--- a/clang/lib/Driver/ToolChains/MinGW.cpp	2021-05-05 03:19:56.148093452 +0100
+++ b/clang/lib/Driver/ToolChains/MinGW.cpp	2021-05-05 03:21:54.686095364 +0100
@@ -326,7 +326,8 @@
 
 // Simplified from Generic_GCC::GCCInstallationDetector::ScanLibDirForGCCTriple.
 static bool findGccVersion(StringRef LibDir, std::string &GccLibDir,
-                           std::string &Ver) {
+                           std::string &Ver,
+                           toolchains::Generic_GCC::GCCVersion &GccVer) {
   auto Version = toolchains::Generic_GCC::GCCVersion::Parse("0.0.0");
   std::error_code EC;
   for (llvm::sys::fs::directory_iterator LI(LibDir, EC), LE; !EC && LI != LE;
@@ -338,6 +339,7 @@
       continue;
     if (CandidateVersion <= Version)
       continue;
+    GccVer = CandidateVersion;
     Ver = std::string(VersionText);
     GccLibDir = LI->path();
   }
@@ -357,7 +359,7 @@
     for (StringRef CandidateArch : Archs) {
       llvm::SmallString<1024> LibDir(Base);
       llvm::sys::path::append(LibDir, CandidateLib, "gcc", CandidateArch);
-      if (findGccVersion(LibDir, GccLibDir, Ver)) {
+      if (findGccVersion(LibDir, GccLibDir, Ver, GccVer)) {
         Arch = std::string(CandidateArch);
         return;
       }
@@ -420,6 +422,11 @@
   getFilePaths().push_back(GccLibDir);
   getFilePaths().push_back(
       (Base + Arch + llvm::sys::path::get_separator() + "lib").str());
+  
+  // Gentoo
+  getFilePaths().push_back(
+      (Base + Arch + llvm::sys::path::get_separator() + "mingw/lib").str());
+  
   getFilePaths().push_back(Base + "lib");
   // openSUSE
   getFilePaths().push_back(Base + Arch + "/sys-root/mingw/lib");
@@ -572,6 +579,11 @@
 
   addSystemInclude(DriverArgs, CC1Args,
                    Base + Arch + llvm::sys::path::get_separator() + "include");
+
+  // Gentoo
+  addSystemInclude(DriverArgs, CC1Args,
+                   Base + Arch + llvm::sys::path::get_separator() + "usr/include");
+  
   addSystemInclude(DriverArgs, CC1Args, Base + "include");
 }
 
@@ -592,7 +604,7 @@
     break;
 
   case ToolChain::CST_Libstdcxx:
-    llvm::SmallVector<llvm::SmallString<1024>, 4> CppIncludeBases;
+    llvm::SmallVector<llvm::SmallString<1024>, 7> CppIncludeBases;
     CppIncludeBases.emplace_back(Base);
     llvm::sys::path::append(CppIncludeBases[0], Arch, "include", "c++");
     CppIncludeBases.emplace_back(Base);
@@ -601,6 +613,15 @@
     llvm::sys::path::append(CppIncludeBases[2], "include", "c++", Ver);
     CppIncludeBases.emplace_back(GccLibDir);
     llvm::sys::path::append(CppIncludeBases[3], "include", "c++");
+    CppIncludeBases.emplace_back(GccLibDir);
+    llvm::sys::path::append(CppIncludeBases[4], "include",
+                            "g++-v" + GccVer.Text);
+    CppIncludeBases.emplace_back(GccLibDir);
+    llvm::sys::path::append(CppIncludeBases[5], "include",
+                            "g++-v" + GccVer.MajorStr + "." + GccVer.MinorStr);
+    CppIncludeBases.emplace_back(GccLibDir);
+    llvm::sys::path::append(CppIncludeBases[6], "include",
+                            "g++-v" + GccVer.MajorStr);
     for (auto &CppIncludeBase : CppIncludeBases) {
       addSystemInclude(DriverArgs, CC1Args, CppIncludeBase);
       CppIncludeBase += Slash;
--- a/clang/lib/Driver/ToolChains/MinGW.h	2021-05-05 03:19:56.148093452 +0100
+++ b/clang/lib/Driver/ToolChains/MinGW.h	2021-05-05 03:21:54.686095364 +0100
@@ -98,6 +98,7 @@
 
   std::string Base;
   std::string GccLibDir;
+  clang::driver::toolchains::Generic_GCC::GCCVersion GccVer;
   std::string Ver;
   std::string Arch;
   mutable std::unique_ptr<tools::gcc::Preprocessor> Preprocessor;