diff --git a/cmd/tools/vast/vast.v b/cmd/tools/vast/vast.v
index a1535812f2..ad094dd080 100644
--- a/cmd/tools/vast/vast.v
+++ b/cmd/tools/vast/vast.v
@@ -493,6 +493,7 @@ fn (t Tree) fn_decl(node ast.FnDecl) &Node {
 	obj.add('no_body', t.bool_node(node.no_body))
 	obj.add('is_builtin', t.bool_node(node.is_builtin))
 	obj.add('is_direct_arr', t.bool_node(node.is_direct_arr))
+	obj.add('ctdefine_idx', t.number_node(node.ctdefine_idx))
 	obj.add('pos', t.position(node.pos))
 	obj.add('body_pos', t.position(node.body_pos))
 	obj.add('return_type_pos', t.position(node.return_type_pos))
@@ -501,7 +502,6 @@ fn (t Tree) fn_decl(node ast.FnDecl) &Node {
 	obj.add('return_type', t.type_node(node.return_type))
 	obj.add('source_file', t.number_node(int(node.source_file)))
 	obj.add('scope', t.number_node(int(node.scope)))
-	obj.add('skip_gen', t.bool_node(node.skip_gen))
 	obj.add('attrs', t.array_node_attr(node.attrs))
 	obj.add('params', t.array_node_arg(node.params))
 	obj.add('generic_names', t.array_node_string(node.generic_names))
@@ -630,6 +630,10 @@ fn (t Tree) attr(node ast.Attr) &Node {
 	obj.add('name', t.string_node(node.name))
 	obj.add('has_arg', t.bool_node(node.has_arg))
 	obj.add('kind', t.enum_node(node.kind))
+	obj.add('ct_expr', t.expr(node.ct_expr))
+	obj.add('ct_opt', t.bool_node(node.ct_opt))
+	obj.add('ct_evaled', t.bool_node(node.ct_evaled))
+	obj.add('ct_skip', t.bool_node(node.ct_skip))
 	obj.add('arg', t.string_node(node.arg))
 	return obj
 }
diff --git a/vlib/v/ast/ast.v b/vlib/v/ast/ast.v
index 88b3b821af..3d50dcdfd1 100644
--- a/vlib/v/ast/ast.v
+++ b/vlib/v/ast/ast.v
@@ -384,7 +384,7 @@ pub:
 	generic_names   []string
 	is_direct_arr   bool // direct array access
 	attrs           []Attr
-	skip_gen        bool // this function doesn't need to be generated (for example [if foo])
+	ctdefine_idx    int = -1 // the index in fn.attrs of `[if xyz]`, when such attribute exists
 pub mut:
 	params          []Param
 	stmts           []Stmt
@@ -392,12 +392,14 @@ pub mut:
 	return_type     Type
 	return_type_pos token.Position // `string` in `fn (u User) name() string` position
 	has_return      bool
-	comments        []Comment // comments *after* the header, but *before* `{`; used for InterfaceDecl
-	next_comments   []Comment // coments that are one line after the decl; used for InterfaceDecl
-	source_file     &File = 0
-	scope           &Scope
-	label_names     []string
-	pos             token.Position // function declaration position
+	//
+	comments      []Comment      // comments *after* the header, but *before* `{`; used for InterfaceDecl
+	next_comments []Comment // coments that are one line after the decl; used for InterfaceDecl
+	//
+	source_file &File = 0
+	scope       &Scope
+	label_names []string
+	pos         token.Position // function declaration position
 }
 
 // break, continue
diff --git a/vlib/v/ast/attr.v b/vlib/v/ast/attr.v
index 0884ce8de5..f91cdb84ac 100644
--- a/vlib/v/ast/attr.v
+++ b/vlib/v/ast/attr.v
@@ -19,7 +19,16 @@ pub:
 	has_arg bool
 	arg     string // [name: arg]
 	kind    AttrKind
+	ct_expr Expr // .kind == comptime_define, for [if !name]
+	ct_opt  bool // true for [if user_defined_name?]
 	pos     token.Position
+pub mut:
+	ct_evaled bool // whether ct_skip has been evaluated already
+	ct_skip   bool // is the comptime expr *false*, filled by checker
+}
+
+pub fn (a Attr) debug() string {
+	return 'Attr{ name: "$a.name", has_arg: $a.has_arg, arg: "$a.arg", kind: $a.kind, ct_expr: $a.ct_expr, ct_opt: $a.ct_opt, ct_skip: $a.ct_skip}'
 }
 
 // str returns the string representation without square brackets
@@ -43,10 +52,10 @@ pub fn (attrs []Attr) contains(str string) bool {
 	return attrs.any(it.name == str)
 }
 
-pub fn (attrs []Attr) find_comptime_define() ?string {
-	for a in attrs {
-		if a.kind == .comptime_define {
-			return a.name
+pub fn (attrs []Attr) find_comptime_define() ?int {
+	for idx in 0 .. attrs.len {
+		if attrs[idx].kind == .comptime_define {
+			return idx
 		}
 	}
 	return none
diff --git a/vlib/v/ast/str.v b/vlib/v/ast/str.v
index 7ecebf5654..bf439549d3 100644
--- a/vlib/v/ast/str.v
+++ b/vlib/v/ast/str.v
@@ -339,6 +339,13 @@ pub fn (x Expr) str() string {
 		ParExpr {
 			return '($x.expr)'
 		}
+		PostfixExpr {
+			// TODO: uncomment after [if x] is deprecated
+			// if x.op == .question {
+			//	return '$x.expr ?'
+			//}
+			return '$x.expr$x.op'
+		}
 		PrefixExpr {
 			return x.op.str() + x.right.str()
 		}
diff --git a/vlib/v/ast/table.v b/vlib/v/ast/table.v
index 6da76c1b94..0337c4603d 100644
--- a/vlib/v/ast/table.v
+++ b/vlib/v/ast/table.v
@@ -77,12 +77,9 @@ pub:
 	is_placeholder  bool
 	is_main         bool // `fn main(){}`
 	is_test         bool // `fn test_abc(){}`
-	is_conditional  bool // `[if abc]fn(){}`
 	is_keep_alive   bool // passed memory must not be freed (by GC) before function returns
 	no_body         bool // a pure declaration like `fn abc(x int)`; used in .vh files, C./JS. fns.
 	mod             string
-	ctdefine        string // compile time define. "myflag", when [if myflag] tag
-	attrs           []Attr
 	pos             token.Position
 	return_type_pos token.Position
 pub mut:
@@ -91,6 +88,10 @@ pub mut:
 	params      []Param
 	source_fn   voidptr // set in the checker, while processing fn declarations
 	usages      int
+	//
+	attrs          []Attr // all fn attributes
+	is_conditional bool   // true for `[if abc]fn(){}`
+	ctdefine_idx   int    // the index of the attribute, containing the compile time define [if mytag]
 }
 
 fn (f &Fn) method_equals(o &Fn) bool {
diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index cdc8df6954..d7a3f49c7c 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -28,11 +28,22 @@ const (
 	valid_comp_if_cpu_features  = ['x64', 'x32', 'little_endian', 'big_endian']
 	valid_comp_if_other         = ['js', 'debug', 'prod', 'test', 'glibc', 'prealloc',
 		'no_bounds_checking', 'freestanding', 'threads']
+	valid_comp_not_user_defined = all_valid_comptime_idents()
 	array_builtin_methods       = ['filter', 'clone', 'repeat', 'reverse', 'map', 'slice', 'sort',
 		'contains', 'index', 'wait', 'any', 'all', 'first', 'last', 'pop']
 	vroot_is_deprecated_message = '@VROOT is deprecated, use @VMODROOT or @VEXEROOT instead'
 )
 
+fn all_valid_comptime_idents() []string {
+	mut res := []string{}
+	res << checker.valid_comp_if_os
+	res << checker.valid_comp_if_compilers
+	res << checker.valid_comp_if_platforms
+	res << checker.valid_comp_if_cpu_features
+	res << checker.valid_comp_if_other
+	return res
+}
+
 [heap]
 pub struct Checker {
 	pref &pref.Preferences // Preferences shared from V struct
@@ -66,6 +77,7 @@ pub mut:
 	inside_ref_lit bool
 	inside_fn_arg  bool // `a`, `b` in `a.f(b)`
 	inside_c_call  bool // true inside C.printf( param ) calls, but NOT in nested calls, unless they are also C.
+	inside_ct_attr bool // true inside [if expr]
 	skip_flags     bool // should `#flag` and `#include` be skipped
 mut:
 	files                            []ast.File
@@ -1860,9 +1872,8 @@ pub fn (mut c Checker) method_call(mut call_expr ast.CallExpr) ast.Type {
 			&& method.no_body {
 			c.error('cannot call a method that does not have a body', call_expr.pos)
 		}
-		if method.return_type == ast.void_type && method.ctdefine.len > 0
-			&& method.ctdefine !in c.pref.compile_defines {
-			call_expr.should_be_skipped = true
+		if method.return_type == ast.void_type && method.is_conditional && method.ctdefine_idx != -1 {
+			call_expr.should_be_skipped = c.evaluate_once_comptime_if_attribute(mut method.attrs[method.ctdefine_idx])
 		}
 		nr_args := if method.params.len == 0 { 0 } else { method.params.len - 1 }
 		min_required_args := method.params.len - if method.is_variadic && method.params.len > 1 {
@@ -2408,9 +2419,8 @@ pub fn (mut c Checker) fn_call(mut call_expr ast.CallExpr) ast.Type {
 			call_expr.pos)
 		return func.return_type
 	}
-	if func.return_type == ast.void_type && func.ctdefine.len > 0
-		&& func.ctdefine !in c.pref.compile_defines {
-		call_expr.should_be_skipped = true
+	if func.return_type == ast.void_type && func.is_conditional && func.ctdefine_idx != -1 {
+		call_expr.should_be_skipped = c.evaluate_once_comptime_if_attribute(mut func.attrs[func.ctdefine_idx])
 	}
 	// dont check number of args for JS functions since arguments are not required
 	if call_expr.language != .js {
@@ -5370,7 +5380,12 @@ pub fn (mut c Checker) ident(mut ident ast.Ident) ast.Type {
 	} else if ident.name == 'errcode' {
 		c.error('undefined ident: `errcode`; did you mean `err.code`?', ident.pos)
 	} else {
-		c.error('undefined ident: `$ident.name`', ident.pos)
+		if c.inside_ct_attr {
+			c.note('`[if $ident.name]` is deprecated. Use `[if $ident.name?]` instead',
+				ident.pos)
+		} else {
+			c.error('undefined ident: `$ident.name`', ident.pos)
+		}
 	}
 	if c.table.known_type(ident.name) {
 		// e.g. `User`  in `json.decode(User, '...')`
@@ -6273,11 +6288,13 @@ fn (mut c Checker) comp_if_branch(cond ast.Expr, pos token.Position) bool {
 						cond.pos)
 					return false
 				}
-				// `$if some_var {}`
+				// `$if some_var {}`, or `[if user_defined_tag] fn abc(){}`
 				typ := c.expr(cond)
 				if cond.obj !is ast.Var && cond.obj !is ast.ConstField
 					&& cond.obj !is ast.GlobalField {
-					c.error('unknown var: `$cname`', pos)
+					if !c.inside_ct_attr {
+						c.error('unknown var: `$cname`', pos)
+					}
 					return false
 				}
 				expr := c.find_obj_definition(cond.obj) or {
@@ -7141,6 +7158,45 @@ fn (mut c Checker) post_process_generic_fns() {
 	}
 }
 
+fn (mut c Checker) evaluate_once_comptime_if_attribute(mut a ast.Attr) bool {
+	if a.ct_evaled {
+		return a.ct_skip
+	}
+	if a.ct_expr is ast.Ident {
+		if a.ct_opt {
+			if a.ct_expr.name in checker.valid_comp_not_user_defined {
+				c.error('optional `[if expression ?]` tags, can be used only for user defined identifiers',
+					a.pos)
+				a.ct_skip = true
+			} else {
+				a.ct_skip = a.ct_expr.name !in c.pref.compile_defines
+			}
+			a.ct_evaled = true
+			return a.ct_skip
+		} else {
+			if a.ct_expr.name !in checker.valid_comp_not_user_defined {
+				// TODO: uncomment after [if x] is deprecated in favour of [if x?], see also vlib/v/ast/str.v:343
+				// c.note('`[if $a.ct_expr.name]` is deprecated. Use `[if $a.ct_expr.name ?]` instead', a.pos)
+				a.ct_skip = a.ct_expr.name !in c.pref.compile_defines
+				a.ct_evaled = true
+				return a.ct_skip
+			} else {
+				if a.ct_expr.name in c.pref.compile_defines {
+					// explicitly allow custom user overrides with `-d linux` for example, for easier testing:
+					a.ct_skip = false
+					a.ct_evaled = true
+					return a.ct_skip
+				}
+			}
+		}
+	}
+	c.inside_ct_attr = true
+	a.ct_skip = c.comp_if_branch(a.ct_expr, a.pos)
+	c.inside_ct_attr = false
+	a.ct_evaled = true
+	return a.ct_skip
+}
+
 fn (mut c Checker) fn_decl(mut node ast.FnDecl) {
 	c.returns = false
 	if node.generic_names.len > 0 && c.table.cur_concrete_types.len == 0 {
@@ -7178,8 +7234,9 @@ fn (mut c Checker) fn_decl(mut node ast.FnDecl) {
 		c.main_fn_decl_node = node
 	}
 	if node.return_type != ast.void_type {
-		if ct_name := node.attrs.find_comptime_define() {
-			c.error('only functions that do NOT return values can have `[if $ct_name]` tags',
+		if ct_attr_idx := node.attrs.find_comptime_define() {
+			sexpr := node.attrs[ct_attr_idx].ct_expr.str()
+			c.error('only functions that do NOT return values can have `[if $sexpr]` tags',
 				node.pos)
 		}
 		if node.generic_names.len > 0 {
@@ -7192,6 +7249,13 @@ fn (mut c Checker) fn_decl(mut node ast.FnDecl) {
 			}
 		}
 	}
+	if node.return_type == ast.void_type {
+		for mut a in node.attrs {
+			if a.kind == .comptime_define {
+				c.evaluate_once_comptime_if_attribute(mut a)
+			}
+		}
+	}
 	if node.is_method {
 		mut sym := c.table.get_type_symbol(node.receiver.typ)
 		if sym.kind == .array && !c.is_builtin_mod && node.name == 'map' {
diff --git a/vlib/v/parser/fn.v b/vlib/v/parser/fn.v
index f541696ccc..89bc039a09 100644
--- a/vlib/v/parser/fn.v
+++ b/vlib/v/parser/fn.v
@@ -197,7 +197,7 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 			else {}
 		}
 	}
-	conditional_ctdefine := p.attrs.find_comptime_define() or { '' }
+	conditional_ctdefine_idx := p.attrs.find_comptime_define() or { -1 }
 	is_pub := p.tok.kind == .key_pub
 	if is_pub {
 		p.next()
@@ -376,12 +376,14 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 			is_unsafe: is_unsafe
 			is_main: is_main
 			is_test: is_test
-			is_conditional: conditional_ctdefine != ''
 			is_keep_alive: is_keep_alive
-			ctdefine: conditional_ctdefine
+			//
+			attrs: p.attrs
+			is_conditional: conditional_ctdefine_idx != -1
+			ctdefine_idx: conditional_ctdefine_idx
+			//
 			no_body: no_body
 			mod: p.mod
-			attrs: p.attrs
 		})
 	} else {
 		if language == .c {
@@ -405,12 +407,14 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 			is_unsafe: is_unsafe
 			is_main: is_main
 			is_test: is_test
-			is_conditional: conditional_ctdefine != ''
 			is_keep_alive: is_keep_alive
-			ctdefine: conditional_ctdefine
+			//
+			attrs: p.attrs
+			is_conditional: conditional_ctdefine_idx != -1
+			ctdefine_idx: conditional_ctdefine_idx
+			//
 			no_body: no_body
 			mod: p.mod
-			attrs: p.attrs
 			language: language
 		})
 	}
@@ -449,8 +453,12 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 		is_variadic: is_variadic
 		is_main: is_main
 		is_test: is_test
-		is_conditional: conditional_ctdefine != ''
 		is_keep_alive: is_keep_alive
+		//
+		attrs: p.attrs
+		is_conditional: conditional_ctdefine_idx != -1
+		ctdefine_idx: conditional_ctdefine_idx
+		//
 		receiver: ast.StructField{
 			name: rec.name
 			typ: rec.typ
@@ -467,7 +475,6 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 		body_pos: body_start_pos
 		file: p.file_name
 		is_builtin: p.builtin_mod || p.mod in util.builtin_module_parts
-		attrs: p.attrs
 		scope: p.scope
 		label_names: p.label_names
 	}
diff --git a/vlib/v/parser/parser.v b/vlib/v/parser/parser.v
index 60585c69f2..8dcc611b0c 100644
--- a/vlib/v/parser/parser.v
+++ b/vlib/v/parser/parser.v
@@ -1520,17 +1520,24 @@ fn (mut p Parser) parse_attr() ast.Attr {
 	mut name := ''
 	mut has_arg := false
 	mut arg := ''
+	mut comptime_cond := ast.empty_expr()
+	mut comptime_cond_opt := false
 	if p.tok.kind == .key_if {
 		kind = .comptime_define
 		p.next()
-		p.check(.name)
-		// TODO: remove this check after bootstrapping
-		// it is only for compatibility with the new
-		// [if user_defined?] syntax.
-		if p.tok.kind == .question {
-			p.next()
+		p.comp_if_cond = true
+		p.inside_if_expr = true
+		p.inside_ct_if_expr = true
+		comptime_cond = p.expr(0)
+		p.comp_if_cond = false
+		p.inside_if_expr = false
+		p.inside_ct_if_expr = false
+		if comptime_cond is ast.PostfixExpr {
+			x := comptime_cond as ast.PostfixExpr
+			comptime_cond_opt = true
+			comptime_cond = x.expr
 		}
-		name = p.prev_tok.lit
+		name = comptime_cond.str()
 	} else if p.tok.kind == .string {
 		name = p.tok.lit
 		kind = .string
@@ -1562,6 +1569,8 @@ fn (mut p Parser) parse_attr() ast.Attr {
 		has_arg: has_arg
 		arg: arg
 		kind: kind
+		ct_expr: comptime_cond
+		ct_opt: comptime_cond_opt
 		pos: apos.extend(p.tok.position())
 	}
 }
diff --git a/vlib/v/parser/pratt.v b/vlib/v/parser/pratt.v
index 9acf319bf0..09849f7e7a 100644
--- a/vlib/v/parser/pratt.v
+++ b/vlib/v/parser/pratt.v
@@ -447,8 +447,10 @@ pub fn (mut p Parser) expr_with_left(left ast.Expr, precedence int, is_stmt_iden
 			// Postfix
 			// detect `f(x++)`, `a[x++]`
 			if p.peek_tok.kind in [.rpar, .rsbr] {
-				p.warn_with_pos('`$p.tok.kind` operator can only be used as a statement',
-					p.peek_tok.position())
+				if !p.inside_ct_if_expr {
+					p.warn_with_pos('`$p.tok.kind` operator can only be used as a statement',
+						p.peek_tok.position())
+				}
 			}
 			if p.tok.kind in [.inc, .dec] && p.prev_tok.line_nr != p.tok.line_nr {
 				p.error_with_pos('$p.tok must be on the same line as the previous token',