patch 8.2.3366: Vim9: debugging elseif does not stop before condition

Problem:    Vim9: debugging elseif does not stop before condition.
Solution:   Move debug statement to after the jump. (closes #8781)

src/testdir/test_vim9_disassemble.vim

@@ -2255,6 +2255,53 @@ def Test_debugged()
         res)
 enddef
 
+def s:DebugElseif()
+  var b = false
+  if b
+    eval 1 + 0
+  silent elseif !b
+    eval 2 + 0
+  endif
+enddef
+
+def Test_debug_elseif()
+  var res = execute('disass debug s:DebugElseif')
+  assert_match('<SNR>\d*_DebugElseif\_s*' ..
+          'var b = false\_s*' ..
+          '0 DEBUG line 1-1 varcount 0\_s*' ..
+          '1 PUSH false\_s*' ..
+          '2 STORE $0\_s*' ..
+
+          'if b\_s*' ..
+          '3 DEBUG line 2-2 varcount 1\_s*' ..
+          '4 LOAD $0\_s*' ..
+          '5 JUMP_IF_FALSE -> 10\_s*' ..
+
+          'eval 1 + 0\_s*' ..
+          '6 DEBUG line 3-3 varcount 1\_s*' ..
+          '7 PUSHNR 1\_s*' ..
+          '8 DROP\_s*' ..
+
+          'silent elseif !b\_s*' ..
+          '9 JUMP -> 20\_s*' ..
+          '10 CMDMOD silent\_s*' ..
+          '11 DEBUG line 4-4 varcount 1\_s*' ..
+          '12 LOAD $0\_s*' ..
+          '13 INVERT -1 (!val)\_s*' ..
+          '14 CMDMOD_REV\_s*' ..
+          '15 JUMP_IF_FALSE -> 20\_s*' ..
+
+          'eval 2 + 0\_s*' ..
+          '16 DEBUG line 5-5 varcount 1\_s*' ..
+          '17 PUSHNR 2\_s*' ..
+          '18 DROP\_s*' ..
+
+          'endif\_s*' ..
+          '19 DEBUG line 6-6 varcount 1\_s*' ..
+          '20 RETURN void*',
+        res)
+enddef
+
 def s:EchoMessages()
   echohl ErrorMsg | echom v:exception | echohl NONE
 enddef

src/version.c

@@ -755,6 +755,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3366,
 /**/
     3365,
 /**/

src/vim9compile.c

@@ -2387,7 +2387,7 @@ misplaced_cmdmod(cctx_T *cctx)
 
 /*
  * Get the index of the current instruction.
- * This compenstates for a preceding ISN_CMDMOD and ISN_PROF_START.
+ * This compensates for a preceding ISN_CMDMOD and ISN_PROF_START.
  */
     static int
 current_instr_idx(cctx_T *cctx)
@@ -7733,6 +7733,8 @@ compile_elseif(char_u *arg, cctx_T *cctx)
     if (cctx->ctx_skip == SKIP_UNKNOWN)
     {
 	int	    moved_cmdmod = FALSE;
+	int	    saved_debug = FALSE;
+	isn_T	    debug_isn;
 
 	// Move any CMDMOD instruction to after the jump
 	if (((isn_T *)instr->ga_data)[instr->ga_len - 1].isn_type == ISN_CMDMOD)
@@ -7745,14 +7747,35 @@ compile_elseif(char_u *arg, cctx_T *cctx)
 	    moved_cmdmod = TRUE;
 	}
 
+	// Remove the already generated ISN_DEBUG, it is written below the
+	// ISN_FOR instruction.
+	if (cctx->ctx_compile_type == CT_DEBUG && instr->ga_len > 0
+		&& ((isn_T *)instr->ga_data)[instr->ga_len - 1]
+							.isn_type == ISN_DEBUG)
+	{
+	    --instr->ga_len;
+	    debug_isn = ((isn_T *)instr->ga_data)[instr->ga_len];
+	    saved_debug = TRUE;
+	}
+
 	if (compile_jump_to_end(&scope->se_u.se_if.is_end_label,
 						    JUMP_ALWAYS, cctx) == FAIL)
 	    return NULL;
 	// previous "if" or "elseif" jumps here
 	isn = ((isn_T *)instr->ga_data) + scope->se_u.se_if.is_if_label;
 	isn->isn_arg.jump.jump_where = instr->ga_len;
+
 	if (moved_cmdmod)
 	    ++instr->ga_len;
+
+	if (saved_debug)
+	{
+	    // move the debug instruction here
+	    if (GA_GROW_FAILS(instr, 1))
+		return NULL;
+	    ((isn_T *)instr->ga_data)[instr->ga_len] = debug_isn;
+	    ++instr->ga_len;
+	}
     }
 
     // compile "expr"; if we know it evaluates to FALSE skip the block