[ty] Add hint that PEP 604 union syntax is only available in 3.10+

Author: sharkdp

crates/ty_python_semantic/resources/mdtest/assignment/annotations.md

@@ -88,6 +88,33 @@ def foo(v: str | int | None, w: str | str | None, x: str | str):
     reveal_type(x)  # revealed: str
 ```
 
+## PEP-604 in non-type-expression context
+
+### In Python 3.10 and later
+
+```toml
+[environment]
+python-version = "3.10"
+```
+
+```py
+IntOrStr = int | str
+```
+
+### Earlier versions
+
+<!-- snapshot-diagnostics -->
+
+```toml
+[environment]
+python-version = "3.9"
+```
+
+```py
+# error: [unsupported-operator]
+IntOrStr = int | str
+```
+
 ## Attribute expressions in type annotations are understood
 
 ```py

crates/ty_python_semantic/resources/mdtest/snapshots/annotations.md_-_Assignment_with_anno…_-_PEP-604_in_non-type-…_-_Earlier_versions_(f2859c9800f37c7).snap

@@ -0,0 +1,32 @@
+---
+source: crates/ty_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: annotations.md - Assignment with annotations - PEP-604 in non-type-expression context - Earlier versions
+mdtest path: crates/ty_python_semantic/resources/mdtest/assignment/annotations.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | # error: [unsupported-operator]
+2 | IntOrStr = int | str
+```
+
+# Diagnostics
+
+```
+error[unsupported-operator]: Operator `|` is unsupported between objects of type `<class 'int'>` and `<class 'str'>`
+ --> src/mdtest_snippet.py:2:12
+  |
+1 | # error: [unsupported-operator]
+2 | IntOrStr = int | str
+  |            ^^^^^^^^^
+  |
+info: Note that `X | Y` PEP 604 union syntax is only available in Python 3.10 and later
+info: rule `unsupported-operator` is enabled by default
+
+```

crates/ty_python_semantic/src/types/infer.rs

@@ -38,7 +38,7 @@ use ruff_db::diagnostic::{Annotation, DiagnosticId, Severity};
 use ruff_db::files::File;
 use ruff_db::parsed::parsed_module;
 use ruff_python_ast::visitor::{Visitor, walk_expr};
-use ruff_python_ast::{self as ast, AnyNodeRef, ExprContext};
+use ruff_python_ast::{self as ast, AnyNodeRef, ExprContext, PythonVersion};
 use ruff_text_size::{Ranged, TextRange};
 use rustc_hash::{FxHashMap, FxHashSet};
 use salsa;
@@ -94,7 +94,7 @@ use crate::types::{
 };
 use crate::unpack::{Unpack, UnpackPosition};
 use crate::util::subscript::{PyIndex, PySlice};
-use crate::{Db, FxOrderSet};
+use crate::{Db, FxOrderSet, Program};
 
 use super::context::{InNoTypeCheck, InferContext};
 use super::diagnostic::{
@@ -5889,11 +5889,18 @@ impl<'db> TypeInferenceBuilder<'db> {
         self.infer_binary_expression_type(binary.into(), false, left_ty, right_ty, *op)
             .unwrap_or_else(|| {
                 if let Some(builder) = self.context.report_lint(&UNSUPPORTED_OPERATOR, binary) {
-                    builder.into_diagnostic(format_args!(
+                    let mut diag = builder.into_diagnostic(format_args!(
                         "Operator `{op}` is unsupported between objects of type `{}` and `{}`",
                         left_ty.display(self.db()),
                         right_ty.display(self.db())
                     ));
+
+                    if op == &ast::Operator::BitOr && left_ty.into_class_literal().is_some()
+                        && right_ty.into_class_literal().is_some() && Program::get(self.db()).python_version(self.db())
+                            < PythonVersion::PY310
+                    {
+                        diag.info("Note that `X | Y` PEP 604 union syntax is only available in Python 3.10 and later");
+                    }
                 }
                 Type::unknown()
             })