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
 
+void checkConstexprCorrectness()
+{
+  static constexpr DependentString s(u"Hello world!");
+  static constexpr DependentString s2;
+  static constexpr DependentString s3(u"Hello again!", 12);
+  static constexpr DependentString s4 = u"Hello again!"_str;
+
+  static_assert(s.length() == 12, "Length should be 12");
+  static_assert(s3.equals(s4), "Strings should be equal");
+  static_assert(s4 == s3, "Strings should 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] == 'H', "First character should be 'H'");
+  static_assert(s[1] == 'e', "Second character should be 'e'");
+
+  static_assert(s2.is_invalid(), "String should be invalid");

[Eric, 2018/03/15 18:33:54]

Does calling empty(), is_writable(), etc. on invalid strings return a
placeholder result or do they throw?
Do we care about this difference in behavior?

[sergei, 2018/03/16 13:43:26]

Could you please create an issue for that? Since now we have native tests one
can cover such cases in tests. Additionally it would be better to add more
checks of arguments, e.g. DependentString(nullptr, 10) looks like an invalid
string or there is perhaps even something wrong.

Basically nothing is supposed to throw here when it runs in JS, however if
someone wants, it's possible to achieve it. For invalid string `empty` returns
true and `is_writable` returns false.

+}
+
 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)));
 }