patch 9.2.0376: Vim9: elseif condition compiled in dead branch

Problem:  When an `if` condition is constant true, the `else` block is
          skipped during compilation. However, any `elseif` condition
          within that skipped block was still compiled. This caused
          errors when the condition referenced variables only declared
          in the skipped block or when it checked for missing features
          (like `has('clipboard')`) (Coacher)
Solution: In compile_elseif(), when scope->se_skip_save is
          already SKIP_YES, skip compiling the elseif condition
          expression using skip_expr_cctx() (Furkan Sahin)

fixes:  #19160
closes: #20021

Signed-off-by: Furkan Sahin <furkan-dev@proton.me>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/testdir/test_vim9_script.vim

@@ -5933,4 +5933,65 @@ def Test_g_variable_shadow_multi_context()
   unlet g:F
 enddef
 
+" Test that dead elseif conditions are skipped without errors inside a :def function
+def Test_skip_elseif_dead_branch()
+  var lines =<< trim END
+    vim9script
+    def CompileTest()
+      if true
+        echo 'ok'
+      else
+        var x = 0
+        if x > 0
+          echo 'pos'
+        elseif x < 0
+          echo 'neg'
+        endif
+      endif
+    enddef
+    CompileTest()
+  END
+  v9.CheckScriptSuccess(lines)
+enddef
+
+" Another variation: using has() in a dead branch that would fail if compiled
+def Test_skip_elseif_with_has_in_dead_branch()
+  var lines =<< trim END
+    vim9script
+    def HasTest()
+      if true
+        echo 'ok'
+      else
+        var x = 0
+        if x > 0
+          echo 'pos'
+        elseif has('clipboard')
+          echo 'neg'
+        endif
+      endif
+    enddef
+    HasTest()
+  END
+  v9.CheckScriptSuccess(lines)
+enddef
+
+def Test_if_false_elseif_true_still_takes_elseif()
+  var lines =<< trim END
+    vim9script
+    var result = ''
+    def F()
+      if false
+        result = 'A'
+      elseif true
+        result = 'B'
+      else
+        result = 'C'
+      endif
+    enddef
+    F()
+    assert_equal('B', result)
+  END
+  v9.CheckScriptSuccess(lines)
+enddef
+
 " vim: ts=8 sw=2 sts=2 expandtab tw=80 fdm=marker

src/version.c

@@ -734,6 +734,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    376,
 /**/
     375,
 /**/

src/vim9cmds.c

@@ -623,6 +623,13 @@ compile_elseif(char_u *arg, cctx_T *cctx)
 	return p;
     }
 
+    if (scope->se_skip_save == SKIP_YES)
+    {
+	// Enclosing outer block is dead, skip this elseif
+	skip_expr_cctx(&p, cctx);
+	return p;
+    }
+
     if (cctx->ctx_skip == SKIP_UNKNOWN)
     {
 	int	    moved_cmdmod = FALSE;