From b252869bb71e7746ef34fd868565f72ae346e796 Mon Sep 17 00:00:00 2001
From: Julian Eder <julian.eder@infineon.com>
Date: Tue, 9 Jun 2026 09:14:31 +0200
Subject: [PATCH] cores/WString: Add missing __FlashStringHelper
 implementations.

The F() macro produces a const __FlashStringHelper* pointer.
WString.h declared the corresponding constructor, assignment
operator, copy helper, concat overload and StringSumHelper
operator+, but WString.cpp was missing all five implementations,
causing undefined-reference linker errors whenever a library
(e.g. SimpleSerialShell) used F() with String operations.

On XMC (non-AVR) pgmspace.h maps strcpy_P / strlen_P to plain
strcpy / strlen, so the implementations are identical to the
char* counterparts.

Fixes #439

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Signed-off-by: Julian Eder <julian.eder@infineon.com>
---
 cores/WString.cpp | 50 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)

diff --git a/cores/WString.cpp b/cores/WString.cpp
index 7fb9b6bf..7c0c3fe4 100644
--- a/cores/WString.cpp
+++ b/cores/WString.cpp
@@ -45,6 +45,11 @@ String::String(const String &value) {
     *this = value;
 }
 
+String::String(const __FlashStringHelper *pstr) {
+    init();
+    *this = pstr;
+}
+
 String::String(char c) {
     init();
     char buf[2];
@@ -159,6 +164,16 @@ String &String::copy(const char *cstr, unsigned int length) {
     return *this;
 }
 
+String &String::copy(const __FlashStringHelper *pstr, unsigned int length) {
+    if (!reserve(length)) {
+        invalidate();
+        return *this;
+    }
+    len = length;
+    strcpy_P(buffer, (PGM_P)pstr);
+    return *this;
+}
+
 String &String::operator=(const String &rhs) {
     if (this == &rhs) {
         return *this;
@@ -183,6 +198,16 @@ String &String::operator=(const char *cstr) {
     return *this;
 }
 
+String &String::operator=(const __FlashStringHelper *pstr) {
+    if (pstr) {
+        copy(pstr, strlen_P((PGM_P)pstr));
+    } else {
+        invalidate();
+    }
+
+    return *this;
+}
+
 /*********************************************/
 /*  concat                                   */
 /*********************************************/
@@ -261,6 +286,23 @@ unsigned char String::concat(double num) {
     return concat(string, strlen(string));
 }
 
+unsigned char String::concat(const __FlashStringHelper *pstr) {
+    if (!pstr) {
+        return 0;
+    }
+    unsigned int length = strlen_P((PGM_P)pstr);
+    if (length == 0) {
+        return 1;
+    }
+    unsigned int newlen = len + length;
+    if (!reserve(newlen)) {
+        return 0;
+    }
+    strcpy_P(buffer + len, (PGM_P)pstr);
+    len = newlen;
+    return 1;
+}
+
 /*********************************************/
 /*  Concatenate                              */
 /*********************************************/
@@ -345,6 +387,14 @@ StringSumHelper &operator+(const StringSumHelper &lhs, double num) {
     return a;
 }
 
+StringSumHelper &operator+(const StringSumHelper &lhs, const __FlashStringHelper *rhs) {
+    StringSumHelper &a = const_cast<StringSumHelper &>(lhs);
+    if (!a.concat(rhs)) {
+        a.invalidate();
+    }
+    return a;
+}
+
 /*********************************************/
 /*  Comparison                               */
 /*********************************************/