fallback default to None during parse late-bound lifetime

Author: bvanjoi

compiler/rustc_parse/messages.ftl

@@ -117,6 +117,8 @@ parse_default_not_followed_by_item = `default` is not followed by an item
     .label = the `default` qualifier
     .note = only `fn`, `const`, `type`, or `impl` items may be prefixed by `default`
 
+parse_default_parameter_in_binder = default parameter is not allowed in this binder
+
 parse_do_catch_syntax_removed = found removed `do catch` syntax
     .note = following RFC #2388, the new non-placeholder syntax is `try`
     .suggestion = replace with the new syntax

compiler/rustc_parse/src/errors.rs

@@ -2897,3 +2897,10 @@ pub(crate) struct TransposeDynOrImplSugg<'a> {
     pub insertion_span: Span,
     pub kw: &'a str,
 }
+
+#[derive(Diagnostic)]
+#[diag(parse_default_parameter_in_binder)]
+pub(crate) struct UnexpectedDefaultParameterInBinder {
+    #[primary_span]
+    pub span: Span,
+}

compiler/rustc_parse/src/parser/ty.rs

@@ -4,7 +4,7 @@ use crate::errors::{
     self, DynAfterMut, ExpectedFnPathFoundFnKeyword, ExpectedMutOrConstInRawPointerType,
     FnPointerCannotBeAsync, FnPointerCannotBeConst, FnPtrWithGenerics, FnPtrWithGenericsSugg,
     InvalidDynKeyword, LifetimeAfterMut, NeedPlusAfterTraitObjectLifetime, NestedCVariadicType,
-    ReturnTypesUseThinArrow,
+    ReturnTypesUseThinArrow, UnexpectedDefaultParameterInBinder,
 };
 use crate::{maybe_recover_from_interpolated_ty_qpath, maybe_whole};
 
@@ -14,8 +14,8 @@ use rustc_ast::token::{self, Delimiter, Token, TokenKind};
 use rustc_ast::util::case::Case;
 use rustc_ast::{
     self as ast, BareFnTy, BoundPolarity, FnRetTy, GenericBound, GenericBounds, GenericParam,
-    Generics, Lifetime, MacCall, MutTy, Mutability, PolyTraitRef, TraitBoundModifier,
-    TraitObjectSyntax, Ty, TyKind,
+    GenericParamKind, Generics, Lifetime, MacCall, MutTy, Mutability, PolyTraitRef,
+    TraitBoundModifier, TraitObjectSyntax, Ty, TyKind,
 };
 use rustc_errors::{Applicability, PResult};
 use rustc_span::symbol::{kw, sym, Ident};
@@ -1033,6 +1033,16 @@ impl<'a> Parser<'a> {
         if self.eat_keyword(kw::For) {
             self.expect_lt()?;
             let params = self.parse_generic_params()?;
+            let params = params
+                .into_iter()
+                .map(|param| match param.kind {
+                    GenericParamKind::Type { ref default } if let Some(ty) = default => {
+                        self.sess.emit_err(UnexpectedDefaultParameterInBinder { span: ty.span });
+                        GenericParam { kind: GenericParamKind::Type { default: None }, ..param }
+                    }
+                    _ => param,
+                })
+                .collect();
             self.expect_gt()?;
             // We rely on AST validation to rule out invalid cases: There must not be type
             // parameters, and the lifetime parameters must not have bounds.

tests/ui/traits/non_lifetime_binders/issue-118697.rs

@@ -0,0 +1,8 @@
+#![allow(incomplete_features)]
+#![feature(non_lifetime_binders)]
+
+type T = dyn for<V = A(&())> Fn(());
+//~^ ERROR default parameter is not allowed in this binder
+//~| ERROR late-bound type parameter not allowed on trait object types
+
+fn main() {}

tests/ui/traits/non_lifetime_binders/issue-118697.stderr

@@ -0,0 +1,14 @@
+error: default parameter is not allowed in this binder
+  --> $DIR/issue-118697.rs:4:22
+   |
+LL | type T = dyn for<V = A(&())> Fn(());
+   |                      ^^^^^^
+
+error: late-bound type parameter not allowed on trait object types
+  --> $DIR/issue-118697.rs:4:18
+   |
+LL | type T = dyn for<V = A(&())> Fn(());
+   |                  ^
+
+error: aborting due to 2 previous errors
+