Add regression test for #79825

src/test/ui/proc-macro/auxiliary/issue-79825.rs

@@ -0,0 +1,14 @@
+// force-host
+// no-prefer-dynamic
+#![crate_type = "proc-macro"]
+
+extern crate proc_macro;
+
+use proc_macro::TokenStream;
+
+#[proc_macro_attribute]
+pub fn assert_input(args: TokenStream, input: TokenStream) -> TokenStream {
+    assert_eq!(input.to_string(), "trait Alias = Sized ;");
+    assert!(args.is_empty());
+    TokenStream::new()
+}

src/test/ui/proc-macro/issue-79825.rs

@@ -0,0 +1,10 @@
+// check-pass
+// aux-build:issue-79825.rs
+#![feature(trait_alias)]
+
+extern crate issue_79825;
+
+#[issue_79825::assert_input]
+trait Alias = Sized;
+
+fn main() {}