compiler: do not try to re-coerce fully coerced numerics.

artiq/compiler/transforms/inferencer.py

@@ -310,9 +310,17 @@ class Inferencer(algorithm.Visitor):
         node_types = []
         for node in nodes:
             if isinstance(node, asttyped.CoerceT):
+                # If we already know exactly what we coerce this value to, use that type,
+                # or we'll get an unification error in case the coerced type is not the same
+                # as the type of the coerced value.
+                # Otherwise, use the potentially more specific subtype when considering possible
+                # coercions, or we may get stuck.
+                if node.type.fold(False, lambda acc, ty: acc or types.is_var(ty)):
                     node_types.append(node.value.type)
                 else:
                     node_types.append(node.type)
+            else:
+                node_types.append(node.type)
         if any(map(types.is_var, node_types)): # not enough info yet
             return
         elif not all(map(builtins.is_numeric, node_types)):

artiq/test/lit/inferencer/coerce_explicit.py

@@ -0,0 +1,12 @@
+# RUN: %python -m artiq.compiler.testbench.inferencer +mono %s >%t
+# RUN: OutputCheck %s --file-to-check=%t
+
+# CHECK-L: n:numpy.int32 =
+n = 0
+# CHECK-L: a:numpy.int32 =
+a = n // 1
+# CHECK-L: b:numpy.int32 =
+b = n // 10
+# CHECK-L: q:numpy.int64 =
+q = (a << 0) + (b << 8)
+core_log(int64(q))