Don't ICE on operator trait methods with generic methods

Emit a fatal error instead.

compiler/rustc_error_messages/locales/en-US/hir_analysis.ftl

@@ -150,3 +150,6 @@ hir_analysis_const_bound_for_non_const_trait =
 hir_analysis_self_in_impl_self =
     `Self` is not valid in the self type of an impl block
     .note = replace `Self` with a different type
+
+hir_analysis_op_trait_generic_params =
+    `{$method_name}` must not have any generic parameters

compiler/rustc_hir_typeck/src/errors.rs

@@ -125,3 +125,11 @@ pub struct AddMissingParenthesesInRange {
     #[suggestion_part(code = ")")]
     pub right: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(hir_analysis_op_trait_generic_params)]
+pub struct OpMethodGenericParams {
+    #[primary_span]
+    pub span: Span,
+    pub method_name: String,
+}

compiler/rustc_hir_typeck/src/method/mod.rs

@@ -10,6 +10,7 @@ mod suggest;
 pub use self::suggest::SelfSource;
 pub use self::MethodError::*;
 
+use crate::errors::OpMethodGenericParams;
 use crate::{Expectation, FnCtxt};
 use rustc_data_structures::sync::Lrc;
 use rustc_errors::{Applicability, Diagnostic};
@@ -443,7 +444,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         };
         let def_id = method_item.def_id;
         let generics = tcx.generics_of(def_id);
-        assert_eq!(generics.params.len(), 0);
+
+        if generics.params.len() != 0 {
+            tcx.sess.emit_fatal(OpMethodGenericParams {
+                span: tcx.def_span(method_item.def_id),
+                method_name: m_name.to_string(),
+            });
+        }
 
         debug!("lookup_in_trait_adjusted: method_item={:?}", method_item);
         let mut obligations = vec![];

src/test/ui/traits/invalid_operator_trait.rs

@@ -0,0 +1,23 @@
+#![crate_type = "lib"]
+#![feature(lang_items)]
+#![feature(no_core)]
+#![no_core]
+
+#[lang="sized"]
+pub trait Sized {
+    // Empty.
+}
+
+#[lang = "add"]
+trait Add<RHS=Self> {
+    type Output;
+
+    fn add<Y>(self, _: RHS) -> Self::Output;
+    //~^ ERROR `add` must not have any generic parameters
+}
+
+#[allow(unreachable_code)]
+fn ice(a: usize) {
+    let r = loop {};
+    r = r + a;
+}

src/test/ui/traits/invalid_operator_trait.stderr

@@ -0,0 +1,8 @@
+error: `add` must not have any generic parameters
+  --> $DIR/invalid_operator_trait.rs:15:5
+   |
+LL |     fn add<Y>(self, _: RHS) -> Self::Output;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+