Work around type normalization issues

Author: Nadrieril

src/librustc_mir_build/hair/pattern/_match.rs

@@ -1870,7 +1870,9 @@ crate fn is_useful<'p, 'tcx>(
         return if any_is_useful { Useful(unreachable_pats) } else { NotUseful };
     }
 
-    let pcx = PatCtxt { ty: v.head().ty, span: v.head().span };
+    // FIXME(Nadrieril): Hack to work around type normalization issues (see #72476).
+    let ty = matrix.heads().next().map(|r| r.ty).unwrap_or(v.head().ty);
+    let pcx = PatCtxt { ty, span: v.head().span };
 
     debug!("is_useful_expand_first_col: pcx={:#?}, expanding {:#?}", pcx, v.head());
 

src/test/ui/pattern/usefulness/issue-72476-associated-type.rs

@@ -0,0 +1,23 @@
+// check-pass
+
+// From https://github.com/rust-lang/rust/issues/72476
+
+trait A {
+    type Projection;
+}
+
+impl A for () {
+    type Projection = bool;
+    // using () instead of bool here does compile though
+}
+
+struct Next<T: A>(T::Projection);
+
+fn f(item: Next<()>) {
+    match item {
+        Next(true) => {}
+        Next(false) => {}
+    }
+}
+
+fn main() {}