[clang-tools-extra] [clang-tidy] Reject malformed -std spellings in `check_clang_tidy.py`. NFC. (PR #195609)

[Zeyi Xu via cfe-commits]

================
@@ -460,24 +460,30 @@ def run(self) -> None:
 
 
 def expand_std(std: str) -> List[str]:
-    split_std, or_later, _ = std.partition("-or-later")
-
-    if not or_later:
-        return [split_std]
-
-    for standard_list in (CPP_STANDARDS, C_STANDARDS):
-        item = next(
-            (
-                i
-                for i, v in enumerate(standard_list)
-                if (split_std in v if isinstance(v, (list, tuple)) else split_std == v)
-            ),
-            None,
-        )
-        if item is not None:
-            return [split_std] + [
-                x if isinstance(x, str) else x[0] for x in standard_list[item + 1 :]
-            ]
+    split_std, or_later, suffix = std.partition("-or-later")
+
+    if or_later:
+        if suffix:
+            # Keep malformed -or-later spellings unchanged so clang diagnoses them.
+            return [std]
+
+        for standard_list in (CPP_STANDARDS, C_STANDARDS):
+            item = next(
+                (
+                    i
+                    for i, v in enumerate(standard_list)
+                    if (
+                        split_std in v
+                        if isinstance(v, (list, tuple))
+                        else split_std == v
+                    )
+                ),
+                None,
+            )
+            if item is not None:
+                return [split_std] + [
+                    x if isinstance(x, str) else x[0] for x in standard_list[item + 1 :]
+                ]
     return [std]
----------------
zeyi2 wrote:

```python
    if not or_later:
        return [split_std]
```

Behavior is the same since `split_std == std` when `partition("-or-later")` does not find the separator.

https://github.com/llvm/llvm-project/pull/195609