diff --git a/artiq/compiler/transforms/artiq_ir_generator.py b/artiq/compiler/transforms/artiq_ir_generator.py
index 09cea92ad..0d17417d9 100644
--- a/artiq/compiler/transforms/artiq_ir_generator.py
+++ b/artiq/compiler/transforms/artiq_ir_generator.py
@@ -284,8 +284,33 @@ class ARTIQIRGenerator(algorithm.Visitor):
         finally:
             self.current_assign = None
 
+    def coerce_to_bool(self, insn, block=None):
+        if builtins.is_bool(insn.type):
+            return insn
+        elif builtins.is_int(insn.type):
+            return self.append(ir.Compare(ast.NotEq(loc=None), insn, ir.Constant(0, insn.type)),
+                               block=block)
+        elif builtins.is_float(insn.type):
+            return self.append(ir.Compare(ast.NotEq(loc=None), insn, ir.Constant(0, insn.type)),
+                               block=block)
+        elif builtins.is_iterable(insn.type):
+            length = self.iterable_len(insn)
+            return self.append(ir.Compare(ast.NotEq(loc=None), length, ir.Constant(0, length.type)),
+                               block=block)
+        else:
+            note = diagnostic.Diagnostic("note",
+                "this expression has type {type}",
+                {"type": types.TypePrinter().name(insn.type)},
+                insn.loc)
+            diag = diagnostic.Diagnostic("warning",
+                "this expression, which is always truthful, is coerced to bool", {},
+                insn.loc, notes=[note])
+            self.engine.process(diag)
+            return ir.Constant(True, builtins.TBool())
+
     def visit_If(self, node):
         cond = self.visit(node.test)
+        cond = self.coerce_to_bool(cond)
         head = self.current_block
 
         if_true = self.add_block()
@@ -838,17 +863,19 @@ class ARTIQIRGenerator(algorithm.Visitor):
                     zip(blocks, [(h,t) for (v,h,t) in blocks[1:]] + [(tail, tail)]):
             phi.add_incoming(value, value_tail)
             if next_value_head != tail:
+                cond = self.coerce_to_bool(value, block=value_tail)
                 if isinstance(node.op, ast.And):
-                    value_tail.append(ir.BranchIf(value, next_value_head, tail))
+                    value_tail.append(ir.BranchIf(cond, next_value_head, tail))
                 else:
-                    value_tail.append(ir.BranchIf(value, tail, next_value_head))
+                    value_tail.append(ir.BranchIf(cond, tail, next_value_head))
             else:
                 value_tail.append(ir.Branch(tail))
         return phi
 
     def visit_UnaryOpT(self, node):
         if isinstance(node.op, ast.Not):
-            return self.append(ir.Select(self.visit(node.operand),
+            cond = self.coerce_to_bool(self.visit(node.operand))
+            return self.append(ir.Select(cond,
                         ir.Constant(False, builtins.TBool()),
                         ir.Constant(True,  builtins.TBool())))
         elif isinstance(node.op, ast.USub):
@@ -1116,9 +1143,7 @@ class ARTIQIRGenerator(algorithm.Visitor):
                 return ir.Constant(False, builtins.TBool())
             elif len(node.args) == 1 and len(node.keywords) == 0:
                 arg = self.visit(node.args[0])
-                return self.append(ir.Select(arg,
-                    ir.Constant(True,  builtins.TBool()),
-                    ir.Constant(False, builtins.TBool())))
+                return self.coerce_to_bool(arg)
             else:
                 assert False
         elif types.is_builtin(typ, "int"):
diff --git a/artiq/compiler/transforms/llvm_ir_generator.py b/artiq/compiler/transforms/llvm_ir_generator.py
index 61b29abee..5c9c78fe4 100644
--- a/artiq/compiler/transforms/llvm_ir_generator.py
+++ b/artiq/compiler/transforms/llvm_ir_generator.py
@@ -295,7 +295,7 @@ class LLVMIRGenerator:
             if builtins.get_int_width(typ) > builtins.get_int_width(value_typ):
                 return self.llbuilder.sext(self.map(insn.value()), self.llty_of_type(typ),
                                            name=insn.name)
-            else: # builtins.get_int_width(typ) < builtins.get_int_width(value_typ):
+            else: # builtins.get_int_width(typ) <= builtins.get_int_width(value_typ):
                 return self.llbuilder.trunc(self.map(insn.value()), self.llty_of_type(typ),
                                             name=insn.name)
         else:
diff --git a/lit-test/compiler/codegen/warning_useless_bool.py b/lit-test/compiler/codegen/warning_useless_bool.py
new file mode 100644
index 000000000..f76e011c8
--- /dev/null
+++ b/lit-test/compiler/codegen/warning_useless_bool.py
@@ -0,0 +1,5 @@
+# RUN: %python -m artiq.compiler.testbench.module +diag %s >%t
+# RUN: OutputCheck %s --file-to-check=%t
+
+# CHECK-L: ${LINE:+1}: warning: this expression, which is always truthful, is coerced to bool
+bool(IndexError())
diff --git a/lit-test/compiler/integration/operator.py b/lit-test/compiler/integration/arithmetics.py
similarity index 93%
rename from lit-test/compiler/integration/operator.py
rename to lit-test/compiler/integration/arithmetics.py
index 4bf0ab7c1..a93dd5945 100644
--- a/lit-test/compiler/integration/operator.py
+++ b/lit-test/compiler/integration/arithmetics.py
@@ -1,7 +1,5 @@
 # RUN: %python -m artiq.compiler.testbench.jit %s
 
-assert (not True) == False
-assert (not False) == True
 assert -(-1) == 1
 assert -(-1.0) == 1.0
 assert +1 == 1
diff --git a/lit-test/compiler/integration/bool.py b/lit-test/compiler/integration/bool.py
new file mode 100644
index 000000000..9df8b97c2
--- /dev/null
+++ b/lit-test/compiler/integration/bool.py
@@ -0,0 +1,20 @@
+# RUN: %python -m artiq.compiler.testbench.jit %s
+
+assert (not 0) == True
+assert (not 1) == False
+
+assert (0 and 0) is 0
+assert (1 and 0) is 0
+assert (0 and 1) is 0
+assert (1 and 2) is 2
+
+assert (0 or 0) is 0
+assert (1 or 0) is 1
+assert (0 or 1) is 1
+assert (1 or 2) is 1
+
+assert bool(False) is False and bool(False) is False
+assert bool(0) is False and bool(1) is True
+assert bool(0.0) is False and bool(1.0) is True
+x = []; assert bool(x) is False; x = [1]; assert bool(x) is True
+assert bool(range(0)) is False and bool(range(1)) is True