[clang-tidy] bugprone-string-constructor: Catch string from nullptr.

Summary: Context: https://twitter.com/willkirkby/status/1084219580799741953

Subscribers: xazax.hun, cfe-commits

Differential Revision: https://reviews.llvm.org/D56657

2 files changed, 27 insertions, 0 deletions

diff --git a/clang-tools-extra/clang-tidy/bugprone/StringConstructorCheck.cpp b/clang-tools-extra/clang-tidy/bugprone/StringConstructorCheck.cpp
index d88823067f2..cc6e2973a0c 100644
--- a/clang-tools-extra/clang-tidy/bugprone/StringConstructorCheck.cpp
+++ b/clang-tools-extra/clang-tidy/bugprone/StringConstructorCheck.cpp
@@ -100,6 +100,15 @@ void StringConstructorCheck::registerMatchers(MatchFinder *Finder) {
                                        integerLiteral().bind("int"))))))
           .bind("constructor"),
       this);
+
+  // Check the literal string constructor with char pointer.
+  // [i.e. string (const char* s);]
+  Finder->addMatcher(
+      cxxConstructExpr(hasDeclaration(cxxMethodDecl(hasName("basic_string"))),
+                       hasArgument(0, expr().bind("from-ptr")),
+                       hasArgument(1, unless(hasType(isInteger()))))
+          .bind("constructor"),
+      this);
 }
 
 void StringConstructorCheck::check(const MatchFinder::MatchResult &Result) {
@@ -128,6 +137,13 @@ void StringConstructorCheck::check(const MatchFinder::MatchResult &Result) {
     if (Lit->getValue().ugt(Str->getLength())) {
       diag(Loc, "length is bigger then string literal size");
     }
+  } else if (const auto *Ptr = Result.Nodes.getNodeAs<Expr>("from-ptr")) {
+    Expr::EvalResult ConstPtr;
+    if (Ptr->EvaluateAsRValue(ConstPtr, Ctx) &&
+        ((ConstPtr.Val.isInt() && ConstPtr.Val.getInt().isNullValue()) ||
+         (ConstPtr.Val.isLValue() && ConstPtr.Val.isNullPointer()))) {
+      diag(Loc, "constructing string from nullptr is undefined behaviour");
+    }
   }
 }
 
diff --git a/clang-tools-extra/test/clang-tidy/bugprone-string-constructor.cpp b/clang-tools-extra/test/clang-tidy/bugprone-string-constructor.cpp
index 51d91305882..3ab4f424087 100644
--- a/clang-tools-extra/test/clang-tidy/bugprone-string-constructor.cpp
+++ b/clang-tools-extra/test/clang-tidy/bugprone-string-constructor.cpp
@@ -9,6 +9,7 @@ template <typename C, typename T = std::char_traits<C>, typename A = std::alloca
 struct basic_string {
   basic_string();
   basic_string(const C*, unsigned int size);
+  basic_string(const C *, const A &allocator = A());
   basic_string(unsigned int size, C c);
 };
 typedef basic_string<char> string;
@@ -45,6 +46,15 @@ void Test() {
   // CHECK-MESSAGES: [[@LINE-1]]:15: warning: length is bigger then string literal size
   std::string q5(kText3,  0x1000000);
   // CHECK-MESSAGES: [[@LINE-1]]:15: warning: suspicious large length parameter
+  std::string q6(nullptr);
+  // CHECK-MESSAGES: [[@LINE-1]]:15: warning: constructing string from nullptr is undefined behaviour
+  std::string q7 = 0;
+  // CHECK-MESSAGES: [[@LINE-1]]:20: warning: constructing string from nullptr is undefined behaviour
+}
+
+std::string StringFromZero() {
+  return 0;
+  // CHECK-MESSAGES: [[@LINE-1]]:10: warning: constructing string from nullptr is undefined behaviour
 }
 
 void Valid() {
@@ -53,4 +63,5 @@ void Valid() {
   std::wstring wstr(4, L'x');
   std::string s1("test", 4);
   std::string s2("test", 3);
+  std::string s3("test");
 }