Report trait/impl sig inconsistency before method/body inconsistency

Closes #15657.

Author: ftxqxd

src/librustc/middle/typeck/check/mod.rs

@@ -758,16 +758,6 @@ pub fn check_item(ccx: &CrateCtxt, it: &ast::Item) {
         debug!("ItemImpl {} with id {}", token::get_ident(it.ident), it.id);
 
         let impl_pty = ty::lookup_item_type(ccx.tcx, ast_util::local_def(it.id));
-        for impl_item in impl_items.iter() {
-            match *impl_item {
-                ast::MethodImplItem(ref m) => {
-                    check_method_body(ccx, &impl_pty.generics, &**m);
-                }
-                ast::TypeImplItem(_) => {
-                    // Nothing to do here.
-                }
-            }
-        }
 
         match *opt_trait_ref {
             Some(ref ast_trait_ref) => {
@@ -782,6 +772,17 @@ pub fn check_item(ccx: &CrateCtxt, it: &ast::Item) {
             None => { }
         }
 
+        for impl_item in impl_items.iter() {
+            match *impl_item {
+                ast::MethodImplItem(ref m) => {
+                    check_method_body(ccx, &impl_pty.generics, &**m);
+                }
+                ast::TypeImplItem(_) => {
+                    // Nothing to do here.
+                }
+            }
+        }
+
       }
       ast::ItemTrait(_, _, _, ref trait_methods) => {
         let trait_def = ty::lookup_trait_def(ccx.tcx, local_def(it.id));