compiler: reject reachable implicit return if not returning TNone.

Fixes #718.

artiq/compiler/transforms/artiq_ir_generator.py

@@ -317,7 +317,15 @@ class ARTIQIRGenerator(algorithm.Visitor):
                     self.current_block.append(ir.Return(ir.Constant(None, builtins.TNone())))
             else:
                 if not self.current_block.is_terminated():
+                    if len(self.current_block.predecessors()) != 0:
+                        diag = diagnostic.Diagnostic("error",
+                            "this function must return a value of type {typ} explicitly",
+                            {"typ": types.TypePrinter().name(typ.ret)},
+                            node.keyword_loc)
+                        self.engine.process(diag)
+
                     self.current_block.append(ir.Unreachable())
+
         finally:
             self.name = old_name
             self.current_args = old_args

artiq/test/lit/codegen/error_illegal_return.py

@@ -0,0 +1,9 @@
+# RUN: %python -m artiq.compiler.testbench.signature +diag %s >%t
+# RUN: OutputCheck %s --file-to-check=%t
+
+# CHECK-L: ${LINE:+1}: error: this function must return a value of type numpy.int32 explicitly
+def foo(x):
+    if x:
+        return 1
+
+foo(True)

artiq/test/lit/codegen/unreachable_implicit_return.py

@@ -0,0 +1,9 @@
+# RUN: %python -m artiq.compiler.testbench.signature %s
+
+def foo(x):
+    if x:
+        return 1
+    else:
+        return 2
+
+foo(True)