[clang-tidy] Fix crash in modernize-use-constraints (#92019)

Improved modernize-use-constraints check by fixing a crash that occurred
in some scenarios and excluded system headers from analysis.

Problem were with DependentNameTypeLoc having null type location as
getQualifierLoc().getTypeLoc().

Fixes #91872

Author: PiotrZSL

clang-tools-extra/clang-tidy/modernize/UseConstraintsCheck.cpp

@@ -41,6 +41,8 @@ AST_MATCHER(FunctionDecl, hasOtherDeclarations) {
 void UseConstraintsCheck::registerMatchers(MatchFinder *Finder) {
   Finder->addMatcher(
       functionTemplateDecl(
+          // Skip external libraries included as system headers
+          unless(isExpansionInSystemHeader()),
           has(functionDecl(unless(hasOtherDeclarations()), isDefinition(),
                            hasReturnTypeLoc(typeLoc().bind("return")))
                   .bind("function")))
@@ -57,6 +59,8 @@ matchEnableIfSpecializationImplTypename(TypeLoc TheType) {
       return std::nullopt;
     }
     TheType = Dep.getQualifierLoc().getTypeLoc();
+    if (TheType.isNull())
+      return std::nullopt;
   }
 
   if (const auto SpecializationLoc =

clang-tools-extra/docs/ReleaseNotes.rst

@@ -322,6 +322,10 @@ Changes in existing checks
   don't remove parentheses used in ``sizeof`` calls when they have array index
   accesses as arguments.
 
+- Improved :doc:`modernize-use-constraints
+  <clang-tidy/checks/modernize/use-constraints>` check by fixing a crash that
+  occurred in some scenarios and excluding system headers from analysis.
+
 - Improved :doc:`modernize-use-nullptr
   <clang-tidy/checks/modernize/use-nullptr>` check to include support for C23,
   which also has introduced the ``nullptr`` keyword.

clang-tools-extra/docs/clang-tidy/checks/modernize/use-constraints.rst

@@ -68,3 +68,7 @@ The tool will replace the above code with,
   // The tool will not emit a diagnostic or attempt to replace the code.
   template <typename T, std::enable_if_t<T::some_trait, int> = 0>
   struct my_class {};
+
+.. note::
+
+  System headers are not analyzed by this check.

clang-tools-extra/test/clang-tidy/checkers/modernize/use-constraints.cpp

@@ -724,3 +724,35 @@ void not_last_param() {
 }
 
 } // namespace enable_if_trailing_type_parameter
+
+
+// Issue fixes:
+
+namespace PR91872 {
+
+enum expression_template_option { value1, value2 };
+
+template <typename T> struct number_category {
+  static const int value = 0;
+};
+
+constexpr int number_kind_complex = 1;
+
+template <typename T, expression_template_option ExpressionTemplates>
+struct number {
+  using type = T;
+};
+
+template <typename T> struct component_type {
+  using type = T;
+};
+
+template <class T, expression_template_option ExpressionTemplates>
+inline typename std::enable_if<
+    number_category<T>::value == number_kind_complex,
+    component_type<number<T, ExpressionTemplates>>>::type::type
+abs(const number<T, ExpressionTemplates> &v) {
+  return {};
+}
+
+}