diff --git a/clang/include/clang/Basic/DiagnosticLexKinds.td b/clang/include/clang/Basic/DiagnosticLexKinds.td
index 45e50c9a8cdb..48d9551d6867 100644
--- a/clang/include/clang/Basic/DiagnosticLexKinds.td
+++ b/clang/include/clang/Basic/DiagnosticLexKinds.td
@@ -268,12 +268,14 @@ def err_pp_hash_error : Error<"%0">;
 }
 
 def pp_include_next_in_primary : Warning<
-  "#include_next in primary source file">,
+  "#include_next in primary source file; "
+  "will search from start of include path">,
   InGroup<DiagGroup<"include-next-outside-header">>;
 def pp_include_macros_out_of_predefines : Error<
   "the #__include_macros directive is only for internal use by -imacros">;
 def pp_include_next_absolute_path : Warning<
-  "#include_next with absolute path">,
+  "#include_next in file found relative to primary source file or found by "
+  "absolute path; will search from start of include path">,
   InGroup<DiagGroup<"include-next-absolute-path">>;
 def ext_c99_whitespace_required_after_macro_name : ExtWarn<
   "ISO C99 requires whitespace after the macro name">, InGroup<C99>;
diff --git a/clang/lib/Lex/PPDirectives.cpp b/clang/lib/Lex/PPDirectives.cpp
index 179b0cbf6960..1850af9cfa4f 100644
--- a/clang/lib/Lex/PPDirectives.cpp
+++ b/clang/lib/Lex/PPDirectives.cpp
@@ -2085,6 +2085,10 @@ void Preprocessor::HandleIncludeNextDirective(SourceLocation HashLoc,
     LookupFromFile = CurPPLexer->getFileEntry();
     Lookup = nullptr;
   } else if (!Lookup) {
+    // The current file was not found by walking the include path. Either it
+    // is the primary file (handled above), or it was found by absolute path,
+    // or it was found relative to such a file.
+    // FIXME: Track enough information so we know which case we're in.
     Diag(IncludeNextTok, diag::pp_include_next_absolute_path);
   } else {
     // Start looking up in the next directory.
diff --git a/clang/test/Preprocessor/Inputs/include-next-1/bar.h b/clang/test/Preprocessor/Inputs/include-next-1/bar.h
new file mode 100644
index 000000000000..1cf97aeb50c5
--- /dev/null
+++ b/clang/test/Preprocessor/Inputs/include-next-1/bar.h
@@ -0,0 +1 @@
+#define BAR 1
diff --git a/clang/test/Preprocessor/Inputs/include-next-1/foo.h b/clang/test/Preprocessor/Inputs/include-next-1/foo.h
new file mode 100644
index 000000000000..7d2753c4e4da
--- /dev/null
+++ b/clang/test/Preprocessor/Inputs/include-next-1/foo.h
@@ -0,0 +1 @@
+#include_next "bar.h"
diff --git a/clang/test/Preprocessor/Inputs/include-next-2/bar.h b/clang/test/Preprocessor/Inputs/include-next-2/bar.h
new file mode 100644
index 000000000000..3ac8411a18d8
--- /dev/null
+++ b/clang/test/Preprocessor/Inputs/include-next-2/bar.h
@@ -0,0 +1 @@
+#define BAR 2
diff --git a/clang/test/Preprocessor/include-next.c b/clang/test/Preprocessor/include-next.c
new file mode 100644
index 000000000000..4b9a0e870cec
--- /dev/null
+++ b/clang/test/Preprocessor/include-next.c
@@ -0,0 +1,29 @@
+// RUN: %clang_cc1 -verify %s -E -o /dev/null -I%S/Inputs/include-next-1 -I%S/Inputs/include-next-2 -DTEST=1
+// RUN: %clang_cc1 -verify %s -E -o /dev/null -I%S/Inputs/include-next-1 -I%S/Inputs/include-next-2 -DTEST=2
+// RUN: %clang_cc1 -verify %s -E -o /dev/null -I%S/Inputs/include-next-1 -I%S/Inputs/include-next-2 -DTEST=3
+
+#if TEST == 1
+// expected-warning@+1 {{#include_next in primary source file}}
+#include_next "bar.h"
+#if BAR != 1
+#error wrong bar
+#endif
+
+#elif TEST == 2
+// expected-no-diagnostics
+#include "foo.h"
+#if BAR != 2
+#error wrong bar
+#endif
+
+#elif TEST == 3
+// expected-warning@foo.h:1 {{#include_next in file found relative to primary source file or found by absolute path}}
+#include "Inputs/include-next-1/foo.h"
+#if BAR != 1
+#error wrong bar
+#endif
+#undef BAR
+
+#else
+#error unknown test
+#endif