Handle nested inline consts in const argument checks

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

@@ -482,16 +482,18 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
     ) -> Result<(), ErrorGuaranteed> {
         let tcx = self.tcx();
         let parent_def_id = self.item_def_id();
+        // In this path, `Some(context)` should be `ConstArgument`: enum
+        // discriminants are handled earlier by resolve. We still use the helper so
+        // nested inline consts are checked in the outer const-argument context.
         if let Res::Def(DefKind::ConstParam, _) = res
-            && matches!(tcx.def_kind(parent_def_id), DefKind::AnonConst | DefKind::InlineConst)
-            && let ty::AnonConstKind::MCG = tcx.anon_const_kind(parent_def_id)
+            && let Some(context) = self.anon_const_forbids_generic_params()
         {
             let folder = ForbidParamUsesFolder {
                 tcx,
                 anon_const_def_id: parent_def_id,
                 span,
                 is_self_alias: false,
-                context: ForbidParamContext::ConstArgument,
+                context,
             };
             return Err(folder.error());
         }

tests/ui/const-generics/mgca/const-param-in-nested-inline-const-arg.rs

@@ -0,0 +1,9 @@
+//@compile-flags: -Znext-solver=globally
+#![feature(generic_const_args, min_generic_const_args)]
+
+struct S<const N: usize>;
+fn foo<const N: usize>(_: S<{ const { const { N } } }>) {}
+//~^ ERROR: generic parameters in const blocks are not allowed; use a named `const` item instead
+
+
+fn main() {}

tests/ui/const-generics/mgca/const-param-in-nested-inline-const-arg.stderr

@@ -0,0 +1,10 @@
+error: generic parameters in const blocks are not allowed; use a named `const` item instead
+  --> $DIR/const-param-in-nested-inline-const-arg.rs:5:47
+   |
+LL | fn foo<const N: usize>(_: S<{ const { const { N } } }>) {}
+   |                                               ^
+   |
+   = help: consider factoring the expression into a `type const` item and use it as the const argument instead
+
+error: aborting due to 1 previous error
+