Emit fatal on defaults for generic params in binders if with nested defs

compiler/rustc_ast_lowering/src/lib.rs

@@ -2220,14 +2220,19 @@ impl<'hir, R: ResolverAstLoweringExt<'hir>> LoweringContext<'_, 'hir, R> {
                 // since later compiler stages cannot handle them (and shouldn't need to be able to).
                 let default = default
                     .as_ref()
-                    .filter(|_| match source {
+                    .filter(|anon_const| match source {
                         hir::GenericParamSource::Generics => true,
                         hir::GenericParamSource::Binder => {
-                            self.dcx().emit_err(errors::GenericParamDefaultInBinder {
-                                span: param.span(),
-                            });
-
-                            false
+                            let err = errors::GenericParamDefaultInBinder { span: param.span() };
+                            if expr::WillCreateDefIdsVisitor
+                                .visit_expr(&anon_const.value)
+                                .is_break()
+                            {
+                                self.dcx().emit_fatal(err)
+                            } else {
+                                self.dcx().emit_err(err);
+                                false
+                            }
                         }
                     })
                     .map(|def| self.lower_anon_const_to_const_arg_and_alloc(def));

tests/crashes/123629.rs

@@ -1,10 +0,0 @@
-//@ known-bug: #123629
-#![feature(generic_assert)]
-
-fn foo()
-where
-    for<const N: usize = { assert!(u) }> ():,
-{
-}
-
-fn main() {}

tests/ui/const-generics/generic_const_exprs/bad-defaults-for-generic-param-in-binder-123629.rs

@@ -0,0 +1,12 @@
+#![feature(generic_assert)]
+
+fn foo()
+where
+    for<const N: usize = { assert!(u) }> ():,
+    //~^ ERROR cannot find value `u` in this scope
+    //~^^ ERROR only lifetime parameters can be used in this context
+    //~^^^ ERROR defaults for generic parameters are not allowed in `for<...>` binders
+{
+}
+
+fn main() {}

tests/ui/const-generics/generic_const_exprs/bad-defaults-for-generic-param-in-binder-123629.stderr

@@ -0,0 +1,26 @@
+error[E0425]: cannot find value `u` in this scope
+  --> $DIR/bad-defaults-for-generic-param-in-binder-123629.rs:5:36
+   |
+LL |     for<const N: usize = { assert!(u) }> ():,
+   |                                    ^ not found in this scope
+
+error[E0658]: only lifetime parameters can be used in this context
+  --> $DIR/bad-defaults-for-generic-param-in-binder-123629.rs:5:15
+   |
+LL |     for<const N: usize = { assert!(u) }> ():,
+   |               ^
+   |
+   = note: see issue #108185 <https://github.com/rust-lang/rust/issues/108185> for more information
+   = help: add `#![feature(non_lifetime_binders)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: defaults for generic parameters are not allowed in `for<...>` binders
+  --> $DIR/bad-defaults-for-generic-param-in-binder-123629.rs:5:9
+   |
+LL |     for<const N: usize = { assert!(u) }> ():,
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0425, E0658.
+For more information about an error, try `rustc --explain E0425`.