parser, cgen: fix anon fn optional call in if expr (#16803)

vlib/v/gen/c/fn.v

@@ -652,7 +652,7 @@ fn (mut g Gen) call_expr(node ast.CallExpr) {
 			g.expr(node.left)
 			g.writeln(';')
 			g.write(tmp_var)
-		} else {
+		} else if node.or_block.kind == .absent {
 			g.expr(node.left)
 		}
 	} else if node.left is ast.IndexExpr && node.name == '' {
@@ -695,6 +695,9 @@ fn (mut g Gen) call_expr(node ast.CallExpr) {
 			g.write('${tmp_opt} = ')
 		} else {
 			g.write('${styp} ${tmp_opt} = ')
+			if node.left is ast.AnonFn {
+				g.expr(node.left)
+			}
 		}
 	}
 	if node.is_method && !node.is_field {

vlib/v/parser/expr.v

@@ -387,11 +387,35 @@ pub fn (mut p Parser) check_expr(precedence int) !ast.Expr {
 					pos := p.tok.pos()
 					args := p.call_args()
 					p.check(.rpar)
+					mut or_kind := ast.OrKind.absent
+					mut or_stmts := []ast.Stmt{} // TODO remove unnecessary allocations by just using .absent
+					mut or_pos := p.tok.pos()
+					if p.tok.kind == .key_orelse {
+						// `foo() or {}``
+						or_kind = .block
+						or_stmts, or_pos = p.or_block(.with_err_var)
+					}
+					if p.tok.kind in [.question, .not] {
+						is_not := p.tok.kind == .not
+						// `foo()?`
+						p.next()
+						if p.inside_defer {
+							p.error_with_pos('error propagation not allowed inside `defer` blocks',
+								p.prev_tok.pos())
+						}
+						or_kind = if is_not { .propagate_result } else { .propagate_option }
+					}
+
 					node = ast.CallExpr{
 						name: 'anon'
 						left: node
 						args: args
 						pos: pos
+						or_block: ast.OrExpr{
+							stmts: or_stmts
+							kind: or_kind
+							pos: or_pos
+						}
 						scope: p.scope
 					}
 				}

vlib/v/tests/anon_fn_optional_call_in_if_expr_test.v

@@ -0,0 +1,35 @@
+fn test_anon_fn_optional_call_in_if_expr_1() {
+	if fn () ?bool {
+		return true
+	}() or { false }
+	{
+		println('ok')
+		assert true
+	} else {
+		assert false
+	}
+}
+
+fn test_anon_fn_optional_call_in_if_expr_2() {
+	if fn () ?bool {
+		return true
+	}()?
+	{
+		println('ok')
+		assert true
+	} else {
+		assert false
+	}
+}
+
+fn test_anon_fn_optional_call_in_if_expr_3() {
+	if fn () !bool {
+		return true
+	}()!
+	{
+		println('ok')
+		assert true
+	} else {
+		assert false
+	}
+}