From 2217d6b560d9030d66259f5b0feb292d5e515cff Mon Sep 17 00:00:00 2001
From: asynts <asynts@gmail.com>
Date: Wed, 7 Oct 2020 14:01:59 +0200
Subject: AK+Format: Add SFINAE wrapper 'FormatIfSupported'.

---
 AK/Format.h             | 44 +++++++++++++++++++++++++++++++++++++++++++-
 AK/Tests/TestFormat.cpp | 18 ++++++++++++++++++
 2 files changed, 61 insertions(+), 1 deletion(-)

diff --git a/AK/Format.h b/AK/Format.h
index cb8d31c39e..d66e9b607c 100644
--- a/AK/Format.h
+++ b/AK/Format.h
@@ -41,7 +41,9 @@ class FormatParser;
 class FormatBuilder;
 
 template<typename T, typename = void>
-struct Formatter;
+struct Formatter {
+    using __no_formatter_defined = void;
+};
 
 constexpr size_t max_format_arguments = 256;
 
@@ -346,6 +348,44 @@ template<typename... Parameters>
 void dbgln(const char* fmtstr, const Parameters&... parameters) { dbgln(StringView { fmtstr }, parameters...); }
 inline void dbgln() { raw_dbg("\n"); }
 
+template<typename T, typename = void>
+struct HasFormatter : TrueType {
+};
+template<typename T>
+struct HasFormatter<T, typename Formatter<T>::__no_formatter_defined> : FalseType {
+};
+
+template<typename T>
+class FormatIfSupported {
+public:
+    explicit FormatIfSupported(const T& value)
+        : m_value(value)
+    {
+    }
+
+    const T& value() const { return m_value; }
+
+private:
+    const T& m_value;
+};
+template<typename T, bool Supported = false>
+struct __FormatIfSupported : Formatter<StringView> {
+    void format(TypeErasedFormatParams& params, FormatBuilder& builder, const FormatIfSupported<T>&)
+    {
+        Formatter<StringView>::format(params, builder, "?");
+    }
+};
+template<typename T>
+struct __FormatIfSupported<T, true> : Formatter<T> {
+    void format(TypeErasedFormatParams& params, FormatBuilder& builder, const FormatIfSupported<T>& value)
+    {
+        Formatter<T>::format(params, builder, value.value());
+    }
+};
+template<typename T>
+struct Formatter<FormatIfSupported<T>> : __FormatIfSupported<T, HasFormatter<T>::value> {
+};
+
 } // namespace AK
 
 #ifndef KERNEL
@@ -361,3 +401,5 @@ using AK::warnln;
 using AK::dbgln;
 using AK::new_dbg;
 using AK::raw_dbg;
+
+using AK::FormatIfSupported;
diff --git a/AK/Tests/TestFormat.cpp b/AK/Tests/TestFormat.cpp
index 0e7e9aae6a..7d426fb4e1 100644
--- a/AK/Tests/TestFormat.cpp
+++ b/AK/Tests/TestFormat.cpp
@@ -196,4 +196,22 @@ TEST_CASE(format_character)
     EXPECT_EQ(String::formatted("{}", true ? a : 'b'), "a");
 }
 
+struct A {
+};
+struct B {
+};
+template<>
+struct AK::Formatter<B> : Formatter<StringView> {
+    void format(TypeErasedFormatParams& params, FormatBuilder& builder, B)
+    {
+        Formatter<StringView>::format(params, builder, "B");
+    }
+};
+
+TEST_CASE(format_if_supported)
+{
+    EXPECT_EQ(String::formatted("{}", FormatIfSupported { A {} }), "?");
+    EXPECT_EQ(String::formatted("{}", FormatIfSupported { B {} }), "B");
+}
+
 TEST_MAIN(Format)
-- 
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