[clang-tidy] Fix broken HeaderFilterRegex when read from config file (#133582)

PR https://github.com/llvm/llvm-project/pull/91400 broke the usage of
HeaderFilterRegex via config file, because it is now created at a
different point in the execution and leads to a different value.

The result of that is that using HeaderFilterRegex only in the config
file does NOT work, in other words clang-tidy stops triggering warnings
on header files, thereby losing a lot of coverage.

This patch reverts the logic so that the header filter is created upon
calling the getHeaderFilter() function.

Additionally, this patch adds 2 unit tests to prevent regressions in the
future:

- One of them, "simple", tests the most basic use case with a single
top-level .clang-tidy file.

- The second one, "inheritance", demonstrates that the subfolder only
gets warnings from headers within it, and not from parent headers.

Fixes #118009
Fixes #121969
Fixes #133453

Co-authored-by: Carlos Gálvez <carlos.galvez@zenseact.com>

clang-tools-extra/clang-tidy/ClangTidyDiagnosticConsumer.cpp

@@ -311,18 +311,7 @@ ClangTidyDiagnosticConsumer::ClangTidyDiagnosticConsumer(
     : Context(Ctx), ExternalDiagEngine(ExternalDiagEngine),
       RemoveIncompatibleErrors(RemoveIncompatibleErrors),
       GetFixesFromNotes(GetFixesFromNotes),
-      EnableNolintBlocks(EnableNolintBlocks) {
-
-  if (Context.getOptions().HeaderFilterRegex &&
-      !Context.getOptions().HeaderFilterRegex->empty())
-    HeaderFilter =
-        std::make_unique<llvm::Regex>(*Context.getOptions().HeaderFilterRegex);
-
-  if (Context.getOptions().ExcludeHeaderFilterRegex &&
-      !Context.getOptions().ExcludeHeaderFilterRegex->empty())
-    ExcludeHeaderFilter = std::make_unique<llvm::Regex>(
-        *Context.getOptions().ExcludeHeaderFilterRegex);
-}
+      EnableNolintBlocks(EnableNolintBlocks) {}
 
 void ClangTidyDiagnosticConsumer::finalizeLastError() {
   if (!Errors.empty()) {
@@ -571,17 +560,30 @@ void ClangTidyDiagnosticConsumer::checkFilters(SourceLocation Location,
   }
 
   StringRef FileName(File->getName());
-  LastErrorRelatesToUserCode =
-      LastErrorRelatesToUserCode || Sources.isInMainFile(Location) ||
-      (HeaderFilter &&
-       (HeaderFilter->match(FileName) &&
-        !(ExcludeHeaderFilter && ExcludeHeaderFilter->match(FileName))));
+  LastErrorRelatesToUserCode = LastErrorRelatesToUserCode ||
+                               Sources.isInMainFile(Location) ||
+                               (getHeaderFilter()->match(FileName) &&
+                                !getExcludeHeaderFilter()->match(FileName));
 
   unsigned LineNumber = Sources.getExpansionLineNumber(Location);
   LastErrorPassesLineFilter =
       LastErrorPassesLineFilter || passesLineFilter(FileName, LineNumber);
 }
 
+llvm::Regex *ClangTidyDiagnosticConsumer::getHeaderFilter() {
+  if (!HeaderFilter)
+    HeaderFilter =
+        std::make_unique<llvm::Regex>(*Context.getOptions().HeaderFilterRegex);
+  return HeaderFilter.get();
+}
+
+llvm::Regex *ClangTidyDiagnosticConsumer::getExcludeHeaderFilter() {
+  if (!ExcludeHeaderFilter)
+    ExcludeHeaderFilter = std::make_unique<llvm::Regex>(
+        *Context.getOptions().ExcludeHeaderFilterRegex);
+  return ExcludeHeaderFilter.get();
+}
+
 void ClangTidyDiagnosticConsumer::removeIncompatibleErrors() {
   // Each error is modelled as the set of intervals in which it applies
   // replacements. To detect overlapping replacements, we use a sweep line

clang-tools-extra/clang-tidy/ClangTidyDiagnosticConsumer.h

@@ -302,6 +302,10 @@ private:
   /// context.
   llvm::Regex *getHeaderFilter();
 
+  /// Returns the \c ExcludeHeaderFilter constructed for the options set in the
+  /// context.
+  llvm::Regex *getExcludeHeaderFilter();
+
   /// Updates \c LastErrorRelatesToUserCode and LastErrorPassesLineFilter
   /// according to the diagnostic \p Location.
   void checkFilters(SourceLocation Location, const SourceManager &Sources);

clang-tools-extra/clang-tidy/ClangTidyOptions.cpp

@@ -194,8 +194,8 @@ ClangTidyOptions ClangTidyOptions::getDefaults() {
   Options.WarningsAsErrors = "";
   Options.HeaderFileExtensions = {"", "h", "hh", "hpp", "hxx"};
   Options.ImplementationFileExtensions = {"c", "cc", "cpp", "cxx"};
-  Options.HeaderFilterRegex = std::nullopt;
-  Options.ExcludeHeaderFilterRegex = std::nullopt;
+  Options.HeaderFilterRegex = "";
+  Options.ExcludeHeaderFilterRegex = "";
   Options.SystemHeaders = false;
   Options.FormatStyle = "none";
   Options.User = std::nullopt;

clang-tools-extra/docs/ReleaseNotes.rst

@@ -94,6 +94,9 @@ Improvements to clang-tidy
 - Improved :program:`clang-tidy-diff.py` script. Add the `-warnings-as-errors`
   argument to treat warnings as errors.
 
+- Fixed bug in :program:`clang-tidy` by which `HeaderFilterRegex` did not take
+  effect when passed via the `.clang-tidy` file.
+
 New checks
 ^^^^^^^^^^
 

clang-tools-extra/test/clang-tidy/infrastructure/header-filter-from-config-file/inheritance/.clang-tidy

@@ -0,0 +1 @@
+HeaderFilterRegex: '.*'

clang-tools-extra/test/clang-tidy/infrastructure/header-filter-from-config-file/inheritance/foo.cpp

@@ -0,0 +1,3 @@
+// RUN: clang-tidy -checks=-*,google-explicit-constructor %s 2>&1 | FileCheck %s
+#include "foo.h"
+// CHECK: foo.h:1:12: warning: single-argument constructors must be marked explicit

clang-tools-extra/test/clang-tidy/infrastructure/header-filter-from-config-file/inheritance/foo.h

@@ -0,0 +1 @@
+struct X { X(int); };

clang-tools-extra/test/clang-tidy/infrastructure/header-filter-from-config-file/inheritance/subfolder/.clang-tidy

@@ -0,0 +1,2 @@
+InheritParentConfig: true
+HeaderFilterRegex: 'subfolder/.*'

clang-tools-extra/test/clang-tidy/infrastructure/header-filter-from-config-file/inheritance/subfolder/bar.cpp

@@ -0,0 +1,8 @@
+// shell is required for the "dirname" command
+// REQUIRES: shell
+// RUN: clang-tidy -checks=-*,google-explicit-constructor %s -- -I "$(dirname %S)" 2>&1 | FileCheck %s
+#include "foo.h"
+// CHECK-NOT: foo.h:1:12: warning: single-argument constructors must be marked explicit
+
+#include "bar.h"
+// CHECK: bar.h:1:13: warning: single-argument constructors must be marked explicit

clang-tools-extra/test/clang-tidy/infrastructure/header-filter-from-config-file/inheritance/subfolder/bar.h

@@ -0,0 +1 @@
+struct XX { XX(int); };

clang-tools-extra/test/clang-tidy/infrastructure/header-filter-from-config-file/simple/.clang-tidy

@@ -0,0 +1 @@
+HeaderFilterRegex: '.*'

clang-tools-extra/test/clang-tidy/infrastructure/header-filter-from-config-file/simple/foo.cpp

@@ -0,0 +1,3 @@
+// RUN: clang-tidy -checks=-*,google-explicit-constructor %s 2>&1 | FileCheck %s
+#include "foo.h"
+// CHECK: foo.h:1:12: warning: single-argument constructors must be marked explicit

clang-tools-extra/test/clang-tidy/infrastructure/header-filter-from-config-file/simple/foo.h

@@ -0,0 +1 @@
+struct X { X(int); };