[Clang] FunctionEffects: Correctly navigate through array types in FunctionEffectsRef::get(). (#121525)

`FunctionEffectsRef::get()` is supposed to strip off layers of
indirection (pointers/references, type sugar) to get to a
`FunctionProtoType` (if any) and return its effects (if any).

It wasn't correctly dealing with situations where the compiler
implicitly converts an array to a pointer.

---------

Co-authored-by: Doug Wyatt <[email protected]>
Co-authored-by: Sirraide <[email protected]>

Author: 3 people

clang/include/clang/AST/Type.h

@@ -8841,13 +8841,16 @@ void FixedPointValueToString(SmallVectorImpl<char> &Str, llvm::APSInt Val,
                              unsigned Scale);
 
 inline FunctionEffectsRef FunctionEffectsRef::get(QualType QT) {
+  const Type *TypePtr = QT.getTypePtr();
   while (true) {
-    QualType Pointee = QT->getPointeeType();
-    if (Pointee.isNull())
+    if (QualType Pointee = TypePtr->getPointeeType(); !Pointee.isNull())
+      TypePtr = Pointee.getTypePtr();
+    else if (TypePtr->isArrayType())
+      TypePtr = TypePtr->getBaseElementTypeUnsafe();
+    else
       break;
-    QT = Pointee;
   }
-  if (const auto *FPT = QT->getAs<FunctionProtoType>())
+  if (const auto *FPT = TypePtr->getAs<FunctionProtoType>())
     return FPT->getFunctionEffects();
   return {};
 }

clang/test/Sema/attr-nonblocking-constraints.cpp

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only -fblocks -fcxx-exceptions -std=c++20 -verify -Wfunction-effects %s
+// RUN: %clang_cc1 -fsyntax-only -fblocks -fcxx-exceptions -std=c++20 -verify -Wfunction-effects -Wno-vla-extension %s
 // These are in a separate file because errors (e.g. incompatible attributes) currently prevent
 // the FXAnalysis pass from running at all.
 
@@ -246,6 +246,23 @@ void PTMFTester::convert() [[clang::nonblocking]]
 	(this->*mConvertFunc)();
 }
 
+// Allow implicit conversion from array to pointer.
+void nb14(unsigned idx) [[clang::nonblocking]]
+{
+	using FP = void (*)() [[clang::nonblocking]];
+	using FPArray = FP[2];
+	auto nb = +[]() [[clang::nonblocking]] {};
+
+	FPArray src{ nb, nullptr };
+	FP f = src[idx]; // This should not generate a warning.
+
+	FP twoDim[2][2] = {};
+	FP g = twoDim[1][1];
+
+	FP vla[idx];
+	FP h = vla[0];
+}
+
 // Block variables
 void nb17(void (^blk)() [[clang::nonblocking]]) [[clang::nonblocking]] {
 	blk();