Add regression test for issue 127545

Author: tesuji

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

@@ -0,0 +1,14 @@
+// Regression test for <https://github.com/rust-lang/rust/issues/127545>.
+#![crate_type = "lib"]
+
+pub fn foo(arg: Option<&Vec<i32>>) -> Option<&[i32]> {
+    arg //~ ERROR 5:5: 5:8: mismatched types [E0308]
+}
+
+pub fn bar(arg: Option<&Vec<i32>>) -> &[i32] {
+    arg.unwrap_or(&[]) //~ ERROR 9:19: 9:22: mismatched types [E0308]
+}
+
+pub fn barzz<'a>(arg: Option<&'a Vec<i32>>, v: &'a [i32]) -> &'a [i32] {
+    arg.unwrap_or(v) //~ ERROR 13:19: 13:20: mismatched types [E0308]
+}

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

@@ -0,0 +1,54 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-127545-option-of-ref.rs:5:5
+   |
+LL | pub fn foo(arg: Option<&Vec<i32>>) -> Option<&[i32]> {
+   |                                       -------------- expected `Option<&[i32]>` because of return type
+LL |     arg
+   |     ^^^ expected `Option<&[i32]>`, found `Option<&Vec<i32>>`
+   |
+   = note: expected enum `Option<&[i32]>`
+              found enum `Option<&Vec<i32>>`
+
+error[E0308]: mismatched types
+  --> $DIR/issue-127545-option-of-ref.rs:9:19
+   |
+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/issue-127545-option-of-ref.rs:9:5
+   |
+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/issue-127545-option-of-ref.rs:13:19
+   |
+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/issue-127545-option-of-ref.rs:13:5
+   |
+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 3 previous errors
+
+For more information about this error, try `rustc --explain E0308`.