[PATCH] Avoid expression parsing crash with unknown language

[Tom Tromey <tromey@adacore.com>]

PR gdb/28093 points out that gdb crashes when language is set to
"unknown" and expression parsing is attempted.  At first I thought
this was a regression due to the expression rewrite, but it turns out
that older versions crash as well.

This patch avoids the crash by changing the default expression parser
to throw an exception.  I think this is preferable -- the current
behavior of silently doing nothing does not really make sense.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28093
---
 gdb/language.c                   | 4 ++--
 gdb/testsuite/gdb.base/langs.exp | 6 ++++++
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/gdb/language.c b/gdb/language.c
index 0d1e3848de8..45ce2ebcc92 100644
--- a/gdb/language.c
+++ b/gdb/language.c
@@ -825,8 +825,8 @@ class auto_or_unknown_language : public language_defn
 
   int parser (struct parser_state *ps) const override
   {
-    /* No parsing is done, just claim success.  */
-    return 1;
+    error (_("expression parsing not implemented for language \"%s\""),
+	   natural_name ());
   }
 
   /* See language.h.  */
diff --git a/gdb/testsuite/gdb.base/langs.exp b/gdb/testsuite/gdb.base/langs.exp
index 15908b0a19a..631401a8eab 100644
--- a/gdb/testsuite/gdb.base/langs.exp
+++ b/gdb/testsuite/gdb.base/langs.exp
@@ -111,6 +111,12 @@ if [runto csub] then {
              "set lang to minimal"
     
     gdb_test "print x" " = 5000" "print parameter value"
+
+    # Ensure this at least does not crash.
+    gdb_test_no_output "set language unknown"
+    gdb_test "print x" \
+	"expression parsing not implemented for language .Unknown." \
+	"expression parsing does not crash for unknown"
 }
 
 set timeout $oldtimeout
-- 
2.26.3