LTA: Check where-clauses for well-formedness at the def site

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -328,16 +328,20 @@ fn check_item<'tcx>(tcx: TyCtxt<'tcx>, item: &'tcx hir::Item<'tcx>) -> Result<()
         hir::ItemKind::TraitAlias(..) => check_trait(tcx, item),
         // `ForeignItem`s are handled separately.
         hir::ItemKind::ForeignMod { .. } => Ok(()),
-        hir::ItemKind::TyAlias(hir_ty, hir_generics) => {
-            if tcx.type_alias_is_lazy(item.owner_id) {
-                // Bounds of lazy type aliases and of eager ones that contain opaque types are respected.
-                // E.g: `type X = impl Trait;`, `type X = (impl Trait, Y);`.
-                let res = check_item_type(tcx, def_id, hir_ty.span, UnsizedHandling::Allow);
-                check_variances_for_type_defn(tcx, item, hir_generics);
-                res
-            } else {
+        hir::ItemKind::TyAlias(hir_ty, hir_generics) if tcx.type_alias_is_lazy(item.owner_id) => {
+            let res = enter_wf_checking_ctxt(tcx, item.span, def_id, |wfcx| {
+                let ty = tcx.type_of(def_id).instantiate_identity();
+                let item_ty = wfcx.normalize(hir_ty.span, Some(WellFormedLoc::Ty(def_id)), ty);
+                wfcx.register_wf_obligation(
+                    hir_ty.span,
+                    Some(WellFormedLoc::Ty(def_id)),
+                    item_ty.into(),
+                );
+                check_where_clauses(wfcx, item.span, def_id);
                 Ok(())
-            }
+            });
+            check_variances_for_type_defn(tcx, item, hir_generics);
+            res
         }
         _ => Ok(()),
     };
@@ -1276,7 +1280,6 @@ fn check_item_fn(
 
 enum UnsizedHandling {
     Forbid,
-    Allow,
     AllowIfForeignTail,
 }
 
@@ -1294,7 +1297,6 @@ fn check_item_type(
 
         let forbid_unsized = match unsized_handling {
             UnsizedHandling::Forbid => true,
-            UnsizedHandling::Allow => false,
             UnsizedHandling::AllowIfForeignTail => {
                 let tail =
                     tcx.struct_tail_for_codegen(item_ty, wfcx.infcx.typing_env(wfcx.param_env));

tests/ui/lazy-type-alias/def-site-param-defaults-wf.rs

@@ -0,0 +1,9 @@
+//! Ensure that we check generic parameter defaults at the definition site for well-formedness.
+#![feature(lazy_type_alias)]
+#![allow(incomplete_features)]
+
+type Alias<T = Vec<str>, const N: usize = {0 - 1}> = T;
+//~^ ERROR evaluation of constant value failed
+//~| ERROR the size for values of type `str` cannot be known at compilation time
+
+fn main() {}

tests/ui/lazy-type-alias/def-site-param-defaults-wf.stderr

@@ -0,0 +1,20 @@
+error[E0080]: evaluation of constant value failed
+  --> $DIR/def-site-param-defaults-wf.rs:5:44
+   |
+LL | type Alias<T = Vec<str>, const N: usize = {0 - 1}> = T;
+   |                                            ^^^^^ attempt to compute `0_usize - 1_usize`, which would overflow
+
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/def-site-param-defaults-wf.rs:5:16
+   |
+LL | type Alias<T = Vec<str>, const N: usize = {0 - 1}> = T;
+   |                ^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `str`
+note: required by an implicit `Sized` bound in `Vec`
+  --> $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0080, E0277.
+For more information about an error, try `rustc --explain E0080`.

tests/ui/lazy-type-alias/def-site-predicates-wf.current.stderr

@@ -0,0 +1,13 @@
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/def-site-predicates-wf.rs:11:5
+   |
+LL |     <Vec<str> as Discard>::Output:;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `str`
+note: required by an implicit `Sized` bound in `Vec`
+  --> $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/lazy-type-alias/def-site-predicates-wf.next.stderr

@@ -0,0 +1,23 @@
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/def-site-predicates-wf.rs:11:5
+   |
+LL |     <Vec<str> as Discard>::Output:;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `str`
+note: required by an implicit `Sized` bound in `Vec`
+  --> $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
+
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/def-site-predicates-wf.rs:16:36
+   |
+LL |     <Vec<str> as Discard>::Output: Sized;
+   |                                    ^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `str`
+note: required by an implicit `Sized` bound in `Vec`
+  --> $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/lazy-type-alias/def-site-predicates-wf.rs

@@ -0,0 +1,21 @@
+//! Ensure that we check the predicates at the definition site for well-formedness.
+#![feature(lazy_type_alias)]
+#![allow(incomplete_features)]
+
+//@ revisions: current next
+//@[next] compile-flags: -Znext-solver
+//@ ignore-compare-mode-next-solver (explicit revisions)
+
+type Alias0 = ()
+where
+    <Vec<str> as Discard>::Output:; //~ ERROR the size for values of type `str` cannot be known at compilation time
+
+// FIXME(#100041): This should error in the current solver, too.
+type Alias1 = ()
+where
+    <Vec<str> as Discard>::Output: Sized; //[next]~ ERROR the size for values of type `str` cannot be known at compilation time
+
+trait Discard { type Output; }
+impl<T> Discard for T { type Output = (); }
+
+fn main() {}