[clang] Support header shadowing diagnostics in Clang header search (llvm#162491)

When including a header file, multiple files with the same name may
exist across different search paths, like:
   |-- main.cpp
  |-- **header.h**
  |-- include
  |  └── **header.h**
The compiler usually picks the first match it finds (typically following
MSVC rules for current/include-chain paths first, then regular -I
paths), which may not be the user’s intended header.
This silent behavior can lead to subtle runtime API mismatches or
increase the cost of resolving errors such as “error: use of undeclared
identifier”, especially in large projects.

Therefore, this patch tries to provide a diagnostic message without
changing the current header selection. It does this by performing an
additional search for duplicate filenames across all search paths (both
MSVC rules and standard paths). This informs the user about a potential
"header shadowing" issue and clarifies which header path was actually
used.

Since header searching is much cheaper than file loading, the added
overhead should be within an acceptable range -- assuming the diagnostic
message is valuable.

Author: Jinjie-Huang

clang/docs/ReleaseNotes.rst

@@ -449,6 +449,9 @@ Improvements to Clang's diagnostics
   comparison operators when mixed with bitwise operators in enum value initializers.
   This can be locally disabled by explicitly casting the initializer value.
 
+- A new warning ``-Wshadow-header`` has been added to detect when a header file
+  is found in multiple search directories (excluding system paths).
+
 Improvements to Clang's time-trace
 ----------------------------------
 

clang/include/clang/Basic/DiagnosticGroups.td

@@ -790,6 +790,8 @@ def ShadowFieldInConstructor : DiagGroup<"shadow-field-in-constructor",
 def ShadowIvar : DiagGroup<"shadow-ivar">;
 def ShadowUncapturedLocal : DiagGroup<"shadow-uncaptured-local">;
 
+def ShadowHeader : DiagGroup<"shadow-header">;
+
 // -Wshadow-all is a catch-all for all shadowing. -Wshadow is just the
 // shadowing that we think is unsafe.
 def Shadow : DiagGroup<"shadow", [ShadowFieldInConstructorModified,

clang/include/clang/Basic/DiagnosticLexKinds.td

@@ -959,6 +959,10 @@ def warn_quoted_include_in_framework_header : Warning<
 def warn_framework_include_private_from_public : Warning<
   "public framework header includes private framework header '%0'"
   >, InGroup<FrameworkIncludePrivateFromPublic>;
+def warn_header_shadowing : Warning<
+  "multiple candidates for header '%0' found; "
+  "directory '%1' chosen, ignoring others including '%2'">,
+  InGroup<ShadowHeader>, DefaultIgnore;
 def warn_deprecated_module_dot_map : Warning<
   "'%0' as a module map name is deprecated, rename it to %select{module.modulemap|module.private.modulemap}1%select{| in the 'Modules' directory of the framework}2">,
   InGroup<DeprecatedModuleDotMap>;

clang/include/clang/Lex/HeaderSearch.h

@@ -465,6 +465,12 @@ class HeaderSearch {
     ExternalSource = ES;
   }
 
+  void diagnoseHeaderShadowing(
+      StringRef Filename, OptionalFileEntryRef FE, bool &DiagnosedShadowing,
+      SourceLocation IncludeLoc, ConstSearchDirIterator FromDir,
+      ArrayRef<std::pair<OptionalFileEntryRef, DirectoryEntryRef>> Includers,
+      bool isAngled, int IncluderLoopIndex, ConstSearchDirIterator MainLoopIt);
+
   /// Set the target information for the header search, if not
   /// already known.
   void setTarget(const TargetInfo &Target);

clang/lib/Lex/HeaderSearch.cpp

@@ -881,6 +881,66 @@ diagnoseFrameworkInclude(DiagnosticsEngine &Diags, SourceLocation IncludeLoc,
         << IncludeFilename;
 }
 
+void HeaderSearch::diagnoseHeaderShadowing(
+    StringRef Filename, OptionalFileEntryRef FE, bool &DiagnosedShadowing,
+    SourceLocation IncludeLoc, ConstSearchDirIterator FromDir,
+    ArrayRef<std::pair<OptionalFileEntryRef, DirectoryEntryRef>> Includers,
+    bool isAngled, int IncluderLoopIndex, ConstSearchDirIterator MainLoopIt) {
+
+  if (Diags.isIgnored(diag::warn_header_shadowing, IncludeLoc) ||
+      DiagnosedShadowing)
+    return;
+  // Ignore diagnostics from system headers.
+  if (MainLoopIt && MainLoopIt->isSystemHeaderDirectory())
+    return;
+
+  DiagnosedShadowing = true;
+
+  // Indicates that file is first found in the includer's directory
+  if (!MainLoopIt) {
+    for (size_t i = IncluderLoopIndex + 1; i < Includers.size(); ++i) {
+      const auto &IncluderAndDir = Includers[i];
+      SmallString<1024> TmpDir = IncluderAndDir.second.getName();
+      llvm::sys::path::append(TmpDir, Filename);
+      if (auto File = getFileMgr().getFileRef(TmpDir, false, false)) {
+        if (&File->getFileEntry() == *FE)
+          continue;
+        Diags.Report(IncludeLoc, diag::warn_header_shadowing)
+            << Filename << (*FE).getDir().getName()
+            << IncluderAndDir.second.getName();
+        return;
+      } else {
+        llvm::errorToErrorCode(File.takeError());
+      }
+    }
+  }
+
+  // Continue searching in the regular search paths
+  ConstSearchDirIterator It =
+      isAngled ? angled_dir_begin() : search_dir_begin();
+  if (MainLoopIt) {
+    It = std::next(MainLoopIt);
+  } else if (FromDir) {
+    It = FromDir;
+  }
+  for (; It != search_dir_end(); ++It) {
+    // Suppress check for system headers, as duplicates are often intentional.
+    if (It->getDirCharacteristic() != SrcMgr::C_User)
+      continue;
+    SmallString<1024> TmpPath = It->getName();
+    llvm::sys::path::append(TmpPath, Filename);
+    if (auto File = getFileMgr().getFileRef(TmpPath, false, false)) {
+      if (&File->getFileEntry() == *FE)
+        continue;
+      Diags.Report(IncludeLoc, diag::warn_header_shadowing)
+          << Filename << (*FE).getDir().getName() << It->getName();
+      return;
+    } else {
+      llvm::errorToErrorCode(File.takeError());
+    }
+  }
+}
+
 /// LookupFile - Given a "foo" or \<foo> reference, look up the indicated file,
 /// return null on failure.  isAngled indicates whether the file reference is
 /// for system \#include's or not (i.e. using <> instead of ""). Includers, if
@@ -930,6 +990,7 @@ OptionalFileEntryRef HeaderSearch::LookupFile(
   // This is the header that MSVC's header search would have found.
   ModuleMap::KnownHeader MSSuggestedModule;
   OptionalFileEntryRef MSFE;
+  bool DiagnosedShadowing = false;
 
   // Check to see if the file is in the #includer's directory. This cannot be
   // based on CurDir, because each includer could be a #include of a
@@ -963,6 +1024,9 @@ OptionalFileEntryRef HeaderSearch::LookupFile(
       if (OptionalFileEntryRef FE = getFileAndSuggestModule(
               TmpDir, IncludeLoc, IncluderAndDir.second, IncluderIsSystemHeader,
               RequestingModule, SuggestedModule)) {
+        diagnoseHeaderShadowing(Filename, FE, DiagnosedShadowing, IncludeLoc,
+                                FromDir, Includers, isAngled,
+                                &IncluderAndDir - Includers.begin(), nullptr);
         if (!Includer) {
           assert(First && "only first includer can have no file");
           return FE;
@@ -1097,6 +1161,9 @@ OptionalFileEntryRef HeaderSearch::LookupFile(
     if (!File)
       continue;
 
+    diagnoseHeaderShadowing(Filename, File, DiagnosedShadowing, IncludeLoc,
+                            FromDir, Includers, isAngled, -1, It);
+
     CurDir = It;
 
     IncludeNames[*File] = Filename;

clang/test/Preprocessor/header-shadowing.c

@@ -0,0 +1,57 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+
+/// Check that:
+/// - Quoted includes ("...") trigger the diagnostic.
+/// - System headers are ignored.
+/// - #include_next does not cause a duplicate warning.
+// RUN: %clang_cc1 -Wshadow-header -Eonly %t/main.c -I %t/include1 -I %t/include2 \
+// RUN: -isystem %t/system1 -isystem %t/system2 2>&1 | FileCheck %s --check-prefix=SHADOWING
+
+// SHADOWING: {{.*}} warning: multiple candidates for header 'header.h' found; directory '{{.*}}include1' chosen, ignoring others including '{{.*}}include2' [-Wshadow-header]
+// SHADOWING: warning: include1/header.h included!
+// SHADOWING-NOT: {{.*}} warning: multiple candidates for header 'header.h' found; directory '{{.*}}include2' chosen, ignoring others including '{{.*}}include1' [-Wshadow-header]
+// SHADOWING: warning: include2/header.h included!
+// SHADOWING-NOT: {{.*}} warning: multiple candidates for header 'stdio.h' found; directory '{{.*}}system1' chosen, ignoring others including '{{.*}}system2' [-Wshadow-header]
+// SHADOWING: warning: system1/stdio.h included!
+
+/// Check that the diagnostic is only performed once in MSVC compatibility mode.
+// RUN: %clang_cc1 -fms-compatibility -Wshadow-header -Eonly %t/t.c 2>&1 | FileCheck %s --check-prefix=SHADOWING-MS
+
+// SHADOWING-MS: {{.*}} warning: multiple candidates for header 't3.h' found; directory '{{.*}}foo' chosen, ignoring others including '{{.*}}' [-Wshadow-header]
+// SHADOWING-MS-NOT: {{.*}} warning: multiple candidates for header 't3.h' found; directory '{{.*}}' chosen, ignoring others including '{{.*}}foo' [-Wshadow-header]
+// SHADOWING-MS: warning: Found foo/t3.h.
+
+//--- main.c
+#include "header.h"
+#include <stdio.h>
+
+//--- include1/header.h
+#warning include1/header.h included!
+#include_next "header.h"
+
+//--- include2/header.h
+#warning include2/header.h included!
+
+//--- system1/stdio.h
+#warning system1/stdio.h included!
+
+//--- system2/stdio.h
+#warning system2/stdio.h included!
+
+
+/// Used to test when running in MSVC compatibility
+//--- t.c
+#include "foo/t1.h"
+
+//--- foo/t1.h
+#include "bar/t2.h"
+
+//--- foo/bar/t2.h
+#include "t3.h"
+
+//--- foo/t3.h
+#warning Found foo/t3.h.
+
+//--- t3.h
+#warning Found t3.h.