Add fallback to FunctionValue that can be used by function implementations

Author: catamorphism

icu4c/source/i18n/messageformat2.cpp

@@ -69,7 +69,7 @@ static UnicodeString functionFallback(const InternalValue& operand,
                 fallbackToUse += var;
             }
             // If it exists, create a BaseValue (FunctionValue) for it
-            LocalPointer<BaseValue> result(BaseValue::create(locale, *val, errorCode));
+            LocalPointer<BaseValue> result(BaseValue::create(locale, fallbackToUse, *val, errorCode));
             // Add fallback and return an InternalValue
             if (U_SUCCESS(errorCode)) {
                 return InternalValue(result.orphan(), fallbackToUse);
@@ -110,6 +110,7 @@ static UnicodeString reserialize(const UnicodeString& s) {
 
     // Create a BaseValue (FunctionValue) that wraps the literal
     LocalPointer<BaseValue> val(BaseValue::create(locale,
+                                                  fallbackToUse,
                                                   Formattable(lit.unquoted()),
                                                   errorCode));
     if (U_SUCCESS(errorCode)) {

icu4c/source/i18n/messageformat2_evaluation.cpp

@@ -26,15 +26,19 @@ using namespace data_model;
 // BaseValue
 // ---------
 
-BaseValue::BaseValue(const Locale& loc, const Formattable& source)
+BaseValue::BaseValue(const Locale& loc, const UnicodeString& fb, const Formattable& source)
     : locale(loc) {
     operand = source;
+    fallback += LEFT_CURLY_BRACE;
+    fallback += fb;
+    fallback += RIGHT_CURLY_BRACE;
 }
 
 /* static */ BaseValue* BaseValue::create(const Locale& locale,
+                                          const UnicodeString& fallback,
                                           const Formattable& source,
                                           UErrorCode& errorCode) {
-    return message2::create<BaseValue>(BaseValue(locale, source), errorCode);
+    return message2::create<BaseValue>(BaseValue(locale, fallback, source), errorCode);
 }
 
 extern UnicodeString formattableToString(const Locale&, const UBiDiDirection, const Formattable&, UErrorCode&);
@@ -50,6 +54,7 @@ BaseValue& BaseValue::operator=(BaseValue&& other) noexcept {
     operand = std::move(other.operand);
     opts = std::move(other.opts);
     locale = other.locale;
+    fallback = other.fallback;
 
     return *this;
 }

icu4c/source/i18n/messageformat2_evaluation.h

@@ -120,7 +120,7 @@ namespace message2 {
     // in a context that expects a FunctionValue.
     class BaseValue : public FunctionValue {
         public:
-            static BaseValue* create(const Locale&, const Formattable&, UErrorCode&);
+            static BaseValue* create(const Locale&, const UnicodeString&, const Formattable&, UErrorCode&);
             // Apply default formatters to the argument value
             UnicodeString formatToString(UErrorCode&) const override;
             UBool isSelectable() const override { return true; }
@@ -130,7 +130,7 @@ namespace message2 {
        private:
             Locale locale;
 
-            BaseValue(const Locale&, const Formattable&);
+            BaseValue(const Locale&, const UnicodeString&, const Formattable&);
     }; // class BaseValue
 
     // A NullValue represents the absence of an argument.

icu4c/source/i18n/messageformat2_function_registry.cpp

@@ -1408,7 +1408,7 @@ StandardFunctions::TestFunctionValue::TestFunctionValue(const TestFunction& pare
     // If FailsFormat is true, attempting to format the placeholder to any
     // formatting target will fail.
     if (failsFormat) {
-        status = U_MF_FORMATTING_ERROR;
+        formattedString = arg.getFallback();
         return;
     }
 
@@ -1451,6 +1451,8 @@ UnicodeString StandardFunctions::TestFunctionValue::formatToString(UErrorCode& s
     }
     if (!canFormat || failsFormat) {
         status = U_MF_FORMATTING_ERROR;
+    }
+    if (!canFormat) {
         return {};
     }
     return formattedString;

icu4c/source/i18n/unicode/messageformat2_function_registry.h

@@ -438,6 +438,15 @@ namespace message2 {
              * @deprecated This API is for technology preview only.
              */
             virtual const UnicodeString& getFunctionName() const { return functionName; }
+            /* Returns a fallback string that can be used as output
+             * if processing this function results in an error.
+             *             *
+             * @returns A string determined by the creator of this FunctionValue.
+             *
+             * @internal ICU 78 technology preview
+             * @deprecated This API is for technology preview only.
+             */
+            virtual const UnicodeString& getFallback() const { return fallback; }
             /**
              * Destructor.
              *
@@ -462,12 +471,19 @@ namespace message2 {
             FunctionOptions opts;
             /**
              * The name of the function that constructed this FunctionValue.
+             *
              * @internal ICU 78 technology preview
              * @deprecated This API is for technology preview only.
              */
             UnicodeString functionName;
-         private:
-
+            /**
+             * Fallback string that can be used if a later function encounters
+             * an error when processing this FunctionValue.
+             *
+             * @internal ICU 78 technology preview
+             * @deprecated This API is for technology preview only.
+             */
+            UnicodeString fallback;
     }; // class FunctionValue
 
 } // namespace message2