compiler: Fix type inference in slice expressions

This was a long-standing issue affecting both lists and
the new NumPy array implementation, just caused by the
generic inference passes not being run on the slice
subexpressions (and thus e.g. ints not being monomorphized).

GitHub: Fixes #1632.

artiq/compiler/transforms/inferencer.py

@@ -216,6 +216,7 @@ class Inferencer(algorithm.Visitor):
                         value.loc, None)
 
     def visit_SliceT(self, node):
+        self.generic_visit(node)
         if (node.lower, node.upper, node.step) == (None, None, None):
             self._unify(node.type, builtins.TInt32(),
                         node.loc, None)

artiq/test/lit/regression/issue_1632.py

@@ -0,0 +1,19 @@
+# RUN: %python -m artiq.compiler.testbench.embedding %s
+
+from artiq.language.core import *
+from artiq.language.types import *
+import numpy as np
+
+class A:
+    def __init__(self):
+        self.n = 2
+
+    @kernel
+    def run(self):
+        print([1, 2, 3][:self.n])
+
+a = A()
+
+@kernel
+def entrypoint():
+    a.run()