compiler: Fix type inference for "ternary" if expressions

Previously, any type would be accepted for the test expression,
leading to internal errors in the code generator if the passed
value wasn't in fact a bool.

artiq/compiler/transforms/inferencer.py

@@ -305,6 +305,7 @@ class Inferencer(algorithm.Visitor):
 
     def visit_IfExpT(self, node):
         self.generic_visit(node)
+        self._unify(node.test.type, builtins.TBool(), node.test.loc, None)
         self._unify(node.body.type, node.orelse.type,
                     node.body.loc, node.orelse.loc)
         self._unify(node.type, node.body.type,

artiq/test/lit/inferencer/if_expr.py

@@ -0,0 +1,6 @@
+# RUN: %python -m artiq.compiler.testbench.inferencer %s >%t
+# RUN: OutputCheck %s --file-to-check=%t
+
+# CHECK-L: def foo(val:bool)->numpy.int?:
+def foo(val):
+    return 1 if val else 0