Recover from default value for a lifetime in generic parameters.

compiler/rustc_error_messages/locales/en-US/parse.ftl

@@ -472,6 +472,9 @@ parse_unexpected_token_after_struct_name_found_other = expected `where`, `{"{"}`
 parse_unexpected_self_in_generic_parameters = unexpected keyword `Self` in generic parameters
     .note = you cannot use `Self` as a generic parameter because it is reserved for associated items
 
+parse_unexpected_default_value_for_lifetime_in_generic_parameters = unexpected default lifetime parameter
+    .label = lifetime parameters cannot have default values
+
 parse_multiple_where_clauses = cannot define duplicate `where` clauses on an item
     .label = previous `where` clause starts here
     .suggestion = consider joining the two `where` clauses into one

compiler/rustc_parse/src/errors.rs

@@ -1589,6 +1589,14 @@ pub(crate) struct UnexpectedSelfInGenericParameters {
     pub span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(parse_unexpected_default_value_for_lifetime_in_generic_parameters)]
+pub(crate) struct UnexpectedDefaultValueForLifetimeInGenericParameters {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(parse_multiple_where_clauses)]
 pub(crate) struct MultipleWhereClauses {

compiler/rustc_parse/src/parser/generics.rs

@@ -1,5 +1,6 @@
 use crate::errors::{
-    MultipleWhereClauses, UnexpectedSelfInGenericParameters, WhereClauseBeforeTupleStructBody,
+    MultipleWhereClauses, UnexpectedDefaultValueForLifetimeInGenericParameters,
+    UnexpectedSelfInGenericParameters, WhereClauseBeforeTupleStructBody,
     WhereClauseBeforeTupleStructBodySugg,
 };
 
@@ -145,6 +146,20 @@ impl<'a> Parser<'a> {
                         } else {
                             (None, Vec::new())
                         };
+
+                        if this.check_noexpect(&token::Eq)
+                            && this.look_ahead(1, |t| t.is_lifetime())
+                        {
+                            let lo = this.token.span;
+                            // Parse `= 'lifetime`.
+                            this.bump(); // `=`
+                            this.bump(); // `'lifetime`
+                            let span = lo.to(this.prev_token.span);
+                            this.sess.emit_err(
+                                UnexpectedDefaultValueForLifetimeInGenericParameters { span },
+                            );
+                        }
+
                         Some(ast::GenericParam {
                             ident: lifetime.ident,
                             id: lifetime.id,

tests/ui/lifetimes/issue-107492-default-value-for-lifetime.rs

@@ -0,0 +1,6 @@
+pub struct DefaultLifetime<'a, 'b = 'static> {
+                                   //~^ ERROR unexpected default lifetime parameter
+    _marker: std::marker::PhantomData<&'a &'b ()>,
+}
+
+fn main(){}

tests/ui/lifetimes/issue-107492-default-value-for-lifetime.stderr

@@ -0,0 +1,8 @@
+error: unexpected default lifetime parameter
+  --> $DIR/issue-107492-default-value-for-lifetime.rs:1:35
+   |
+LL | pub struct DefaultLifetime<'a, 'b = 'static> {
+   |                                   ^^^^^^^^^ lifetime parameters cannot have default values
+
+error: aborting due to previous error
+