Auto merge of #86987 - lcnr:const-default-eval-bound, r=oli-obk

only check cg defaults wf once instantiated

the previous fixmes here didn't make too much sense as I didn't yet fully understand the code further below.
That code only runs if the predicates using our generic param default are fully concrete after substituting our default, which never happens if our default is generic.

r? `@oli-obk` `@BoxyUwU`

compiler/rustc_typeck/src/check/wfcheck.rs

@@ -738,15 +738,19 @@ fn check_where_clauses<'tcx, 'fcx>(
                 }
             }
             GenericParamDefKind::Const { .. } => {
-                // FIXME(const_generics_defaults): Figure out if this
-                // is the behavior we want, see the comment further below.
                 if is_our_default(&param) {
+                    // FIXME(const_generics_defaults): This
+                    // is incorrect when dealing with unused substs, for example
+                    // for `struct Foo<const N: usize, const M: usize = { 1 - 2 }>`
+                    // we should eagerly error.
                     let default_ct = tcx.const_param_default(param.def_id);
-                    fcx.register_wf_obligation(
-                        default_ct.into(),
-                        tcx.def_span(param.def_id),
-                        ObligationCauseCode::MiscObligation,
-                    );
+                    if !default_ct.needs_subst() {
+                        fcx.register_wf_obligation(
+                            default_ct.into(),
+                            tcx.def_span(param.def_id),
+                            ObligationCauseCode::MiscObligation,
+                        );
+                    }
                 }
             }
             // Doesn't have defaults.
@@ -783,14 +787,6 @@ fn check_where_clauses<'tcx, 'fcx>(
                 tcx.mk_param_from_def(param)
             }
             GenericParamDefKind::Const { .. } => {
-                // FIXME(const_generics_defaults): I(@lcnr) feel like always
-                // using the const parameter is the right choice here, even
-                // if it needs substs.
-                //
-                // Before stabilizing this we probably want to get some tests
-                // where this makes a difference and figure out what's the exact
-                // behavior we want here.
-
                 // If the param has a default, ...
                 if is_our_default(param) {
                     let default_ct = tcx.const_param_default(param.def_id);

compiler/rustc_typeck/src/collect.rs

@@ -2308,6 +2308,16 @@ fn const_evaluatable_predicates_of<'tcx>(
                 ));
             }
         }
+
+        fn visit_const_param_default(&mut self, _param: HirId, _ct: &'tcx hir::AnonConst) {
+            // Do not look into const param defaults,
+            // these get checked when they are actually instantiated.
+            //
+            // We do not want the following to error:
+            //
+            //     struct Foo<const N: usize, const M: usize = { N + 1 }>;
+            //     struct Bar<const N: usize>(Foo<N, 3>);
+        }
     }
 
     let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);

src/test/ui/const-generics/const_evaluatable_checked/unused-complex-default-expr.rs

@@ -0,0 +1,6 @@
+// check-pass
+#![feature(const_generics, const_evaluatable_checked, const_generics_defaults)]
+#![allow(incomplete_features)]
+struct Foo<const N: usize, const M: usize = { N + 1 }>;
+struct Bar<const N: usize>(Foo<N, 3>);
+fn main() {}

src/test/ui/const-generics/defaults/complex-generic-default-expr.min.stderr

@@ -1,5 +1,5 @@
 error: generic parameters may not be used in const operations
-  --> $DIR/complex-generic-default-expr.rs:6:47
+  --> $DIR/complex-generic-default-expr.rs:7:47
    |
 LL | struct Foo<const N: usize, const M: usize = { N + 1 }>;
    |                                               ^ cannot perform const operation using `N`

src/test/ui/const-generics/defaults/complex-generic-default-expr.rs

@@ -1,14 +1,13 @@
 // revisions: full min
+//[full] check-pass
 #![cfg_attr(full, feature(const_generics))]
 #![feature(const_generics_defaults)]
 #![allow(incomplete_features)]
 
 struct Foo<const N: usize, const M: usize = { N + 1 }>;
-//[full]~^ ERROR constant expression depends on a generic parameter
-//[min]~^^ ERROR generic parameters may not be used in const operations
+//[min]~^ ERROR generic parameters may not be used in const operations
 
 struct Bar<T, const TYPE_SIZE: usize = { std::mem::size_of::<T>() }>(T);
-//[full]~^ ERROR constant expression depends on a generic parameter
-//[min]~^^ ERROR generic parameters may not be used in const operations
+//[min]~^ ERROR generic parameters may not be used in const operations
 
 fn main() {}