Fix type annotations with mixed tuples

The type checker/inferer visits every node in an AST tree, including
function return annotations. This means for a function definition like

    def f() -> TTuple([TInt32, TBool]):
      ...

We attempt to type check the list [TInt32, TBool], which generates the
unification constraint builtins.TBool ~ builtins.TInt. This causes an
internal error due to compiler weirdness.

We can avoid this by just nulling-out the return annotation in the
embedding stage. The return type isn't actually used anywhere (it's
extracted via the inspect module instead), so this is entirely safe.

Arguments aren't affected by this, as we already nulled out the
annotation (see visit_arg in embedding.py).

Signed-off-by: Jonathan Coates <jonathan.coates@oxionics.com>

artiq/compiler/embedding.py

@@ -447,7 +447,7 @@ class StitchingASTTypedRewriter(ASTTypedRewriter):
         node = asttyped.QuotedFunctionDefT(
             typing_env=extractor.typing_env, globals_in_scope=extractor.global_,
             signature_type=types.TVar(), return_type=types.TVar(),
-            name=node.name, args=node.args, returns=node.returns,
+            name=node.name, args=node.args, returns=None,
             body=node.body, decorator_list=node.decorator_list,
             keyword_loc=node.keyword_loc, name_loc=node.name_loc,
             arrow_loc=node.arrow_loc, colon_loc=node.colon_loc, at_locs=node.at_locs,

artiq/test/lit/embedding/mixed_tuple.py

@@ -0,0 +1,16 @@
+# RUN: %python -m artiq.compiler.testbench.embedding %s
+
+from artiq.language.core import *
+from artiq.language.types import *
+
+@kernel
+def consume_tuple(x: TTuple([TInt32, TBool])):
+    print(x)
+
+@kernel
+def return_tuple() -> TTuple([TInt32, TBool]):
+    return (123, False)
+
+@kernel
+def entrypoint():
+    consume_tuple(return_tuple())