Add test cases

clang-tools-extra/clang-tidy/cppcoreguidelines/DeclareNotNullCheck.cpp

 
 using namespace clang::ast_matchers;
 
 namespace clang {
 namespace tidy {
 namespace cppcoreguidelines {
 
 void DeclareNotNullCheck::registerMatchers(MatchFinder *Finder) {
-  auto PointerType = hasType(pointerType());
-  Finder->addMatcher(parmVarDecl(PointerType).bind("param"), this);
+  auto NullCondition = expr(ignoringImpCasts(anyOf(
+      binaryOperator(hasOperatorName("=="),
+                     hasLHS(ignoringImpCasts(cxxNullPtrLiteralExpr())),
+                     hasRHS(ignoringImpCasts(expr().bind("exp")))),
+      binaryOperator(hasOperatorName("=="),
+                     hasLHS(ignoringImpCasts(expr().bind("exp"))),
+                     hasRHS(ignoringImpCasts(cxxNullPtrLiteralExpr()))))));
+  auto functMatcher =
+      functionDecl(
+          hasAnyParameter(parmVarDecl(hasType(pointerType())).bind("param")))
+          .bind("funct");
+  Finder->addMatcher(ifStmt(hasParent(compoundStmt(hasParent(functMatcher))),
+                            hasCondition(NullCondition)),
+                     this);
 }
 
 void DeclareNotNullCheck::check(const MatchFinder::MatchResult &Result) {
+  const FunctionDecl *Funct = Result.Nodes.getNodeAs<FunctionDecl>("funct");
   const ParmVarDecl *Par = Result.Nodes.getNodeAs<ParmVarDecl>("param");
-  diag(Par->getLocation(), "Found a pointer passed as a param") << Par;
+  const Expr *Ex = Result.Nodes.getNodeAs<Expr>("exp");
+  diag(Ex->getExprLoc(), "pointer parameter checked against nullptr") << Par;
+  diag(Par->getLocation(), "consider declaring as not_null",
+       DiagnosticIDs::Note);
 }
 
 } // namespace cppcoreguidelines
 } // namespace tidy
 } // namespace clang

clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines/declare-not-null.cpp

 // RUN: %check_clang_tidy %s cppcoreguidelines-declare-not-null %t
 
-void f(int *a);
-// CHECK-MESSAGES: :[[@LINE-1]]:13: warning: Found a pointer passed as a param [cppcoreguidelines-declare-not-null]
-void f2(int a);
+namespace gsl {
+template <typename T>
+struct not_null {};
+} // namespace gsl
+
+
+void f(int *a){
+    if(a == nullptr)
+        return;
+    else a++;
+    return;
+}
+// CHECK-MESSAGES: [[@LINE-5]]:8: warning: pointer parameter checked against nullptr
+// CHECK-MESSAGES: [[@LINE-7]]:13: note: consider declaring as not_null
+
+void f2(gsl::not_null<const int*>a){
+    return;
+}
+
+void f3(int a);
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