Fix coercion of two closures to a function pointer

Fixes #8604.

crates/hir_ty/src/infer/coerce.rs

@@ -40,32 +40,45 @@ impl<'a> InferenceContext<'a> {
     /// Merge two types from different branches, with possible coercion.
     ///
     /// Mostly this means trying to coerce one to the other, but
-    ///  - if we have two function types for different functions, we need to
+    ///  - if we have two function types for different functions or closures, we need to
     ///    coerce both to function pointers;
     ///  - if we were concerned with lifetime subtyping, we'd need to look for a
     ///    least upper bound.
     pub(super) fn coerce_merge_branch(&mut self, ty1: &Ty, ty2: &Ty) -> Ty {
+        // Special case: two function types. Try to coerce both to
+        // 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 (ty1.kind(&Interner), ty2.kind(&Interner)) {
+            (TyKind::FnDef(..), TyKind::FnDef(..))
+            | (TyKind::Closure(..), TyKind::FnDef(..))
+            | (TyKind::FnDef(..), TyKind::Closure(..))
+            | (TyKind::Closure(..), TyKind::Closure(..)) => {
+                // FIXME: we're ignoring safety here. To be more correct, if we have one FnDef and one Closure,
+                // we should be coercing the closure to a fn pointer of the safety of the FnDef
+                cov_mark::hit!(coerce_fn_reification);
+                let sig = ty1.callable_sig(self.db).expect("FnDef without callable sig");
+                Some(sig)
+            }
+            _ => None,
+        };
+        if let Some(sig) = sig {
+            let target_ty = TyKind::Function(sig.to_fn_ptr()).intern(&Interner);
+            let result1 = self.coerce_inner(ty1.clone(), &target_ty);
+            let result2 = self.coerce_inner(ty2.clone(), &target_ty);
+            if let (Ok(_result1), Ok(_result2)) = (result1, result2) {
+                // TODO deal with the goals
+                return target_ty;
+            }
+        }
+
         if self.coerce(ty1, ty2) {
             ty2.clone()
         } else if self.coerce(ty2, ty1) {
             ty1.clone()
         } else {
-            if let (TyKind::FnDef(..), TyKind::FnDef(..)) =
-                (ty1.kind(&Interner), ty2.kind(&Interner))
-            {
-                cov_mark::hit!(coerce_fn_reification);
-                // Special case: two function types. Try to coerce both to
-                // 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 sig1 = ty1.callable_sig(self.db).expect("FnDef without callable sig");
-                let sig2 = ty2.callable_sig(self.db).expect("FnDef without callable sig");
-                let ptr_ty1 = TyBuilder::fn_ptr(sig1);
-                let ptr_ty2 = TyBuilder::fn_ptr(sig2);
-                self.coerce_merge_branch(&ptr_ty1, &ptr_ty2)
-            } else {
-                cov_mark::hit!(coerce_merge_fail_fallback);
-                ty1.clone()
-            }
+            // FIXME record a type mismatch
+            cov_mark::hit!(coerce_merge_fail_fallback);
+            ty1.clone()
         }
     }
 
@@ -236,8 +249,7 @@ impl<'a> InferenceContext<'a> {
         Ok(result)
     }
 
-    /// Attempts to coerce from the type of a Rust function item into a closure
-    /// or a function pointer.
+    /// Attempts to coerce from the type of a Rust function item into a function pointer.
     fn coerce_from_fn_item(&mut self, from_ty: Ty, to_ty: &Ty) -> InferResult {
         match to_ty.kind(&Interner) {
             TyKind::Function(_) => {

crates/hir_ty/src/lib.rs

@@ -167,6 +167,7 @@ pub fn make_canonical<T: HasInterner<Interner = Interner>>(
     Canonical { value, binders: chalk_ir::CanonicalVarKinds::from_iter(&Interner, kinds) }
 }
 
+// FIXME: get rid of this, just replace it by FnPointer
 /// A function signature as seen by type inference: Several parameter types and
 /// one return type.
 #[derive(Clone, PartialEq, Eq, Debug)]

crates/hir_ty/src/tests/coercion.rs

@@ -1,6 +1,6 @@
 use expect_test::expect;
 
-use super::{check_infer, check_infer_with_mismatches};
+use super::{check_infer, check_infer_with_mismatches, check_types};
 
 #[test]
 fn infer_block_expr_type_mismatch() {
@@ -858,3 +858,18 @@ fn coerce_unsize_generic() {
         "]],
     );
 }
+
+#[test]
+fn infer_two_closures_lub() {
+    check_types(
+        r#"
+fn foo(c: i32) {
+    let add = |a: i32, b: i32| a + b;
+    let sub = |a, b| a - b;
+            //^ |i32, i32| -> i32
+    if c > 42 { add } else { sub };
+  //^ fn(i32, i32) -> i32
+}
+        "#,
+    )
+}

crates/hir_ty/src/tests/simple.rs

@@ -1039,42 +1039,6 @@ fn infer_in_elseif() {
     )
 }
 
-#[test]
-fn infer_closure_unify() {
-    check_infer(
-        r#"
-        fn foo(f: bool) {
-            let a = |x| x;
-            let b = |x| x;
-            let id = if f { a } else { b };
-            id(123);
-        }
-        "#,
-        expect![[r#"
-            7..8 'f': bool
-            16..106 '{     ...23); }': ()
-            26..27 'a': |i32| -> i32
-            30..35 '|x| x': |i32| -> i32
-            31..32 'x': i32
-            34..35 'x': i32
-            45..46 'b': |i32| -> i32
-            49..54 '|x| x': |i32| -> i32
-            50..51 'x': i32
-            53..54 'x': i32
-            64..66 'id': |i32| -> i32
-            69..90 'if f {... { b }': |i32| -> i32
-            72..73 'f': bool
-            74..79 '{ a }': |i32| -> i32
-            76..77 'a': |i32| -> i32
-            85..90 '{ b }': |i32| -> i32
-            87..88 'b': |i32| -> i32
-            96..98 'id': |i32| -> i32
-            96..103 'id(123)': i32
-            99..102 '123': i32
-        "#]],
-    )
-}
-
 #[test]
 fn infer_if_match_with_return() {
     check_infer(