compiler/rustc_hir_analysis/src/collect/type_of.rs

@@ -62,10 +62,25 @@ fn anon_const_type_of<'tcx>(icx: &ItemCtxt<'tcx>, def_id: LocalDefId) -> Ty<'tcx
             return ty;
         }
 
-        Node::Field(&hir::FieldDef { default: Some(c), def_id: field_def_id, .. })
-            if c.hir_id == hir_id =>
-        {
-            tcx.type_of(field_def_id).instantiate_identity()
+        Node::Field(&hir::FieldDef {
+            default: Some(c), def_id: field_def_id, ty: hir_ty, ..
+        }) if c.hir_id == hir_id => {
+            let ty = tcx.type_of(field_def_id).instantiate_identity();
+            if !ty.has_param() || tcx.features().generic_const_exprs() {
+                ty
+            } else {
+                let mut diag = tcx.dcx().struct_span_err(
+                    c.span,
+                    format!("default value for field cannot depend on generic parameters"),
+                );
+                diag.span_label(hir_ty.span, "this field references generic parameters");
+                let adt_def_id = tcx.parent(field_def_id.to_def_id());
+                let adt = tcx.def_descr(adt_def_id);
+                diag.span_label(tcx.def_span(adt_def_id), format!("in this {adt}"));
+                // FIXME: We could give a feature error for GCE, but it's best not to point users
+                // to that feature since it's broken.
+                Ty::new_error(tcx, diag.emit())
+            }
         }
 
         _ => Ty::new_error_with_message(

tests/ui/structs/default-field-values/do-not-ice-on-invalid-lifetime.rs

@@ -1,6 +1,9 @@
 #![feature(default_field_values)]
-struct A<'a> { //~ ERROR lifetime parameter `'a` is never used
-    x: Vec<A> = Vec::new(), //~ ERROR missing lifetime specifier
+struct A<'a> {
+    //~^ ERROR lifetime parameter `'a` is never used
+    x: Vec<A> = Vec::new(),
+    //~^ ERROR missing lifetime specifier
+    //~| ERROR default value for field cannot depend on generic parameters
 }
 
 fn main() {}

tests/ui/structs/default-field-values/do-not-ice-on-invalid-lifetime.stderr

@@ -1,5 +1,5 @@
 error[E0106]: missing lifetime specifier
-  --> $DIR/do-not-ice-on-invalid-lifetime.rs:3:12
+  --> $DIR/do-not-ice-on-invalid-lifetime.rs:4:12
    |
 LL |     x: Vec<A> = Vec::new(),
    |            ^ expected named lifetime parameter
@@ -9,6 +9,17 @@ help: consider using the `'a` lifetime
 LL |     x: Vec<A<'a>> = Vec::new(),
    |             ++++
 
+error: default value for field cannot depend on generic parameters
+  --> $DIR/do-not-ice-on-invalid-lifetime.rs:4:17
+   |
+LL | struct A<'a> {
+   | ------------ in this struct
+LL |
+LL |     x: Vec<A> = Vec::new(),
+   |        ------   ^^^^^^^^^^
+   |        |
+   |        this field references generic parameters
+
 error[E0392]: lifetime parameter `'a` is never used
   --> $DIR/do-not-ice-on-invalid-lifetime.rs:2:10
    |
@@ -17,7 +28,7 @@ LL | struct A<'a> {
    |
    = help: consider removing `'a`, referring to it in a field, or using a marker such as `PhantomData`
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 
 Some errors have detailed explanations: E0106, E0392.
 For more information about an error, try `rustc --explain E0106`.

tests/ui/structs/default-field-values/field-references-param.rs

@@ -0,0 +1,16 @@
+// Make sure we don't ICE when a field default references a generic parameter via inference.
+
+#![feature(default_field_values)]
+
+struct W<const X: usize>;
+
+impl<const X: usize> W<X> {
+    const fn new() -> Self { W }
+}
+
+struct Z<const X: usize> {
+    x: W<X> = W::new(),
+    //~^ ERROR default value for field cannot depend on generic parameters
+}
+
+fn main() {}

tests/ui/structs/default-field-values/field-references-param.stderr

@@ -0,0 +1,12 @@
+error: default value for field cannot depend on generic parameters
+  --> $DIR/field-references-param.rs:12:15
+   |
+LL | struct Z<const X: usize> {
+   | ------------------------ in this struct
+LL |     x: W<X> = W::new(),
+   |        ----   ^^^^^^^^
+   |        |
+   |        this field references generic parameters
+
+error: aborting due to 1 previous error
+