[clang-tools-extra] [clang-tidy] Fix `modernize-use-constraints` crash on uses of nonstandard `enable_if`s (PR #152938)

[Victor Chernyakin via cfe-commits]

https://github.com/localspook updated https://github.com/llvm/llvm-project/pull/152938

>From 728fc710260d529a604bf3692b18c9131061e070 Mon Sep 17 00:00:00 2001
From: Victor Chernyakin <chernyakin.victor.j at outlook.com>
Date: Sun, 10 Aug 2025 12:25:26 -0700
Subject: [PATCH 1/4] [clang-tidy] Fix `modernize-use-constraints` crash on
 uses of nonstandard `enable_if`s

---
 .../modernize/UseConstraintsCheck.cpp         | 10 ++++++
 clang-tools-extra/docs/ReleaseNotes.rst       |  5 +++
 .../checkers/modernize/use-constraints.cpp    | 31 ++++++++++++++++++-
 3 files changed, 45 insertions(+), 1 deletion(-)

diff --git a/clang-tools-extra/clang-tidy/modernize/UseConstraintsCheck.cpp b/clang-tools-extra/clang-tidy/modernize/UseConstraintsCheck.cpp
index e9b96c4016af6..ab25fb675552a 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseConstraintsCheck.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/UseConstraintsCheck.cpp
@@ -8,6 +8,7 @@
 
 #include "UseConstraintsCheck.h"
 #include "clang/AST/ASTContext.h"
+#include "clang/AST/DeclTemplate.h"
 #include "clang/ASTMatchers/ASTMatchFinder.h"
 #include "clang/Lex/Lexer.h"
 
@@ -78,6 +79,15 @@ matchEnableIfSpecializationImplTypename(TypeLoc TheType) {
     if (!TD || TD->getName() != "enable_if")
       return std::nullopt;
 
+    const TemplateParameterList *Params = TD->getTemplateParameters();
+    if (Params->size() != 2)
+      return std::nullopt;
+
+    const auto *FirstParam =
+        dyn_cast<NonTypeTemplateParmDecl>(Params->getParam(0));
+    if (!FirstParam || !FirstParam->getType()->isBooleanType())
+      return std::nullopt;
+
     int NumArgs = SpecializationLoc.getNumArgs();
     if (NumArgs != 1 && NumArgs != 2)
       return std::nullopt;
diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
index 2de2818172850..f6aedb299e456 100644
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -136,6 +136,11 @@ Changes in existing checks
 - Improved :doc:`misc-header-include-cycle
   <clang-tidy/checks/misc/header-include-cycle>` check performance.
 
+- Fixed a :doc:`modernize-use-constraints
+  <clang-tidy/checks/modernize/use-constraints>` crash on uses of
+  nonstandard ``enable_if``s with a signature different from
+  ``std::enable_if`` (such as ``boost::enable_if``).
+
 - Improved :doc:`modernize-use-designated-initializers
   <clang-tidy/checks/modernize/use-designated-initializers>` check to
   suggest using designated initializers for aliased aggregate types.
diff --git a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-constraints.cpp b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-constraints.cpp
index 3bcd5cd74024e..d61073c1aac3f 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-constraints.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-constraints.cpp
@@ -1,4 +1,4 @@
-// RUN: %check_clang_tidy -std=c++20 %s modernize-use-constraints %t -- -- -fno-delayed-template-parsing
+// RUN: %check_clang_tidy -std=c++20-or-later %s modernize-use-constraints %t -- -- -fno-delayed-template-parsing
 
 // NOLINTBEGIN
 namespace std {
@@ -756,3 +756,32 @@ abs(const number<T, ExpressionTemplates> &v) {
 }
 
 }
+
+template <typename T>
+struct some_type_trait {
+  static constexpr bool value = true;
+};
+
+// Fix-its are offered even for a nonstandard enable_if.
+namespace nonstd {
+
+template <bool Condition, typename T = void>
+struct enable_if : std::enable_if<Condition, T> {};
+
+}
+
+template <typename T>
+typename nonstd::enable_if<some_type_trait<T>::value, void>::type nonstd_enable_if() {}
+// CHECK-MESSAGES: :[[@LINE-1]]:1: warning: use C++20 requires constraints instead of enable_if [modernize-use-constraints]
+// CHECK-FIXES: {{^}}void nonstd_enable_if() requires some_type_trait<T>::value {}{{$}}
+
+// But only if the nonstandard enable_if has the same signature as the standard one.
+namespace boost {
+
+template <typename Condition, typename T = void>
+struct enable_if : std::enable_if<Condition::value, T> {};
+
+}
+
+template <typename T>
+typename boost::enable_if<some_type_trait<T>, void>::type boost_enable_if() {}

>From a189367029572320d44ad3fb787571d1e4e984f1 Mon Sep 17 00:00:00 2001
From: Victor Chernyakin <chernyakin.victor.j at outlook.com>
Date: Sun, 10 Aug 2025 13:22:55 -0700
Subject: [PATCH 2/4] Adjust wording

---
 clang-tools-extra/docs/ReleaseNotes.rst                     | 6 +++---
 .../test/clang-tidy/checkers/modernize/use-constraints.cpp  | 4 ++--
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
index f6aedb299e456..51deab4dfd234 100644
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -136,9 +136,9 @@ Changes in existing checks
 - Improved :doc:`misc-header-include-cycle
   <clang-tidy/checks/misc/header-include-cycle>` check performance.
 
-- Fixed a :doc:`modernize-use-constraints
-  <clang-tidy/checks/modernize/use-constraints>` crash on uses of
-  nonstandard ``enable_if``s with a signature different from
+- Improved :doc:`modernize-use-constraints
+  <clang-tidy/checks/modernize/use-constraints>` check by fixing a crash on
+  uses of non-standard ``enable_if`` with a signature different from
   ``std::enable_if`` (such as ``boost::enable_if``).
 
 - Improved :doc:`modernize-use-designated-initializers
diff --git a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-constraints.cpp b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-constraints.cpp
index d61073c1aac3f..54e66882ff9fe 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-constraints.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-constraints.cpp
@@ -762,7 +762,7 @@ struct some_type_trait {
   static constexpr bool value = true;
 };
 
-// Fix-its are offered even for a nonstandard enable_if.
+// Fix-its are offered even for a non-standard enable_if.
 namespace nonstd {
 
 template <bool Condition, typename T = void>
@@ -775,7 +775,7 @@ typename nonstd::enable_if<some_type_trait<T>::value, void>::type nonstd_enable_
 // CHECK-MESSAGES: :[[@LINE-1]]:1: warning: use C++20 requires constraints instead of enable_if [modernize-use-constraints]
 // CHECK-FIXES: {{^}}void nonstd_enable_if() requires some_type_trait<T>::value {}{{$}}
 
-// But only if the nonstandard enable_if has the same signature as the standard one.
+// But only if the non-standard enable_if has the same signature as the standard one.
 namespace boost {
 
 template <typename Condition, typename T = void>

>From 3911696efef576c43268e3582c9d4d8cc2e7e8c5 Mon Sep 17 00:00:00 2001
From: Victor Chernyakin <chernyakin.victor.j at outlook.com>
Date: Sun, 10 Aug 2025 15:08:10 -0700
Subject: [PATCH 3/4] Remove check for zero template arguments, handle
 `enable_if_t`

---
 .../clang-tidy/modernize/UseConstraintsCheck.cpp   | 13 +++++++------
 .../checkers/modernize/use-constraints.cpp         | 14 ++++++++++++++
 2 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/clang-tools-extra/clang-tidy/modernize/UseConstraintsCheck.cpp b/clang-tools-extra/clang-tidy/modernize/UseConstraintsCheck.cpp
index ab25fb675552a..10f6eb5aa3271 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseConstraintsCheck.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/UseConstraintsCheck.cpp
@@ -79,12 +79,8 @@ matchEnableIfSpecializationImplTypename(TypeLoc TheType) {
     if (!TD || TD->getName() != "enable_if")
       return std::nullopt;
 
-    const TemplateParameterList *Params = TD->getTemplateParameters();
-    if (Params->size() != 2)
-      return std::nullopt;
-
-    const auto *FirstParam =
-        dyn_cast<NonTypeTemplateParmDecl>(Params->getParam(0));
+    const auto *FirstParam = dyn_cast<NonTypeTemplateParmDecl>(
+        TD->getTemplateParameters()->getParam(0));
     if (!FirstParam || !FirstParam->getType()->isBooleanType())
       return std::nullopt;
 
@@ -118,6 +114,11 @@ matchEnableIfSpecializationImplTrait(TypeLoc TheType) {
     if (!Specialization->isTypeAlias())
       return std::nullopt;
 
+    const auto *FirstParam = dyn_cast<NonTypeTemplateParmDecl>(
+        TD->getTemplateParameters()->getParam(0));
+    if (!FirstParam || !FirstParam->getType()->isBooleanType())
+      return std::nullopt;
+
     if (const auto *AliasedType =
             dyn_cast<DependentNameType>(Specialization->getAliasedType())) {
       ElaboratedTypeKeyword Keyword = AliasedType->getKeyword();
diff --git a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-constraints.cpp b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-constraints.cpp
index 54e66882ff9fe..7a8721061be2c 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-constraints.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-constraints.cpp
@@ -768,6 +768,9 @@ namespace nonstd {
 template <bool Condition, typename T = void>
 struct enable_if : std::enable_if<Condition, T> {};
 
+template <bool Condition, typename T = void>
+using enable_if_t = typename enable_if<Condition, T>::type;
+
 }
 
 template <typename T>
@@ -775,13 +778,24 @@ typename nonstd::enable_if<some_type_trait<T>::value, void>::type nonstd_enable_
 // CHECK-MESSAGES: :[[@LINE-1]]:1: warning: use C++20 requires constraints instead of enable_if [modernize-use-constraints]
 // CHECK-FIXES: {{^}}void nonstd_enable_if() requires some_type_trait<T>::value {}{{$}}
 
+template <typename T>
+nonstd::enable_if_t<some_type_trait<T>::value, void> nonstd_enable_if_t() {}
+// CHECK-MESSAGES: :[[@LINE-1]]:1: warning: use C++20 requires constraints instead of enable_if [modernize-use-constraints]
+// CHECK-FIXES: {{^}}void nonstd_enable_if_t() requires some_type_trait<T>::value {}{{$}}
+
 // But only if the non-standard enable_if has the same signature as the standard one.
 namespace boost {
 
 template <typename Condition, typename T = void>
 struct enable_if : std::enable_if<Condition::value, T> {};
 
+template <typename Condition, typename T = void>
+using enable_if_t = typename enable_if<Condition, T>::type;
+
 }
 
 template <typename T>
 typename boost::enable_if<some_type_trait<T>, void>::type boost_enable_if() {}
+
+template <typename T>
+boost::enable_if_t<some_type_trait<T>, void> boost_enable_if_t() {}

>From 3d273ebd1712f120e93f91ff5d2bf96c978a4ff7 Mon Sep 17 00:00:00 2001
From: Victor Chernyakin <chernyakin.victor.j at outlook.com>
Date: Mon, 11 Aug 2025 20:51:43 -0700
Subject: [PATCH 4/4] Add asserts

---
 .../clang-tidy/modernize/UseConstraintsCheck.cpp              | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/clang-tools-extra/clang-tidy/modernize/UseConstraintsCheck.cpp b/clang-tools-extra/clang-tidy/modernize/UseConstraintsCheck.cpp
index 10f6eb5aa3271..4dc1a0a2c47a4 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseConstraintsCheck.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/UseConstraintsCheck.cpp
@@ -79,6 +79,8 @@ matchEnableIfSpecializationImplTypename(TypeLoc TheType) {
     if (!TD || TD->getName() != "enable_if")
       return std::nullopt;
 
+    assert(!TD->getTemplateParameters()->empty() &&
+           "found template with no template parameters?");
     const auto *FirstParam = dyn_cast<NonTypeTemplateParmDecl>(
         TD->getTemplateParameters()->getParam(0));
     if (!FirstParam || !FirstParam->getType()->isBooleanType())
@@ -114,6 +116,8 @@ matchEnableIfSpecializationImplTrait(TypeLoc TheType) {
     if (!Specialization->isTypeAlias())
       return std::nullopt;
 
+    assert(!TD->getTemplateParameters()->empty() &&
+           "found template with no template parameters?");
     const auto *FirstParam = dyn_cast<NonTypeTemplateParmDecl>(
         TD->getTemplateParameters()->getParam(0));
     if (!FirstParam || !FirstParam->getType()->isBooleanType())