diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 069223a..b2c3f78 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -23,9 +23,7 @@ jobs:
     strategy:
       matrix:
         rust:
-          - 1.31
-          - 1.33
-          - 1.39
+          - 1.45
           - 1.46
           - stable
           - beta
diff --git a/CHANGELOG.md b/CHANGELOG.md
index c00401d..970a06d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,16 @@ This project adheres to [Semantic Versioning](https://semver.org).
 
 ## [Unreleased]
 
+* Accept `const_fn` attribute with no arguments and functions without `const` keyword.
+  This allows `const_fn` to be used as an optional dependency.
+
+  ```rust
+  #[cfg_attr(feature = "...", const_fn::const_fn)]
+  pub fn func() {
+      /* ... */
+  }
+  ```
+
 ## [0.4.3] - 2020-11-02
 
 * [`const_fn` no longer fails to compile if unable to determine rustc version. Instead, it now displays a warning.](https://github.com/taiki-e/const_fn/pull/31)
diff --git a/src/ast.rs b/src/ast.rs
index b156493..3622a2f 100644
--- a/src/ast.rs
+++ b/src/ast.rs
@@ -1,4 +1,4 @@
-use proc_macro::{Delimiter, Literal, Span, TokenStream, TokenTree};
+use proc_macro::{Delimiter, Ident, Literal, Span, TokenStream, TokenTree};
 
 use crate::{
     iter::TokenIter,
@@ -32,17 +32,6 @@ pub(crate) fn parse_input(input: TokenStream) -> Result<Func> {
             "#[const_fn] attribute may only be used on functions"
         ));
     }
-    if !sig
-        .iter()
-        .any(|tt| if let TokenTree::Ident(i) = tt { i.to_string() == "const" } else { false })
-    {
-        let span = sig
-            .iter()
-            .position(|tt| if let TokenTree::Ident(i) = tt { i.to_string() == "fn" } else { false })
-            .map(|i| sig[i].span())
-            .unwrap();
-        return Err(error!(span, "#[const_fn] attribute may only be used on const functions"));
-    }
 
     Ok(Func { attrs, sig, body: body.unwrap(), print_const: true })
 }
@@ -66,10 +55,22 @@ impl ToTokens for Func {
 
 fn parse_signature(input: &mut TokenIter) -> Vec<TokenTree> {
     let mut sig = Vec::new();
+    let mut has_const = false;
     loop {
         match input.peek() {
             Some(TokenTree::Group(group)) if group.delimiter() == Delimiter::Brace => break,
             None => break,
+            Some(TokenTree::Ident(i)) if !has_const => {
+                match &*i.to_string() {
+                    "const" => has_const = true,
+                    "async" | "unsafe" | "extern" | "fn" => {
+                        sig.push(TokenTree::Ident(Ident::new("const", i.span())));
+                        has_const = true;
+                    }
+                    _ => {}
+                }
+                sig.push(input.next().unwrap());
+            }
             Some(_) => sig.push(input.next().unwrap()),
         }
     }
diff --git a/src/lib.rs b/src/lib.rs
index f216da6..0eabb4e 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -119,6 +119,7 @@ fn expand(arg: Arg, mut func: Func) -> TokenStream {
             func.print_const = VERSION.nightly;
             func.to_token_stream()
         }
+        Arg::Always => func.to_token_stream(),
     }
 }
 
@@ -131,6 +132,8 @@ enum Arg {
     Cfg(TokenStream),
     // `const_fn(feature = "...")`
     Feature(TokenStream),
+    // `const_fn`
+    Always,
 }
 
 fn parse_arg(tokens: TokenStream) -> Result<Arg> {
@@ -139,6 +142,7 @@ fn parse_arg(tokens: TokenStream) -> Result<Arg> {
     let next = iter.next();
     let next_span = tt_span(next.as_ref());
     match next {
+        None => return Ok(Arg::Always),
         Some(TokenTree::Ident(i)) => match &*i.to_string() {
             "nightly" => {
                 parse_as_empty(iter)?;
@@ -182,7 +186,7 @@ fn parse_arg(tokens: TokenStream) -> Result<Arg> {
                 };
             }
         }
-        _ => {}
+        Some(_) => {}
     }
 
     Err(error!(next_span, "expected one of: `nightly`, `cfg`, `feature`, string literal"))
diff --git a/test_suite/tests/test.rs b/test_suite/tests/test.rs
index dc17fcb..bf36031 100644
--- a/test_suite/tests/test.rs
+++ b/test_suite/tests/test.rs
@@ -1,5 +1,76 @@
-#![cfg_attr(const_unstable, feature(const_fn))]
+#![cfg_attr(const_unstable, feature(const_fn, const_extern_fn))]
 #![warn(rust_2018_idioms, single_use_lifetimes)]
+#![allow(clippy::missing_safety_doc)] // this is test
+
+pub mod syntax {
+    #![allow(dead_code)]
+
+    use const_fn::const_fn;
+
+    // const
+    #[const_fn]
+    fn const_non_const() {}
+    #[const_fn]
+    pub fn const_non_const_pub() {}
+    #[const_fn]
+    const fn const_const() {}
+    #[const_fn]
+    pub const fn const_const_pub() {}
+    const _: () = const_non_const();
+    const _: () = const_non_const_pub();
+    const _: () = const_const();
+    const _: () = const_const_pub();
+
+    // const unsafe
+    #[const_fn]
+    unsafe fn const_unsafe_non_const() {}
+    #[const_fn]
+    pub unsafe fn const_unsafe_non_const_pub() {}
+    #[const_fn]
+    const unsafe fn const_unsafe_const() {}
+    #[const_fn]
+    pub const unsafe fn const_unsafe_const_pub() {}
+    const _: () = unsafe { const_unsafe_non_const() };
+    const _: () = unsafe { const_unsafe_non_const_pub() };
+    const _: () = unsafe { const_unsafe_const() };
+    const _: () = unsafe { const_unsafe_const_pub() };
+
+    // const extern
+    #[const_fn(cfg(const_unstable))]
+    extern "C" fn const_extern_non_const() {}
+    #[const_fn(cfg(const_unstable))]
+    pub extern "C" fn const_extern_non_const_pub() {}
+    #[const_fn(cfg(const_unstable))]
+    const extern "C" fn const_extern_const() {}
+    #[const_fn(cfg(const_unstable))]
+    pub const extern "C" fn const_extern_const_pub() {}
+    #[cfg(const_unstable)]
+    const _: () = const_extern_non_const();
+    #[cfg(const_unstable)]
+    const _: () = const_extern_non_const_pub();
+    #[cfg(const_unstable)]
+    const _: () = const_extern_const();
+    #[cfg(const_unstable)]
+    const _: () = const_extern_const_pub();
+
+    // const unsafe extern
+    #[const_fn(cfg(const_unstable))]
+    unsafe extern "C" fn const_unsafe_extern_non_const() {}
+    #[const_fn(cfg(const_unstable))]
+    pub unsafe extern "C" fn const_unsafe_extern_non_const_pub() {}
+    #[const_fn(cfg(const_unstable))]
+    const unsafe extern "C" fn const_unsafe_extern_const() {}
+    #[const_fn(cfg(const_unstable))]
+    pub const unsafe extern "C" fn const_unsafe_extern_const_pub() {}
+    #[cfg(const_unstable)]
+    const _: () = unsafe { const_unsafe_extern_non_const() };
+    #[cfg(const_unstable)]
+    const _: () = unsafe { const_unsafe_extern_non_const_pub() };
+    #[cfg(const_unstable)]
+    const _: () = unsafe { const_unsafe_extern_const() };
+    #[cfg(const_unstable)]
+    const _: () = unsafe { const_unsafe_extern_const_pub() };
+}
 
 pub mod version {
     use const_fn::const_fn;