1 files changed, 49 insertions, 0 deletions

diff --git a/src/test/ui/issues/issue-52126-assign-op-invariance.rs b/src/test/ui/issues/issue-52126-assign-op-invariance.rs
new file mode 100644
index 000000000..c96cfdf3c
--- /dev/null
+++ b/src/test/ui/issues/issue-52126-assign-op-invariance.rs
@@ -0,0 +1,49 @@
+// Issue 52126: With respect to variance, the assign-op's like += were
+// accidentally lumped together with other binary op's. In both cases
+// we were coercing the LHS of the op to the expected supertype.
+//
+// The problem is that since the LHS of += is modified, we need the
+// parameter to be invariant with respect to the overall type, not
+// covariant.
+
+use std::collections::HashMap;
+use std::ops::AddAssign;
+
+pub fn main() {
+    panics();
+}
+
+pub struct Counter<'l> {
+    map: HashMap<&'l str, usize>,
+}
+
+impl<'l> AddAssign for Counter<'l>
+{
+    fn add_assign(&mut self, rhs: Counter<'l>) {
+        rhs.map.into_iter().for_each(|(key, val)| {
+            let count = self.map.entry(key).or_insert(0);
+            *count += val;
+        });
+    }
+}
+
+/// Often crashes, if not prints invalid strings.
+pub fn panics() {
+    let mut acc = Counter{map: HashMap::new()};
+    for line in vec!["123456789".to_string(), "12345678".to_string()] {
+        let v: Vec<&str> = line.split_whitespace().collect();
+        //~^ ERROR `line` does not live long enough
+        // println!("accumulator before add_assign {:?}", acc.map);
+        let mut map = HashMap::new();
+        for str_ref in v {
+            let e = map.entry(str_ref);
+            println!("entry: {:?}", e);
+            let count = e.or_insert(0);
+            *count += 1;
+        }
+        let cnt2 = Counter{map};
+        acc += cnt2;
+        // println!("accumulator after add_assign {:?}", acc.map);
+        // line gets dropped here but references are kept in acc.map
+    }
+}