Index: clang/lib/Tooling/InterpolatingCompilationDatabase.cpp
===================================================================
--- clang/lib/Tooling/InterpolatingCompilationDatabase.cpp
+++ clang/lib/Tooling/InterpolatingCompilationDatabase.cpp
@@ -329,7 +329,7 @@
       StringRef Path = Strings.save(StringRef(OriginalPaths[I]).lower());
 
       Paths.emplace_back(Path, I);
-      Types.push_back(foldType(guessType(Path)));
+      Types.push_back(foldType(guessType(OriginalPaths[I])));
       Stems.emplace_back(sys::path::stem(Path), I);
       auto Dir = ++sys::path::rbegin(Path), DirEnd = sys::path::rend(Path);
       for (int J = 0; J < DirectorySegmentsIndexed && Dir != DirEnd; ++J, ++Dir)
Index: clang/unittests/Tooling/CompilationDatabaseTest.cpp
===================================================================
--- clang/unittests/Tooling/CompilationDatabaseTest.cpp
+++ clang/unittests/Tooling/CompilationDatabaseTest.cpp
@@ -845,6 +845,14 @@
   EXPECT_EQ(getProxy("foo/bar/baz/shout.C"), "FOO/BAR/BAZ/SHOUT.cc");
 }
 
+TEST_F(InterpolateTest, LanguagePreference) {
+  add("foo/bar/baz/exact.C");
+  add("foo/bar/baz/exact.c");
+  add("other/random/path.cpp");
+  // Proxies for ".H" files are ".C" files, and not ".c files".
+  EXPECT_EQ(getProxy("foo/bar/baz/exact.H"), "foo/bar/baz/exact.C");
+}
+
 TEST_F(InterpolateTest, Aliasing) {
   add("foo.cpp", "-faligned-new");