Explain that default field values aren't compatible with type parameters unless going through the type parameter

```
error[E0308]: mismatched types
  --> $DIR/struct-type-parameter-with-default.rs:5:12
   |
LL | struct Foo<T = String> {
   |            ---------- expected this type parameter
LL |     x: T = String::new(),
   |            ^^^^^^^^^^^^^ expected type parameter `T`, found `String`
   |
   = note: expected type parameter `T`
                      found struct `String`
   = note: the type of default fields referencing type parameters can't be assumed inside the struct defining them
```

compiler/rustc_hir_typeck/src/lib.rs

@@ -205,7 +205,7 @@ fn typeck_with_inspect<'tcx>(
         }
 
         fcx.check_expr_coercible_to_type_or_error(body.value, expected_type, None, |err, _| {
-            extend_err_with_const_context(err, tcx, node);
+            extend_err_with_const_context(err, tcx, node, expected_type);
         });
 
         fcx.write_ty(id, expected_type);
@@ -276,7 +276,12 @@ fn typeck_with_inspect<'tcx>(
     typeck_results
 }
 
-fn extend_err_with_const_context(err: &mut Diag<'_>, tcx: TyCtxt<'_>, node: hir::Node<'_>) {
+fn extend_err_with_const_context(
+    err: &mut Diag<'_>,
+    tcx: TyCtxt<'_>,
+    node: hir::Node<'_>,
+    expected_ty: Ty<'_>,
+) {
     match node {
         hir::Node::ImplItem(hir::ImplItem { kind: hir::ImplItemKind::Const(ty, _), .. })
         | hir::Node::TraitItem(hir::TraitItem {
@@ -378,6 +383,15 @@ fn extend_err_with_const_context(err: &mut Diag<'_>, tcx: TyCtxt<'_>, node: hir:
             // Point at `char` in `pattern_type!(char is 1..=1)`.
             err.span_label(ty.span, "the pattern must match the type");
         }
+        hir::Node::AnonConst(anon)
+            if let hir::Node::Field(_) = tcx.parent_hir_node(anon.hir_id)
+                && let ty::Param(_) = expected_ty.kind() =>
+        {
+            err.note(
+                "the type of default fields referencing type parameters can't be assumed inside \
+                 the struct defining them",
+            );
+        }
         _ => {}
     }
 }

tests/ui/structs/default-field-values/struct-type-parameter-with-default.rs

@@ -0,0 +1,11 @@
+// Test for #147748, providing additional clarification that default field values aren't compatible
+// with type parameters unless going through the type parameter.
+#![feature(default_field_values)]
+struct Foo<T = String> { //~ NOTE: expected this type parameter
+    x: T = String::new(),
+    //~^ ERROR: mismatched types
+    //~| NOTE: expected type parameter
+    //~| NOTE: expected type parameter
+    //~| NOTE: the type of default fields referencing type parameters can't be assumed inside the struct defining them
+}
+fn main() {}

tests/ui/structs/default-field-values/struct-type-parameter-with-default.stderr

@@ -0,0 +1,15 @@
+error[E0308]: mismatched types
+  --> $DIR/struct-type-parameter-with-default.rs:5:12
+   |
+LL | struct Foo<T = String> {
+   |            ---------- expected this type parameter
+LL |     x: T = String::new(),
+   |            ^^^^^^^^^^^^^ expected type parameter `T`, found `String`
+   |
+   = note: expected type parameter `T`
+                      found struct `String`
+   = note: the type of default fields referencing type parameters can't be assumed inside the struct defining them
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.