diagnostics: Fix add_misspelling_candidates [PR115440]

The option_map array for most entries contains just non-NULL opt0
    { "-Wno-", NULL, "-W", false, true },
    { "-fno-", NULL, "-f", false, true },
    { "-gno-", NULL, "-g", false, true },
    { "-mno-", NULL, "-m", false, true },
    { "--debug=", NULL, "-g", false, false },
    { "--machine-", NULL, "-m", true, false },
    { "--machine-no-", NULL, "-m", false, true },
    { "--machine=", NULL, "-m", false, false },
    { "--machine=no-", NULL, "-m", false, true },
    { "--machine", "", "-m", false, false },
    { "--machine", "no-", "-m", false, true },
    { "--optimize=", NULL, "-O", false, false },
    { "--std=", NULL, "-std=", false, false },
    { "--std", "", "-std=", false, false },
    { "--warn-", NULL, "-W", true, false },
    { "--warn-no-", NULL, "-W", false, true },
    { "--", NULL, "-f", true, false },
    { "--no-", NULL, "-f", false, true }
and so add_misspelling_candidates works correctly for it, but 3 out of
these,
    { "--machine", "", "-m", false, false },
    { "--machine", "no-", "-m", false, true },
and
    { "--std", "", "-std=", false, false },
use non-NULL opt1.  That says that
--machine foo
should map to
-mfoo
and
--machine no-foo
should map to
-mno-foo
and
--std c++17
should map to
-std=c++17
add_misspelling_canidates was not handling this, so it hapilly
registered say
--stdc++17
or
--machineavx512
(twice) as spelling alternatives, when those options aren't recognized.
Instead we support
--std c++17
or
--machine avx512
--machine no-avx512

The following patch fixes that.  On this particular testcase, we no longer
suggest anything, even when among the suggestion is say that
--std c++17
or
-std=c++17
etc.

2024-06-17  Jakub Jelinek  <jakub@redhat.com>

	PR driver/115440
	* opts-common.cc (add_misspelling_candidates): If opt1 is non-NULL,
	add a space and opt1 to the alternative suggestion text.

	* g++.dg/cpp1z/pr115440.C: New test.

gcc/opts-common.cc

@@ -524,6 +524,7 @@ add_misspelling_candidates (auto_vec<char *> *candidates,
   for (unsigned i = 0; i < ARRAY_SIZE (option_map); i++)
     {
       const char *opt0 = option_map[i].opt0;
+      const char *opt1 = option_map[i].opt1;
       const char *new_prefix = option_map[i].new_prefix;
       size_t new_prefix_len = strlen (new_prefix);
 
@@ -532,8 +533,9 @@ add_misspelling_candidates (auto_vec<char *> *candidates,
 
       if (strncmp (opt_text, new_prefix, new_prefix_len) == 0)
 	{
-	  char *alternative = concat (opt0 + 1, opt_text + new_prefix_len,
-				      NULL);
+	  char *alternative
+	    = concat (opt0 + 1, opt1 ? " " : "", opt1 ? opt1 : "",
+		      opt_text + new_prefix_len, NULL);
 	  candidates->safe_push (alternative);
 	}
     }

gcc/testsuite/g++.dg/cpp1z/pr115440.C

@@ -0,0 +1,8 @@
+// PR driver/115440
+// { dg-do compile { target c++17_only } }
+// { dg-options "--c++17" }
+
+int i;
+
+// { dg-bogus "unrecognized command-line option '--c\\\+\\\+17'; did you mean '--stdc\\\+\\\+17'" "" { target *-*-* } 0 }
+// { dg-error "unrecognized command-line option '--c\\\+\\\+17'" "" { target *-*-* } 0 }