Add regression test for issue 127545

Author: tesuji

tests/ui/mismatched_types/transforming-option-ref-issue-127545.rs

@@ -0,0 +1,10 @@
+// Regression test for <https://github.com/rust-lang/rust/issues/127545>.
+#![crate_type = "lib"]
+
+pub fn bar(arg: Option<&Vec<i32>>) -> &[i32] {
+    arg.unwrap_or(&[]) //~ ERROR 5:19: 5:22: mismatched types [E0308]
+}
+
+pub fn barzz<'a>(arg: Option<&'a Vec<i32>>, v: &'a [i32]) -> &'a [i32] {
+    arg.unwrap_or(v) //~ ERROR 9:19: 9:20: mismatched types [E0308]
+}

tests/ui/mismatched_types/transforming-option-ref-issue-127545.stderr

@@ -0,0 +1,43 @@
+error[E0308]: mismatched types
+  --> $DIR/transforming-option-ref-issue-127545.rs:5:19
+   |
+LL |     arg.unwrap_or(&[])
+   |         --------- ^^^ expected `&Vec<i32>`, found `&[_; 0]`
+   |         |
+   |         arguments to this method are incorrect
+   |
+   = note: expected reference `&Vec<i32>`
+              found reference `&[_; 0]`
+help: the return type of this call is `&[_; 0]` due to the type of the argument passed
+  --> $DIR/transforming-option-ref-issue-127545.rs:5:5
+   |
+LL |     arg.unwrap_or(&[])
+   |     ^^^^^^^^^^^^^^---^
+   |                   |
+   |                   this argument influences the return type of `unwrap_or`
+note: method defined here
+  --> $SRC_DIR/core/src/option.rs:LL:COL
+
+error[E0308]: mismatched types
+  --> $DIR/transforming-option-ref-issue-127545.rs:9:19
+   |
+LL |     arg.unwrap_or(v)
+   |         --------- ^ expected `&Vec<i32>`, found `&[i32]`
+   |         |
+   |         arguments to this method are incorrect
+   |
+   = note: expected reference `&Vec<i32>`
+              found reference `&'a [i32]`
+help: the return type of this call is `&'a [i32]` due to the type of the argument passed
+  --> $DIR/transforming-option-ref-issue-127545.rs:9:5
+   |
+LL |     arg.unwrap_or(v)
+   |     ^^^^^^^^^^^^^^-^
+   |                   |
+   |                   this argument influences the return type of `unwrap_or`
+note: method defined here
+  --> $SRC_DIR/core/src/option.rs:LL:COL
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.