From 0cb5bc6707dbf605c2fa2a369233048cefe0ce8b Mon Sep 17 00:00:00 2001
From: Fredrik Roubert <roubert@google.com>
Date: Wed, 11 Sep 2024 18:52:52 +0200
Subject: [PATCH] ICU-22843 Disambiguate UnicodeString::readOnlyAlias() for
 MSVC.

Both std::u16string_view and std::wstring_view are possible matches for
UnicodeString as a template parameter, but adding an explicit overload
avoids both having to make that choice and taking the detour through
creating any string view at all.
---
 icu4c/source/common/unicode/unistr.h | 24 ++++++++++++++++++++++++
 icu4c/source/common/unistr.cpp       | 10 ++++++++++
 2 files changed, 34 insertions(+)

diff --git a/icu4c/source/common/unicode/unistr.h b/icu4c/source/common/unicode/unistr.h
index 41743f4f541c..477dea301ab0 100644
--- a/icu4c/source/common/unicode/unistr.h
+++ b/icu4c/source/common/unicode/unistr.h
@@ -3600,6 +3600,29 @@ class U_COMMON_API UnicodeString : public Replaceable
   static inline UnicodeString readOnlyAlias(const S &text) {
     return readOnlyAliasFromU16StringView(internal::toU16StringView(text));
   }
+
+  /**
+   * Readonly-aliasing factory method.
+   * Aliases the same buffer as the input `text`.
+   *
+   * The text will be used for the UnicodeString object, but
+   * it will not be released when the UnicodeString is destroyed.
+   * This has copy-on-write semantics:
+   * When the string is modified, then the buffer is first copied into
+   * newly allocated memory.
+   * The aliased buffer is never modified.
+   *
+   * In an assignment to another UnicodeString, when using the copy constructor
+   * or the assignment operator, the text will be copied.
+   * When using fastCopyFrom(), the text will be aliased again,
+   * so that both strings then alias the same readonly-text.
+   *
+   * @param text The UnicodeString to alias.
+   * @draft ICU 76
+   */
+  static inline UnicodeString readOnlyAlias(const UnicodeString &text) {
+    return readOnlyAliasFromUnicodeString(text);
+  }
 #endif  // U_HIDE_DRAFT_API
 
   /**
@@ -3730,6 +3753,7 @@ class U_COMMON_API UnicodeString : public Replaceable
 
 private:
   static UnicodeString readOnlyAliasFromU16StringView(std::u16string_view text);
+  static UnicodeString readOnlyAliasFromUnicodeString(const UnicodeString &text);
 
   // For char* constructors. Could be made public.
   UnicodeString &setToUTF8(StringPiece utf8);
diff --git a/icu4c/source/common/unistr.cpp b/icu4c/source/common/unistr.cpp
index ca577ecb1560..a9d5f81cb2f0 100644
--- a/icu4c/source/common/unistr.cpp
+++ b/icu4c/source/common/unistr.cpp
@@ -308,6 +308,16 @@ UnicodeString UnicodeString::readOnlyAliasFromU16StringView(std::u16string_view
   return result;
 }
 
+UnicodeString UnicodeString::readOnlyAliasFromUnicodeString(const UnicodeString &text) {
+  UnicodeString result;
+  if (text.isBogus()) {
+    result.setToBogus();
+  } else {
+    result.setTo(false, text.getBuffer(), text.length());
+  }
+  return result;
+}
+
 #if U_CHARSET_IS_UTF8
 
 UnicodeString::UnicodeString(const char *codepageData) {