Add regression test for #81804

src/test/ui/parser/issue-81804.rs

@@ -0,0 +1,9 @@
+// error-pattern: this file contains an unclosed delimiter
+// error-pattern: this file contains an unclosed delimiter
+// error-pattern: expected pattern, found `=`
+// error-pattern: expected one of `)`, `,`, `->`, `where`, or `{`, found `]`
+// error-pattern: expected item, found `]`
+
+fn main() {}
+
+fn p([=(}

src/test/ui/parser/issue-81804.stderr

@@ -0,0 +1,41 @@
+error: this file contains an unclosed delimiter
+  --> $DIR/issue-81804.rs:9:11
+   |
+LL | fn p([=(}
+   |     --    ^
+   |     ||
+   |     |unclosed delimiter
+   |     unclosed delimiter
+
+error: this file contains an unclosed delimiter
+  --> $DIR/issue-81804.rs:9:11
+   |
+LL | fn p([=(}
+   |     --    ^
+   |     ||
+   |     |unclosed delimiter
+   |     unclosed delimiter
+
+error: expected pattern, found `=`
+  --> $DIR/issue-81804.rs:9:7
+   |
+LL | fn p([=(}
+   |       ^ expected pattern
+
+error: expected one of `)`, `,`, `->`, `where`, or `{`, found `]`
+  --> $DIR/issue-81804.rs:9:8
+   |
+LL | fn p([=(}
+   |        ^ -^
+   |        | |
+   |        | help: `)` may belong here
+   |        unclosed delimiter
+
+error: expected item, found `]`
+  --> $DIR/issue-81804.rs:9:11
+   |
+LL | fn p([=(}
+   |           ^ expected item
+
+error: aborting due to 5 previous errors
+