fixup extern operator+ dependency on insert

Author: mcspr

cores/esp8266/WString.h

@@ -509,6 +509,8 @@ inline String operator +(char lhs, String &&rhs) {
     return std::move(rhs.insert(0, lhs));
 }
 
+// Similarly, allow c-strings and `F(...)` as both lhs and rhs
+
 String operator +(const char *lhs, const String &rhs);
 
 inline String operator +(const char *lhs, String &&rhs) {
@@ -520,7 +522,7 @@ inline String operator +(const __FlashStringHelper *lhs, const String &rhs) {
 }
 
 inline String operator +(const __FlashStringHelper *lhs, String &&rhs) {
-    return std::move(rhs.insert(0, reinterpret_cast<const char *>(lhs)));
+    return reinterpret_cast<const char *>(lhs) + std::move(rhs);
 }
 
 extern const String emptyString;

tests/host/core/test_string.cpp

@@ -600,7 +600,8 @@ TEST_CASE("String chaining", "[core][String]")
         String tmp(chunks[3]);
         tmp.reserve(2 * all.length());
         auto*  ptr = tmp.c_str();
-        String result("~1" + String(&chunks[0][0] + 2) + FPSTR(chunks[1]) + chunks[2] + std::move(tmp));
+        String result("~1" + String(&chunks[0][0] + 2) + FPSTR(chunks[1]) + chunks[2]
+                      + std::move(tmp));
         REQUIRE(result == all);
         REQUIRE(static_cast<const void*>(result.c_str()) == static_cast<const void*>(ptr));
     }