Check for Default trait in initial expression

tests/ui/manual_unwrap_or_default.fixed

@@ -16,6 +16,16 @@ fn main() {
 
     let x: Option<Vec<String>> = None;
     x.unwrap_or_default();
+
+    // Issue #12564
+    // No error as &Vec<_> doesn't implement std::default::Default
+    let mut map = std::collections::HashMap::from([(0, vec![0; 3]), (1, vec![1; 3]), (2, vec![2])]);
+    let x: &[_] = if let Some(x) = map.get(&0) { x } else { &[] };
+    // Same code as above written using match.
+    let x: &[_] = match map.get(&0) {
+        Some(x) => x,
+        None => &[],
+    };
 }
 
 // Issue #12531

tests/ui/manual_unwrap_or_default.rs

@@ -37,6 +37,16 @@ fn main() {
     } else {
         Vec::default()
     };
+
+    // Issue #12564
+    // No error as &Vec<_> doesn't implement std::default::Default
+    let mut map = std::collections::HashMap::from([(0, vec![0; 3]), (1, vec![1; 3]), (2, vec![2])]);
+    let x: &[_] = if let Some(x) = map.get(&0) { x } else { &[] };
+    // Same code as above written using match.
+    let x: &[_] = match map.get(&0) {
+        Some(x) => x,
+        None => &[],
+    };
 }
 
 // Issue #12531

tests/ui/manual_unwrap_or_default.stderr

@@ -53,7 +53,7 @@ LL | |     };
    | |_____^ help: replace it with: `x.unwrap_or_default()`
 
 error: match can be simplified with `.unwrap_or_default()`
-  --> tests/ui/manual_unwrap_or_default.rs:46:20
+  --> tests/ui/manual_unwrap_or_default.rs:56:20
    |
 LL |           Some(_) => match *b {
    |  ____________________^