[clang-tidy] Accept string literal decay in conditional operator

[estan]

Summary

The cppcoreguidelines-pro-bounds-array-to-pointer-decay check currently accepts

const char *b = i ? "foo" : "foobar";

but not

const char *a = i ? "foo" : "bar";

This is because the AST is slightly different in the latter case (see https://godbolt.org/z/MkHVvs).

This eliminates the inconsistency by making it accept the latter form as well.

Fixes https://github.com/llvm/llvm-project/issues/31155.

[estan]

Format code with git-clang-format

[estan]

Add entry in clang-tools-extra/docs/ReleaseNotes.rst

[aaron.ballman]

LGTM, thanks!

[estan]

Thanks @aaron.ballman. I don't have commit/push rights, so someone please commit for me when you got the time.

[aaron.ballman]

In D116814#3234768, @estan wrote:

Thanks @aaron.ballman. I don't have commit/push rights, so someone please commit for me when you got the time.

Happy to! What name and email address would you like me to use for patch attribution?

[estan]

Ah right, Elvis Stansvik <[e-mail]>

[aaron.ballman]

Thank you for the fix! I commit on your behalf in 36af07334246a87a0f3fb9bbc85dedfca04b00d1.

[estan]

👍

[estan]

BTW how are backports to release branches made in LLVM? (sorry if it's in the docs somewhere!)

[aaron.ballman]

In D116814#3235240, @estan wrote:

BTW how are backports to release branches made in LLVM? (sorry if it's in the docs somewhere!)

We document the process here: https://llvm.org/docs/HowToReleaseLLVM.html#merge-requests, but it could be pretty late to make it to 13.0.1.

[estan]

Ah thanks. Yea no worries.

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

 } // namespace
 
 void ProBoundsArrayToPointerDecayCheck::registerMatchers(MatchFinder *Finder) {
   // The only allowed array to pointer decay
   // 1) just before array subscription
   // 2) inside a range-for over an array
   // 3) if it converts a string literal to a pointer
   Finder->addMatcher(
-      traverse(TK_AsIs,
-               implicitCastExpr(
-                   unless(hasParent(arraySubscriptExpr())),
-                   unless(hasParentIgnoringImpCasts(explicitCastExpr())),
-                   unless(isInsideOfRangeBeginEndStmt()),
-                   unless(hasSourceExpression(ignoringParens(stringLiteral()))))
-                   .bind("cast")),
+      traverse(
+          TK_AsIs,
+              unless(hasSourceExpression(ignoringParens(stringLiteral()))),
+              unless(hasSourceExpression(ignoringParens(conditionalOperator(
+                  allOf(hasTrueExpression(stringLiteral()),
+                        hasFalseExpression(stringLiteral())))))))
       this);
 }
 
 void ProBoundsArrayToPointerDecayCheck::check(
     const MatchFinder::MatchResult &Result) {
   const auto *MatchedCast = Result.Nodes.getNodeAs<ImplicitCastExpr>("cast");
   if (MatchedCast->getCastKind() != CK_ArrayToPointerDecay)
     return;

clang-tools-extra/docs/ReleaseNotes.rst

 Improvements to clang-rename
 ----------------------------
 
 The improvements are...
 
 Improvements to clang-tidy
 --------------------------
 
+- Make the `cppcoreguidelines-pro-bounds-array-to-pointer-decay` check accept
+  string literal to pointer decay in conditional operator even if operands are
+  of the same length.
+
 - Ignore warnings from macros defined in system headers, if not using the
   `-system-headers` flag.
 
 - Added support for globbing in `NOLINT*` expressions, to simplify suppressing
   multiple warnings in the same line.
 
 - Added support for `NOLINTBEGIN` ... `NOLINTEND` comments to suppress
   Clang-Tidy warnings over multiple lines.

clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines-pro-bounds-array-to-pointer-decay.cpp

const char *g2() {
     return ("clang"); // OK, ParenExpr hides the literal-pointer decay
 }
 
 void f2(void *const *);
 void bug25362() {
   void *a[2];
   f2(static_cast<void *const*>(a)); // OK, explicit cast
 }
+
+void issue31155(int i) {
+  const char *a = i ? "foo" : "bar";    // OK, decay string literal to pointer
+  const char *b = i ? "foo" : "foobar"; // OK, decay string literal to pointer
+
+  char arr[1];
+  const char *c = i ? arr : "bar";
+  // CHECK-MESSAGES: :[[@LINE-1]]:23: warning: do not implicitly decay an array into a pointer
+  const char *d = i ? "foo" : arr;
+  // CHECK-MESSAGES: :[[@LINE-1]]:31: warning: do not implicitly decay an array into a pointer
+}