Make sure we can produce ConstArgHasWrongType errors for valtree consts

compiler/rustc_trait_selection/src/solve/fulfill.rs

@@ -264,9 +264,10 @@ fn fulfillment_error_for_no_solution<'tcx>(
                     infcx.tcx.type_of(uv.def).instantiate(infcx.tcx, uv.args)
                 }
                 ty::ConstKind::Param(param_ct) => param_ct.find_ty_from_env(obligation.param_env),
-                _ => span_bug!(
+                ty::ConstKind::Value(ty, _) => ty,
+                kind => span_bug!(
                     obligation.cause.span,
-                    "ConstArgHasWrongType failed but we don't know how to compute type"
+                    "ConstArgHasWrongType failed but we don't know how to compute type for {kind:?}"
                 ),
             };
             FulfillmentErrorCode::Select(SelectionError::ConstArgHasWrongType {

tests/ui/const-generics/type-mismatch-in-nested-goal.current.stderr

@@ -0,0 +1,45 @@
+error: the constant `N` is not of type `bool`
+  --> $DIR/type-mismatch-in-nested-goal.rs:9:50
+   |
+LL | fn needs_a<const N: usize>(_: [u8; N]) where (): A<N> {}
+   |                                                  ^^^^ expected `bool`, found `usize`
+   |
+note: required by a const generic parameter in `A`
+  --> $DIR/type-mismatch-in-nested-goal.rs:5:9
+   |
+LL | trait A<const B: bool> {}
+   |         ^^^^^^^^^^^^^ required by this const generic parameter in `A`
+
+error: the constant `true` is not of type `usize`
+  --> $DIR/type-mismatch-in-nested-goal.rs:13:13
+   |
+LL |     needs_a([]);
+   |     ------- ^^ expected `usize`, found `bool`
+   |     |
+   |     required by a bound introduced by this call
+   |
+note: required by a const generic parameter in `needs_a`
+  --> $DIR/type-mismatch-in-nested-goal.rs:9:12
+   |
+LL | fn needs_a<const N: usize>(_: [u8; N]) where (): A<N> {}
+   |            ^^^^^^^^^^^^^^ required by this const generic parameter in `needs_a`
+
+error[E0308]: mismatched types
+  --> $DIR/type-mismatch-in-nested-goal.rs:13:13
+   |
+LL |     needs_a([]);
+   |     ------- ^^ expected an array with a size of true, found one with a size of 0
+   |     |
+   |     arguments to this function are incorrect
+   |
+   = note: expected array `[u8; true]`
+              found array `[_; 0]`
+note: function defined here
+  --> $DIR/type-mismatch-in-nested-goal.rs:9:4
+   |
+LL | fn needs_a<const N: usize>(_: [u8; N]) where (): A<N> {}
+   |    ^^^^^^^                 ----------
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/const-generics/type-mismatch-in-nested-goal.next.stderr

@@ -0,0 +1,45 @@
+error: the constant `N` is not of type `bool`
+  --> $DIR/type-mismatch-in-nested-goal.rs:9:50
+   |
+LL | fn needs_a<const N: usize>(_: [u8; N]) where (): A<N> {}
+   |                                                  ^^^^ expected `bool`, found `usize`
+   |
+note: required by a const generic parameter in `A`
+  --> $DIR/type-mismatch-in-nested-goal.rs:5:9
+   |
+LL | trait A<const B: bool> {}
+   |         ^^^^^^^^^^^^^ required by this const generic parameter in `A`
+
+error: the constant `true` is not of type `usize`
+  --> $DIR/type-mismatch-in-nested-goal.rs:13:13
+   |
+LL |     needs_a([]);
+   |     ------- ^^ expected `usize`, found `bool`
+   |     |
+   |     required by a bound introduced by this call
+   |
+note: required by a const generic parameter in `needs_a`
+  --> $DIR/type-mismatch-in-nested-goal.rs:9:12
+   |
+LL | fn needs_a<const N: usize>(_: [u8; N]) where (): A<N> {}
+   |            ^^^^^^^^^^^^^^ required by this const generic parameter in `needs_a`
+
+error[E0308]: mismatched types
+  --> $DIR/type-mismatch-in-nested-goal.rs:13:13
+   |
+LL |     needs_a([]);
+   |     ------- ^^ expected an array with a size of true, found one with a size of 0
+   |     |
+   |     arguments to this function are incorrect
+   |
+   = note: expected array `[u8; true]`
+              found array `[_; 0]`
+note: function defined here
+  --> $DIR/type-mismatch-in-nested-goal.rs:9:4
+   |
+LL | fn needs_a<const N: usize>(_: [u8; N]) where (): A<N> {}
+   |    ^^^^^^^                 ----------
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/const-generics/type-mismatch-in-nested-goal.rs

@@ -0,0 +1,17 @@
+//@ revisions: current next
+//@[next] compile-flags: -Znext-solver
+//@ ignore-compare-mode-next-solver (explicit revisions)
+
+trait A<const B: bool> {}
+
+impl A<true> for () {}
+
+fn needs_a<const N: usize>(_: [u8; N]) where (): A<N> {}
+//~^ ERROR the constant `N` is not of type `bool`
+
+pub fn main() {
+    needs_a([]);
+    //~^ ERROR the constant `true` is not of type `usize`
+    //~| ERROR mismatched types
+    // FIXME(const_generics): we should hide this error as we've already errored above
+}