Add additional string case functions

Add function to perform case-insensitive comparison of two string-view
instancess. Add functions to perform case conversion on string_view.

Source/cmList.cxx

@@ -114,9 +114,9 @@ protected:
 
   StringFilter GetCaseFilter(CaseSensitivity sensitivity)
   {
-    return (sensitivity == CaseSensitivity::INSENSITIVE)
-      ? cmSystemTools::LowerCase
-      : nullptr;
+    constexpr std::string (*filter)(std::string const&) =
+      cmSystemTools::LowerCase;
+    return (sensitivity == CaseSensitivity::INSENSITIVE) ? filter : nullptr;
   }
 
   using ComparisonFunction =

Source/cmStringAlgorithms.cxx

@@ -8,6 +8,16 @@
 #include <cstdio>
 #include <cstdlib>
 
+bool cmStrCaseEq(cm::string_view s1, cm::string_view s2)
+{
+  if (s1.size() != s2.size()) {
+    return false;
+  }
+
+  return std::equal(s1.begin(), s1.end(), s2.begin(),
+                    [](char a, char b) { return tolower(a) == tolower(b); });
+}
+
 std::string cmTrimWhitespace(cm::string_view str)
 {
   // XXX(clang-tidy): This declaration and the next cannot be `const auto*`

Source/cmStringAlgorithms.h

@@ -43,6 +43,13 @@ private:
   std::string const Test_;
 };
 
+/**
+ * Test if two strings are identical, ignoring case.
+ *
+ * Note that this is not guaranteed to work correctly on non-ASCII strings.
+ */
+bool cmStrCaseEq(cm::string_view a, cm::string_view b);
+
 /** Returns true if the character @a ch is a whitespace character.  **/
 inline bool cmIsSpace(char ch)
 {

Source/cmSystemTools.cxx

@@ -387,6 +387,28 @@ void cmSystemTools::ExpandRegistryValues(std::string& source,
 }
 #endif
 
+// Return a lower case string
+std::string cmSystemTools::LowerCase(cm::string_view s)
+{
+  std::string n;
+  n.resize(s.size());
+  for (size_t i = 0; i < s.size(); i++) {
+    n[i] = static_cast<std::string::value_type>(tolower(s[i]));
+  }
+  return n;
+}
+
+// Return an upper case string
+std::string cmSystemTools::UpperCase(cm::string_view s)
+{
+  std::string n;
+  n.resize(s.size());
+  for (size_t i = 0; i < s.size(); i++) {
+    n[i] = static_cast<std::string::value_type>(toupper(s[i]));
+  }
+  return n;
+}
+
 std::string cmSystemTools::HelpFileName(cm::string_view str)
 {
   std::string name(str);

Source/cmSystemTools.h

@@ -40,6 +40,26 @@ public:
   using Superclass = cmsys::SystemTools;
   using Encoding = cmProcessOutput::Encoding;
 
+  /**
+   * Return a lower case string
+   */
+  static std::string LowerCase(cm::string_view);
+  static std::string LowerCase(char const* s)
+  {
+    return LowerCase(cm::string_view{ s });
+  }
+  using cmsys::SystemTools::LowerCase;
+
+  /**
+   * Return an upper case string
+   */
+  static std::string UpperCase(cm::string_view);
+  static std::string UpperCase(char const* s)
+  {
+    return UpperCase(cm::string_view{ s });
+  }
+  using cmsys::SystemTools::UpperCase;
+
   /**
    * Look for and replace registry values in a string
    */