[PATCH] Fix ICE with spelling hints within explicit namespace aliases (PR c++/79298)

[David Malcolm]

PR c++/79298 reports a segfault when handling a qualified name lookup error,
where consider_binding_level dereferences a NULL cp_binding_level *.

The root cause is that suggest_alternative_in_explicit_scope assumes that
NAMESPACE_LEVEL (scope) is non-NULL, but this can be NULL for namespace aliases.

This patch adds a use of ORIGINAL_NAMESPACE to look through any namespace
aliases.

Successfully bootstrapped&regrtested on x86_64-pc-linux-gnu.

OK for trunk?

gcc/cp/ChangeLog:
        PR c++/79298
        * name-lookup.c (suggest_alternative_in_explicit_scope): Resolve
        any namespace aliases.

gcc/testsuite/ChangeLog:
        PR c++/79298
        * g++.dg/spellcheck-pr79298.C: New test case.
---
 gcc/cp/name-lookup.c                      |  3 +++
 gcc/testsuite/g++.dg/spellcheck-pr79298.C | 17 +++++++++++++++++
 2 files changed, 20 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/spellcheck-pr79298.C

diff --git a/gcc/cp/name-lookup.c b/gcc/cp/name-lookup.c
index a3cb7ee..994f7f0 100644
--- a/gcc/cp/name-lookup.c
+++ b/gcc/cp/name-lookup.c
@@ -4545,6 +4545,9 @@ bool
 suggest_alternative_in_explicit_scope (location_t location, tree name,
                                       tree scope)
 {
+  /* Resolve any namespace aliases.  */
+  scope = ORIGINAL_NAMESPACE (scope);
+
   cp_binding_level *level = NAMESPACE_LEVEL (scope);
 
   best_match <tree, tree> bm (name);
diff --git a/gcc/testsuite/g++.dg/spellcheck-pr79298.C 
b/gcc/testsuite/g++.dg/spellcheck-pr79298.C
new file mode 100644
index 0000000..4d7bbf9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/spellcheck-pr79298.C
@@ -0,0 +1,17 @@
+// Ensure that we can offer suggestions for misspellings via a
+// namespace alias.
+
+namespace N { int x; int color; }
+namespace M = N; 
+namespace O = M; 
+
+int foo () 
+{
+  return M::y; // { dg-error ".y. is not a member of .M." }
+}
+
+int bar () 
+{
+  return O::colour; // { dg-error ".colour. is not a member of .O." }
+  // { dg-message "suggested alternative: .color." "" { target *-*-* } .-1 }
+}
-- 
1.8.5.3