Fixed compilation error when a type annotation contains a type guarde…

…d by `if TYPE_CHECKING:`

Fixes #331.

docs/versionhistory.rst

@@ -10,6 +10,8 @@ This library adheres to `Semantic Versioning 2.0 <https://semver.org/#semantic-v
   generated by older versions
 - Fixed typed variable positional and keyword arguments causing compilation errors on
   Python 3.7 and 3.8
+- Fixed compilation error when a type annotation contains a type guarded by
+  ``if TYPE_CHECKING:``
 
 **4.0.0rc1** (2023-04-02)
 

src/typeguard/_transformer.py

@@ -3,6 +3,7 @@
 import ast
 import builtins
 import sys
+from _ast import AST, Expression
 from ast import (
     Add,
     AnnAssign,
@@ -330,6 +331,13 @@ def __init__(self, transformer: TypeguardTransformer):
         self._memo = transformer._memo
         self.level = 0
 
+    def visit(self, node: AST) -> Any:
+        new_node = super().visit(node)
+        if isinstance(new_node, Expression) and not hasattr(new_node, "body"):
+            return None
+
+        return new_node
+
     def visit_BinOp(self, node: BinOp) -> Any:
         self.level += 1
         self.generic_visit(node)
@@ -385,15 +393,21 @@ def visit_Constant(self, node: Constant) -> Any:
         if isinstance(node.value, str):
             expression = ast.parse(node.value, mode="eval")
             new_node = self.visit(expression)
-            return copy_location(new_node.body, node)
+            if new_node:
+                return copy_location(new_node.body, node)
+            else:
+                return None
 
         return node
 
     def visit_Str(self, node: Str) -> Any:
         # Only used on Python 3.7
         expression = ast.parse(node.s, mode="eval")
         new_node = self.visit(expression)
-        return copy_location(new_node.body, node)
+        if new_node:
+            return copy_location(new_node.body, node)
+        else:
+            return None
 
 
 class TypeguardTransformer(NodeTransformer):

tests/dummymodule.py

@@ -2,6 +2,7 @@
 import sys
 from contextlib import contextmanager
 from typing import (
+    TYPE_CHECKING,
     Any,
     AsyncGenerator,
     Callable,
@@ -33,6 +34,9 @@
     typeguard_ignore,
 )
 
+if TYPE_CHECKING:
+    from nonexistent import Imaginary
+
 P = ParamSpec("P")
 
 
@@ -306,3 +310,9 @@ def typed_variable_args(
     *args: str, **kwargs: int
 ) -> Tuple[Tuple[str, ...], Dict[str, int]]:
     return args, kwargs
+
+
+@typechecked
+def guarded_type_hint(x: "Imaginary") -> "Imaginary":
+    y: Imaginary = x
+    return y

tests/test_instrumentation.py

@@ -330,3 +330,7 @@ def test_kwargs_fail(self, dummymodule):
             r"instance of int",
         ):
             dummymodule.typed_variable_args("foo", "bar", a="baz")
+
+
+def test_guarded_type(dummymodule):
+    assert dummymodule.guarded_type_hint("foo") == "foo"