Issue 6378 - [emscripten] Make DependentString constexpr

test/compiled/String.cpp

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 #include <string>
 #include <gtest/gtest.h>
 #include "compiled/String.h"
 
 ABP_NS_USING
 
+constexpr void checkConstexprTemplateCtor()
+{
+  constexpr DependentString s(u"Test string");
+}
+
+constexpr void checkConstexprInvalidCtor()
+{
+  constexpr DependentString s;
+  static_assert(s.is_invalid(), "String should be invalid");
+}
+
+constexpr void checkConstexprCtor()
+{
+  constexpr DependentString s(u"Test string", 11);
+}
+
+constexpr void checkConstexprFuncsAndOps()
+{
+  constexpr DependentString s = u"Test string"_str;
+  constexpr DependentString s2 = u"Test string"_str;
+  constexpr DependentString s3 = u"Another test string"_str;
+
+  static_assert(s.length() == s2.length(), "Length should be equal");
+  static_assert(s.equals(s2), "Strings should be equal");
+  static_assert(s == s2, "Strings should be equal");
+  static_assert(s != s3, "Strings should not be equal");
+
+  static_assert(!s.empty(), "String should not be empty");
+  static_assert(!s.is_writable(), "String should not be writable");
+  static_assert(!s.is_deleted(), "String should not be deleted");
+
+  static_assert(s.data()[0] == 'T', "First character should be 'T'");
+  static_assert(s[1] == 'e', "Second character should be 'e'");
+
+  static_assert(s.find(u'e') == 1, "'e' should be at pos 1");
+  static_assert(s.find(s2) == 0, "String should be found at pos 0");
+  static_assert(s.rfind(u'e') == 1, "'e' should be at pos 1");
+}
+
 TEST(TestString, constructInvalidDependentString)
 {
   DependentString s;
   EXPECT_TRUE(s.is_invalid());
 
   DependentString s2(s);
   EXPECT_TRUE(s2.is_invalid());
 }
 
 TEST(TestString, constructInvalidOwnedString)
@@ skipping 134 matching lines @@
   EXPECT_FALSE(lexical_cast<bool>(ABP_TEXT("some-string"_str)));
   EXPECT_FALSE(lexical_cast<bool>(ABP_TEXT(""_str)));
   EXPECT_FALSE(lexical_cast<bool>(DependentString()));
 }
 
 TEST(TestStringLexicalCast, toOwnedString)
 {
   EXPECT_EQ(ABP_TEXT("some-string"_str), lexical_cast<OwnedString>(ABP_TEXT("some-string"_str)));
   EXPECT_EQ(ABP_TEXT(""_str), lexical_cast<OwnedString>(ABP_TEXT(""_str)));
 }