fix ice for associated constant generics

compiler/rustc_span/src/source_map.rs

@@ -956,7 +956,7 @@ impl SourceMap {
     }
 
     pub fn generate_fn_name_span(&self, span: Span) -> Option<Span> {
-        let prev_span = self.span_extend_to_prev_str(span, "fn", true, true).unwrap_or(span);
+        let prev_span = self.span_extend_to_prev_str(span, "fn", true, true)?;
         if let Ok(snippet) = self.span_to_snippet(prev_span) {
             debug!(
                 "generate_fn_name_span: span={:?}, prev_span={:?}, snippet={:?}",

src/test/ui/generics/issue-98432.rs

@@ -0,0 +1,9 @@
+struct Struct<T>(T);
+
+impl<T> Struct<T> {
+    const CONST: fn() = || {
+        struct _Obligation where T:; //~ ERROR can't use generic parameters from outer function
+    };
+}
+
+fn main() {}

src/test/ui/generics/issue-98432.stderr

@@ -0,0 +1,14 @@
+error[E0401]: can't use generic parameters from outer function
+  --> $DIR/issue-98432.rs:5:34
+   |
+LL | impl<T> Struct<T> {
+   |      - type parameter from outer function
+LL |     const CONST: fn() = || {
+LL |         struct _Obligation where T:;
+   |                                  ^ use of generic parameter from outer function
+   |
+   = help: try using a local generic parameter instead
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0401`.