: Fixes #1497.

artiq/compiler/validators/escape.py

@@ -103,7 +103,7 @@ class RegionOf(algorithm.Visitor):
             # The cache is borrow checked dynamically
             return Global()
         else:
-            self.visit_sometimes_allocating(node)
+            return self.visit_sometimes_allocating(node)
 
     # Value lives as long as the object/container, if it's mutable,
     # or else forever

artiq/test/lit/escape/error_call.py

@@ -0,0 +1,14 @@
+# RUN: %python -m artiq.compiler.testbench.embedding +diag %s 2>%t
+# RUN: OutputCheck %s --file-to-check=%t
+
+from artiq.experiment import *
+
+@kernel
+def leak(a):
+    return a
+
+@kernel
+def entrypoint():
+    # CHECK-L: ${LINE:+2}: error: cannot return an allocated value that does not live forever
+    # CHECK-L: ${LINE:+1}: note: ... to this point
+    return leak([1, 2, 3])