checker: fix const init with comptime (#17528)

vlib/v/checker/comptime.v

@@ -388,6 +388,27 @@ fn (mut c Checker) eval_comptime_const_expr(expr ast.Expr, nlevel int) ?ast.Comp
 				}
 			}
 		}
+		ast.IfExpr {
+			if !expr.is_comptime {
+				return none
+			}
+			for i in 0 .. expr.branches.len {
+				branch := expr.branches[i]
+				if !expr.has_else || i < expr.branches.len - 1 {
+					if c.comptime_if_branch(branch.cond, branch.pos) == .eval {
+						last_stmt := branch.stmts.last()
+						if last_stmt is ast.ExprStmt {
+							return c.eval_comptime_const_expr(last_stmt.expr, nlevel + 1)
+						}
+					}
+				} else {
+					last_stmt := branch.stmts.last()
+					if last_stmt is ast.ExprStmt {
+						return c.eval_comptime_const_expr(last_stmt.expr, nlevel + 1)
+					}
+				}
+			}
+		}
 		// ast.ArrayInit {}
 		// ast.PrefixExpr {
 		//	c.note('prefixexpr: $expr', expr.pos)

vlib/v/tests/comptime_const_def_test.v

@@ -0,0 +1,12 @@
+const (
+	buf_size = $if true {
+		2
+	} $else {
+		1
+	}
+)
+
+fn test_main() {
+	mut buf := [buf_size]u8{}
+	assert buf.len == 2
+}