diff --git a/src/eval.c b/src/eval.c
index a678d32..285558d 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -191,7 +191,7 @@
 	if (!skip)
 	{
 	    if (in_vim9script())
-		retval = tv2bool(&tv);
+		retval = tv_get_bool_chk(&tv, error);
 	    else
 		retval = (tv_get_number_chk(&tv, error) != 0);
 	    clear_tv(&tv);
@@ -2143,6 +2143,7 @@
 	evalarg_T	local_evalarg;
 	int		orig_flags;
 	int		evaluate;
+	int		vim9script = in_vim9script();
 
 	if (evalarg == NULL)
 	{
@@ -2156,7 +2157,7 @@
 	    *arg = eval_next_line(evalarg_used);
 	else
 	{
-	    if (evaluate && in_vim9script() && !VIM_ISWHITE(p[-1]))
+	    if (evaluate && vim9script && !VIM_ISWHITE(p[-1]))
 	    {
 		error_white_both(p, 1);
 		clear_tv(rettv);
@@ -2170,8 +2171,10 @@
 	{
 	    int		error = FALSE;
 
-	    if (in_vim9script() || op_falsy)
+	    if (op_falsy)
 		result = tv2bool(rettv);
+	    else if (vim9script)
+		result = tv_get_bool_chk(rettv, &error);
 	    else if (tv_get_number_chk(rettv, &error) != 0)
 		result = TRUE;
 	    if (error || !op_falsy || !result)
@@ -2185,7 +2188,7 @@
 	 */
 	if (op_falsy)
 	    ++*arg;
-	if (evaluate && in_vim9script() && !IS_WHITE_OR_NUL((*arg)[1]))
+	if (evaluate && vim9script && !IS_WHITE_OR_NUL((*arg)[1]))
 	{
 	    error_white_both(p, 1);
 	    clear_tv(rettv);
@@ -2220,7 +2223,7 @@
 		*arg = eval_next_line(evalarg_used);
 	    else
 	    {
-		if (evaluate && in_vim9script() && !VIM_ISWHITE(p[-1]))
+		if (evaluate && vim9script && !VIM_ISWHITE(p[-1]))
 		{
 		    error_white_both(p, 1);
 		    clear_tv(rettv);
@@ -2233,7 +2236,7 @@
 	    /*
 	     * Get the third variable.  Recursive!
 	     */
-	    if (evaluate && in_vim9script() && !IS_WHITE_OR_NUL((*arg)[1]))
+	    if (evaluate && vim9script && !IS_WHITE_OR_NUL((*arg)[1]))
 	    {
 		error_white_both(p, 1);
 		clear_tv(rettv);
diff --git a/src/testdir/test_expr.vim b/src/testdir/test_expr.vim
index 1086534..754f856 100644
--- a/src/testdir/test_expr.vim
+++ b/src/testdir/test_expr.vim
@@ -42,6 +42,16 @@
   call assert_false(has('patch-9.9.1'))
 endfunc
 
+func Test_op_trinary()
+  call assert_equal('yes', 1 ? 'yes' : 'no')
+  call assert_equal('no', 0 ? 'yes' : 'no')
+  call assert_equal('no', 'x' ? 'yes' : 'no')
+  call assert_equal('yes', '1x' ? 'yes' : 'no')
+
+  call assert_fails('echo [1] ? "yes" : "no"', 'E745:')
+  call assert_fails('echo {} ? "yes" : "no"', 'E728:')
+endfunc
+
 func Test_op_falsy()
   call assert_equal(v:true, v:true ?? 456)
   call assert_equal(123, 123 ?? 456)
diff --git a/src/testdir/test_vim9_cmd.vim b/src/testdir/test_vim9_cmd.vim
index 82d5f32..599c287 100644
--- a/src/testdir/test_vim9_cmd.vim
+++ b/src/testdir/test_vim9_cmd.vim
@@ -68,6 +68,74 @@
   CheckScriptSuccess(lines)
 enddef
 
+def Test_condition_types()
+  var lines =<< trim END
+      if 'text'
+      endif
+  END
+  CheckDefAndScriptFailure(lines, 'E1030:', 1)
+
+  lines =<< trim END
+      if [1]
+      endif
+  END
+  CheckDefFailure(lines, 'E1012:', 1)
+  CheckScriptFailure(['vim9script'] + lines, 'E745:', 2)
+
+  lines =<< trim END
+      g:cond = 'text'
+      if g:cond
+      endif
+  END
+  CheckDefExecAndScriptFailure(lines, 'E1030:', 2)
+
+  lines =<< trim END
+      g:cond = 0
+      if g:cond
+      elseif 'text'
+      endif
+  END
+  CheckDefFailure(lines, 'E1012:', 3)
+  CheckScriptFailure(['vim9script'] + lines, 'E1030:', 4)
+
+  lines =<< trim END
+      if g:cond
+      elseif [1]
+      endif
+  END
+  CheckDefFailure(lines, 'E1012:', 2)
+  CheckScriptFailure(['vim9script'] + lines, 'E745:', 3)
+
+  lines =<< trim END
+      g:cond = 'text'
+      if 0
+      elseif g:cond
+      endif
+  END
+  CheckDefExecAndScriptFailure(lines, 'E1030:', 3)
+
+  lines =<< trim END
+      while 'text'
+      endwhile
+  END
+  CheckDefFailure(lines, 'E1012:', 1)
+  CheckScriptFailure(['vim9script'] + lines, 'E1030:', 2)
+
+  lines =<< trim END
+      while [1]
+      endwhile
+  END
+  CheckDefFailure(lines, 'E1012:', 1)
+  CheckScriptFailure(['vim9script'] + lines, 'E745:', 2)
+
+  lines =<< trim END
+      g:cond = 'text'
+      while g:cond
+      endwhile
+  END
+  CheckDefExecAndScriptFailure(lines, 'E1030:', 2)
+enddef
+
 def Test_if_linebreak()
   var lines =<< trim END
       vim9script
diff --git a/src/testdir/test_vim9_expr.vim b/src/testdir/test_vim9_expr.vim
index 5158e10..2cbb285 100644
--- a/src/testdir/test_vim9_expr.vim
+++ b/src/testdir/test_vim9_expr.vim
@@ -18,27 +18,27 @@
 			'one' :
 			'two')
   if has('float')
-    assert_equal('one', 0.1 ? 'one' : 'two')
+    assert_equal('one', !!0.1 ? 'one' : 'two')
   endif
-  assert_equal('one', 'x' ? 'one' : 'two')
-  assert_equal('one', 'x'
+  assert_equal('one', !!'x' ? 'one' : 'two')
+  assert_equal('one', !!'x'
   			? 'one'
 			: 'two')
-  assert_equal('one', 0z1234 ? 'one' : 'two')
-  assert_equal('one', [0] ? 'one' : 'two')
-  assert_equal('one', #{x: 0} ? 'one' : 'two')
+  assert_equal('one', !!0z1234 ? 'one' : 'two')
+  assert_equal('one', !![0] ? 'one' : 'two')
+  assert_equal('one', !!#{x: 0} ? 'one' : 'two')
   var name = 1
   assert_equal('one', name ? 'one' : 'two')
 
   assert_equal('two', false ? 'one' : 'two')
   assert_equal('two', 0 ? 'one' : 'two')
   if has('float')
-    assert_equal('two', 0.0 ? 'one' : 'two')
+    assert_equal('two', !!0.0 ? 'one' : 'two')
   endif
-  assert_equal('two', '' ? 'one' : 'two')
-  assert_equal('two', 0z ? 'one' : 'two')
-  assert_equal('two', [] ? 'one' : 'two')
-  assert_equal('two', {} ? 'one' : 'two')
+  assert_equal('two', !!'' ? 'one' : 'two')
+  assert_equal('two', !!0z ? 'one' : 'two')
+  assert_equal('two', !![] ? 'one' : 'two')
+  assert_equal('two', !!{} ? 'one' : 'two')
   name = 0
   assert_equal('two', name ? 'one' : 'two')
 
@@ -117,6 +117,24 @@
   END
   CheckScriptFailure(lines, 'E1004:', 2)
 
+  lines =<< trim END
+      vim9script
+      var name = 'x' ? 1 : 2
+  END
+  CheckScriptFailure(lines, 'E1030:', 2)
+
+  lines =<< trim END
+      vim9script
+      var name = [] ? 1 : 2
+  END
+  CheckScriptFailure(lines, 'E745:', 2)
+
+  lines =<< trim END
+      vim9script
+      var name = {} ? 1 : 2
+  END
+  CheckScriptFailure(lines, 'E728:', 2)
+
   # check after failure eval_flags is reset
   lines =<< trim END
       vim9script
@@ -152,6 +170,15 @@
   call CheckDefFailure(["var x = 1 ? 'one' :'two'"], msg, 1)
   call CheckDefFailure(["var x = 1 ? 'one':'two'"], msg, 1)
 
+  call CheckDefFailure(["var x = 'x' ? 'one' : 'two'"], 'E1030:', 1)
+  call CheckDefFailure(["var x = 0z1234 ? 'one' : 'two'"], 'E974:', 1)
+  call CheckDefExecFailure(["var x = [] ? 'one' : 'two'"], 'E745:', 1)
+  call CheckDefExecFailure(["var x = {} ? 'one' : 'two'"], 'E728:', 1)
+
+  if has('float')
+    call CheckDefFailure(["var x = 0.1 ? 'one' : 'two'"], 'E805:', 1)
+  endif
+
   " missing argument detected even when common type is used
   call CheckDefFailure([
 	\ 'var X = FuncOne',
diff --git a/src/testdir/test_vim9_script.vim b/src/testdir/test_vim9_script.vim
index 6dbabe3..7fc5223 100644
--- a/src/testdir/test_vim9_script.vim
+++ b/src/testdir/test_vim9_script.vim
@@ -543,7 +543,7 @@
   CheckDefFailure(['endtry'], 'E602:')
   CheckDefFailure(['while 1', 'endtry'], 'E170:')
   CheckDefFailure(['for i in range(5)', 'endtry'], 'E170:')
-  CheckDefFailure(['if 2', 'endtry'], 'E171:')
+  CheckDefFailure(['if 1', 'endtry'], 'E171:')
   CheckDefFailure(['try', 'echo 1', 'endtry'], 'E1032:')
 
   CheckDefFailure(['throw'], 'E1015:')
diff --git a/src/testdir/vim9.vim b/src/testdir/vim9.vim
index 2e4b03f..aeeb5a3 100644
--- a/src/testdir/vim9.vim
+++ b/src/testdir/vim9.vim
@@ -52,3 +52,10 @@
   CheckDefFailure(lines, error, lnum)
   CheckScriptFailure(['vim9script'] + lines, error, lnum + 1)
 enddef
+
+" Check that a command fails both when executed in a :def function and when
+" used in Vim9 script.
+def CheckDefExecAndScriptFailure(lines: list<string>, error: string, lnum = -3)
+  CheckDefExecFailure(lines, error, lnum)
+  CheckScriptFailure(['vim9script'] + lines, error, lnum + 1)
+enddef
diff --git a/src/version.c b/src/version.c
index 914e4f0..58eb3d6 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1798,
+/**/
     1797,
 /**/
     1796,
diff --git a/src/vim9compile.c b/src/vim9compile.c
index f7dc9df..8d362ab 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -4212,7 +4212,17 @@
 	    // the condition is a constant, we know whether the ? or the :
 	    // expression is to be evaluated.
 	    has_const_expr = TRUE;
-	    const_value = tv2bool(&ppconst->pp_tv[ppconst_used]);
+	    if (op_falsy)
+		const_value = tv2bool(&ppconst->pp_tv[ppconst_used]);
+	    else
+	    {
+		int error = FALSE;
+
+		const_value = tv_get_bool_chk(&ppconst->pp_tv[ppconst_used],
+								       &error);
+		if (error)
+		    return FAIL;
+	    }
 	    cctx->ctx_skip = save_skip == SKIP_YES ||
 		 (op_falsy ? const_value : !const_value) ? SKIP_YES : SKIP_NOT;
 
@@ -5638,6 +5648,23 @@
 }
 
 /*
+ * Check that the top of the type stack has a type that can be used as a
+ * condition.  Give an error and return FAIL if not.
+ */
+    static int
+bool_on_stack(cctx_T *cctx)
+{
+    garray_T	*stack = &cctx->ctx_type_stack;
+    type_T	*type;
+
+    type = ((type_T **)stack->ga_data)[stack->ga_len - 1];
+    if (type != &t_bool && type != &t_number && type != &t_any
+	    && need_type(type, &t_bool, -1, cctx, FALSE) == FAIL)
+	return FAIL;
+    return OK;
+}
+
+/*
  * compile "if expr"
  *
  * "if expr" Produces instructions:
@@ -5689,8 +5716,14 @@
 	clear_ppconst(&ppconst);
     else if (instr->ga_len == instr_count && ppconst.pp_used == 1)
     {
+	int error = FALSE;
+	int v;
+
 	// The expression results in a constant.
-	cctx->ctx_skip = tv2bool(&ppconst.pp_tv[0]) ? SKIP_NOT : SKIP_YES;
+	v = tv_get_bool_chk(&ppconst.pp_tv[0], &error);
+	if (error)
+	    return NULL;
+	cctx->ctx_skip = v ? SKIP_NOT : SKIP_YES;
 	clear_ppconst(&ppconst);
     }
     else
@@ -5699,6 +5732,8 @@
 	cctx->ctx_skip = SKIP_UNKNOWN;
 	if (generate_ppconst(cctx, &ppconst) == FAIL)
 	    return NULL;
+	if (bool_on_stack(cctx) == FAIL)
+	    return NULL;
     }
 
     scope = new_scope(cctx, IF_SCOPE);
@@ -5764,9 +5799,15 @@
 	clear_ppconst(&ppconst);
     else if (instr->ga_len == instr_count && ppconst.pp_used == 1)
     {
+	int error = FALSE;
+	int v;
+
 	// The expression results in a constant.
 	// TODO: how about nesting?
-	cctx->ctx_skip = tv2bool(&ppconst.pp_tv[0]) ? SKIP_NOT : SKIP_YES;
+	v = tv_get_bool_chk(&ppconst.pp_tv[0], &error);
+	if (error)
+	    return NULL;
+	cctx->ctx_skip = v ? SKIP_NOT : SKIP_YES;
 	clear_ppconst(&ppconst);
 	scope->se_u.se_if.is_if_label = -1;
     }
@@ -5776,6 +5817,8 @@
 	cctx->ctx_skip = SKIP_UNKNOWN;
 	if (generate_ppconst(cctx, &ppconst) == FAIL)
 	    return NULL;
+	if (bool_on_stack(cctx) == FAIL)
+	    return NULL;
 
 	// "where" is set when ":elseif", "else" or ":endif" is found
 	scope->se_u.se_if.is_if_label = instr->ga_len;
@@ -6037,6 +6080,9 @@
     if (compile_expr0(&p, cctx) == FAIL)
 	return NULL;
 
+    if (bool_on_stack(cctx) == FAIL)
+	return FAIL;
+
     // "while_end" is set when ":endwhile" is found
     if (compile_jump_to_end(&scope->se_u.se_while.ws_end_label,
 						  JUMP_IF_FALSE, cctx) == FAIL)
diff --git a/src/vim9execute.c b/src/vim9execute.c
index b1065ca..7afc5c2 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -1908,6 +1908,7 @@
 		    {
 			tv = STACK_TV_BOT(-1);
 			if (when == JUMP_IF_COND_FALSE
+				|| when == JUMP_IF_FALSE
 				|| when == JUMP_IF_COND_TRUE)
 			{
 			    SOURCING_LNUM = iptr->isn_lnum;
@@ -3403,7 +3404,7 @@
 }
 
 /*
- * Return TRUE when "tv" is not falsey: non-zero, non-empty string, non-empty
+ * Return TRUE when "tv" is not falsy: non-zero, non-empty string, non-empty
  * list, etc.  Mostly like what JavaScript does, except that empty list and
  * empty dictionary are FALSE.
  */