Fix handling of diverging branches in match coercion

Fixes #7626.

crates/hir_ty/src/infer/coerce.rs

@@ -45,6 +45,10 @@ impl<'a> InferenceContext<'a> {
     ///  - 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 {
+        let ty1 = self.resolve_ty_shallow(ty1);
+        let ty1 = ty1.as_ref();
+        let ty2 = self.resolve_ty_shallow(ty2);
+        let ty2 = ty2.as_ref();
         // 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
@@ -71,12 +75,17 @@ impl<'a> InferenceContext<'a> {
             }
         }
 
-        if self.coerce(ty1, ty2) {
-            ty2.clone()
-        } else if self.coerce(ty2, ty1) {
+        // It might not seem like it, but order is important here: ty1 is our
+        // "previous" type, ty2 is the "new" one being added. If the previous
+        // type is a type variable and the new one is `!`, trying it the other
+        // way around first would mean we make the type variable `!`, instead of
+        // just marking it as possibly diverging.
+        if self.coerce(ty2, ty1) {
             ty1.clone()
+        } else if self.coerce(ty1, ty2) {
+            ty2.clone()
         } else {
-            // FIXME record a type mismatch
+            // TODO record a type mismatch
             cov_mark::hit!(coerce_merge_fail_fallback);
             ty1.clone()
         }

crates/hir_ty/src/tests/coercion.rs

@@ -873,3 +873,42 @@ fn foo(c: i32) {
         "#,
     )
 }
+
+#[test]
+fn infer_match_diverging_branch_1() {
+    check_types(
+        r#"
+enum Result<T> { Ok(T), Err }
+fn parse<T>() -> T { loop {} }
+
+fn test() -> i32 {
+    let a = match parse() {
+        Ok(val) => val,
+        Err => return 0,
+    };
+    a
+  //^ i32
+}
+        "#,
+    )
+}
+
+#[test]
+fn infer_match_diverging_branch_2() {
+    // same as 1 except for order of branches
+    check_types(
+        r#"
+enum Result<T> { Ok(T), Err }
+fn parse<T>() -> T { loop {} }
+
+fn test() -> i32 {
+    let a = match parse() {
+        Err => return 0,
+        Ok(val) => val,
+    };
+    a
+  //^ i32
+}
+        "#,
+    )
+}