Add a regression test for #80779

Author: JohnTitor

src/test/ui/typeck/issue-80779.rs

@@ -0,0 +1,13 @@
+// Regression test for #80779.
+
+pub struct T<'a>(&'a str);
+
+pub fn f<'a>(val: T<'a>) -> _ {
+    //~^ ERROR: the type placeholder `_` is not allowed within types on item signatures
+    g(val)
+}
+
+pub fn g(_: T<'static>) -> _ {}
+//~^ ERROR: the type placeholder `_` is not allowed within types on item signatures
+
+fn main() {}

src/test/ui/typeck/issue-80779.stderr

@@ -0,0 +1,21 @@
+error[E0121]: the type placeholder `_` is not allowed within types on item signatures
+  --> $DIR/issue-80779.rs:10:28
+   |
+LL | pub fn g(_: T<'static>) -> _ {}
+   |                            ^
+   |                            |
+   |                            not allowed in type signatures
+   |                            help: replace with the correct return type: `()`
+
+error[E0121]: the type placeholder `_` is not allowed within types on item signatures
+  --> $DIR/issue-80779.rs:5:29
+   |
+LL | pub fn f<'a>(val: T<'a>) -> _ {
+   |                             ^
+   |                             |
+   |                             not allowed in type signatures
+   |                             help: replace with the correct return type: `()`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0121`.