eagerly check nested obligations when coercing fndefs

compiler/rustc_hir_typeck/src/coercion.rs

@@ -1190,9 +1190,23 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     (ty::FnDef(..), ty::FnDef(..)) => {
                         // Don't reify if the function types have a LUB, i.e., they
                         // are the same function and their parameters have a LUB.
-                        match self
-                            .commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty))
-                        {
+                        match self.commit_if_ok(|_| {
+                            // We need to eagerly handle nested obligations due to lazy norm.
+                            if self.next_trait_solver() {
+                                let ocx = ObligationCtxt::new(self);
+                                let value = ocx.lub(cause, self.param_env, prev_ty, new_ty)?;
+                                if ocx.select_where_possible().is_empty() {
+                                    Ok(InferOk {
+                                        value,
+                                        obligations: ocx.into_pending_obligations(),
+                                    })
+                                } else {
+                                    Err(TypeError::Mismatch)
+                                }
+                            } else {
+                                self.at(cause, self.param_env).lub(prev_ty, new_ty)
+                            }
+                        }) {
                             // We have a LUB of prev_ty and new_ty, just return it.
                             Ok(ok) => return Ok(self.register_infer_ok_obligations(ok)),
                             Err(_) => {

compiler/rustc_trait_selection/src/traits/engine.rs

@@ -188,6 +188,20 @@ where
             .map(|infer_ok| self.register_infer_ok_obligations(infer_ok))
     }
 
+    /// Computes the least-upper-bound, or mutual supertype, of two values.
+    pub fn lub<T: ToTrace<'tcx>>(
+        &self,
+        cause: &ObligationCause<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+        expected: T,
+        actual: T,
+    ) -> Result<T, TypeError<'tcx>> {
+        self.infcx
+            .at(cause, param_env)
+            .lub(expected, actual)
+            .map(|infer_ok| self.register_infer_ok_obligations(infer_ok))
+    }
+
     #[must_use]
     pub fn select_where_possible(&self) -> Vec<E> {
         self.engine.borrow_mut().select_where_possible(self.infcx)

tests/ui/traits/next-solver/coercion/fn-def-coerce-nested-obligations.rs

@@ -0,0 +1,16 @@
+//@ compile-flags: -Znext-solver
+//@ check-pass
+
+// Make sure that we consider nested obligations when checking whether
+// we should coerce fn definitions to function pointers.
+
+fn foo<const N: usize>() {}
+fn bar<T>() {}
+fn main() {
+    let _ = if true { foo::<{ 0 + 0 }> } else { foo::<1> };
+    let _ = if true {
+        bar::<for<'a> fn(<Vec<&'a ()> as IntoIterator>::Item)>
+    } else {
+        bar::<fn(i32)>
+    };
+}