Coerce two `FnDef`s to fn pointers even if they are the same, if they are subtypes

That's because they can be the same function but still different substs, which mandates them to coerce to fn pointers in order to unify.

2 files changed, 21 insertions, 1 deletions

diff --git a/src/tools/rust-analyzer/crates/hir-ty/src/infer/coerce.rs b/src/tools/rust-analyzer/crates/hir-ty/src/infer/coerce.rs
index 366c3cb0f17..2fe90a8a924 100644
--- a/src/tools/rust-analyzer/crates/hir-ty/src/infer/coerce.rs
+++ b/src/tools/rust-analyzer/crates/hir-ty/src/infer/coerce.rs
@@ -125,7 +125,11 @@ impl CoerceMany {
         // pointers to have a chance at getting a match. See
         // https://github.com/rust-lang/rust/blob/7b805396bf46dce972692a6846ce2ad8481c5f85/src/librustc_typeck/check/coercion.rs#L877-L916
         let sig = match (self.merged_ty().kind(Interner), expr_ty.kind(Interner)) {
-            (TyKind::FnDef(x, _), TyKind::FnDef(y, _)) if x == y => None,
+            (TyKind::FnDef(x, _), TyKind::FnDef(y, _))
+                if x == y && ctx.table.unify(&self.merged_ty(), &expr_ty) =>
+            {
+                None
+            }
             (TyKind::Closure(x, _), TyKind::Closure(y, _)) if x == y => None,
             (TyKind::FnDef(..) | TyKind::Closure(..), TyKind::FnDef(..) | TyKind::Closure(..)) => {
                 // FIXME: we're ignoring safety here. To be more correct, if we have one FnDef and one Closure,
diff --git a/src/tools/rust-analyzer/crates/hir-ty/src/tests/coercion.rs b/src/tools/rust-analyzer/crates/hir-ty/src/tests/coercion.rs
index 273571901ad..7992f1feeeb 100644
--- a/src/tools/rust-analyzer/crates/hir-ty/src/tests/coercion.rs
+++ b/src/tools/rust-analyzer/crates/hir-ty/src/tests/coercion.rs
@@ -942,3 +942,19 @@ fn main() {
 "#,
     )
 }
+
+#[test]
+fn regression_18626() {
+    check_no_mismatches(
+        r#"
+fn f() {
+    trait T {
+        fn f() {}
+    }
+    impl T for i32 {}
+    impl T for u32 {}
+    &[i32::f, u32::f] as &[fn()];
+}
+    "#,
+    );
+}