Match up array coercion for rust-lang/rust#140283

Author: Veykril

crates/hir-ty/src/infer/expr.rs

@@ -1164,7 +1164,9 @@ impl InferenceContext<'_> {
         expected: &Expectation,
     ) -> chalk_ir::Ty<Interner> {
         let elem_ty = match expected.to_option(&mut self.table).as_ref().map(|t| t.kind(Interner)) {
-            Some(TyKind::Array(st, _) | TyKind::Slice(st)) => st.clone(),
+            // Avoid using a type variable as the coerce_to type, as it may resolve
+            // during the first coercion instead of being based on the common type.
+            Some(TyKind::Array(st, _) | TyKind::Slice(st)) if !st.is_ty_var() => st.clone(),
             _ => self.table.new_type_var(),
         };
 

crates/hir-ty/src/tests/coercion.rs

@@ -96,7 +96,7 @@ fn foo<T>(x: &[T]) -> &[T] { x }
 fn test() {
     let x = if true {
         foo(&[1])
-         // ^^^^ adjustments: Deref(None), Borrow(Ref('?8, Not)), Pointer(Unsize)
+         // ^^^^ adjustments: Deref(None), Borrow(Ref('?7, Not)), Pointer(Unsize)
     } else {
         &[1]
     };
@@ -148,7 +148,7 @@ fn foo<T>(x: &[T]) -> &[T] { x }
 fn test(i: i32) {
     let x = match i {
         2 => foo(&[2]),
-              // ^^^^ adjustments: Deref(None), Borrow(Ref('?8, Not)), Pointer(Unsize)
+              // ^^^^ adjustments: Deref(None), Borrow(Ref('?7, Not)), Pointer(Unsize)
         1 => &[1],
         _ => &[3],
     };
@@ -957,3 +957,35 @@ fn f() {
     "#,
     );
 }
+
+#[test]
+fn coerce_array_elements() {
+    check_no_mismatches(
+        r#"
+// Check that least upper bound coercions don't resolve type variable merely based on the first
+// coercion. Check issue rust-lang/rust#136420.
+
+fn foo() {}
+fn bar() {}
+
+fn infer<T>(_: T) {}
+
+fn infer_array_element<T>(_: [T; 2]) {}
+
+fn main() {
+    infer(if false {
+        foo
+    } else {
+        bar
+    });
+
+    infer(match false {
+        true => foo,
+        false => bar,
+    });
+
+    infer_array_element([foo, bar]);
+}
+    "#,
+    );
+}