Auto merge of #139646 - lcnr:default-is-fully-concrete, r=BoxyUwU

check types of const param defaults

fixes #139643 by checking that the type of a const parameter default matches the type of the parameter as long as both types are fully concrete

r? `@BoxyUwU`

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -1483,6 +1483,39 @@ fn check_where_clauses<'tcx>(wfcx: &WfCheckingCtxt<'_, 'tcx>, span: Span, def_id
                         .then(|| WellFormedLoc::Ty(param.def_id.expect_local())),
                     default.as_term().unwrap(),
                 );
+            } else {
+                // If we've got a generic const parameter we still want to check its
+                // type is correct in case both it and the param type are fully concrete.
+                let GenericArgKind::Const(ct) = default.unpack() else {
+                    continue;
+                };
+
+                let ct_ty = match ct.kind() {
+                    ty::ConstKind::Infer(_)
+                    | ty::ConstKind::Placeholder(_)
+                    | ty::ConstKind::Bound(_, _) => unreachable!(),
+                    ty::ConstKind::Error(_) | ty::ConstKind::Expr(_) => continue,
+                    ty::ConstKind::Value(cv) => cv.ty,
+                    ty::ConstKind::Unevaluated(uv) => {
+                        infcx.tcx.type_of(uv.def).instantiate(infcx.tcx, uv.args)
+                    }
+                    ty::ConstKind::Param(param_ct) => param_ct.find_ty_from_env(wfcx.param_env),
+                };
+
+                let param_ty = tcx.type_of(param.def_id).instantiate_identity();
+                if !ct_ty.has_param() && !param_ty.has_param() {
+                    let cause = traits::ObligationCause::new(
+                        tcx.def_span(param.def_id),
+                        wfcx.body_def_id,
+                        ObligationCauseCode::WellFormed(None),
+                    );
+                    wfcx.register_obligation(Obligation::new(
+                        tcx,
+                        cause,
+                        wfcx.param_env,
+                        ty::ClauseKind::ConstArgHasType(ct, param_ty),
+                    ));
+                }
             }
         }
     }