[clang] Check if clang::FieldDecl has constant-integer bit width before getting the width (PR #148692)

[via cfe-commits]

https://github.com/higher-performance created https://github.com/llvm/llvm-project/pull/148692

This avoids crashing due to template-dependent bit widths, such as in the following:
```
template<unsigned N>
struct S {
        explicit S(bool x) : x(x) { }
        int x : N;
};
```

>From 715aacbcafd894b72364f6fb7181057f6f1fa4c6 Mon Sep 17 00:00:00 2001
From: higher-performance <higher.performance.github at gmail.com>
Date: Mon, 14 Jul 2025 14:05:07 -0400
Subject: [PATCH] Check if clang::FieldDecl has constant-integer bit width
 before getting the width, to avoid crashing inside templates

---
 clang/include/clang/AST/Decl.h                | 5 +++++
 clang/include/clang/ASTMatchers/ASTMatchers.h | 3 ++-
 clang/lib/AST/Decl.cpp                        | 9 ++++++---
 3 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/clang/include/clang/AST/Decl.h b/clang/include/clang/AST/Decl.h
index f70a039bf3517..f88c206ca4d44 100644
--- a/clang/include/clang/AST/Decl.h
+++ b/clang/include/clang/AST/Decl.h
@@ -3234,6 +3234,11 @@ class FieldDecl : public DeclaratorDecl, public Mergeable<FieldDecl> {
     return hasInClassInitializer() ? InitAndBitWidth->BitWidth : BitWidth;
   }
 
+  /// Determines whether the bit width of this field is a constant integer.
+  /// This may not always be the case, such as inside template-dependent
+  /// expressions.
+  bool hasConstantIntegerBitWidth() const;
+
   /// Computes the bit width of this field, if this is a bit field.
   /// May not be called on non-bitfields.
   /// Note that in order to successfully use this function, the bitwidth
diff --git a/clang/include/clang/ASTMatchers/ASTMatchers.h b/clang/include/clang/ASTMatchers/ASTMatchers.h
index bae004896c11f..fcce9b5c38b23 100644
--- a/clang/include/clang/ASTMatchers/ASTMatchers.h
+++ b/clang/include/clang/ASTMatchers/ASTMatchers.h
@@ -709,7 +709,8 @@ AST_MATCHER(FieldDecl, isBitField) {
 /// fieldDecl(hasBitWidth(2))
 ///   matches 'int a;' and 'int c;' but not 'int b;'.
 AST_MATCHER_P(FieldDecl, hasBitWidth, unsigned, Width) {
-  return Node.isBitField() && Node.getBitWidthValue() == Width;
+  return Node.isBitField() && Node.hasConstantIntegerBitWidth() &&
+         Node.getBitWidthValue() == Width;
 }
 
 /// Matches non-static data members that have an in-class initializer.
diff --git a/clang/lib/AST/Decl.cpp b/clang/lib/AST/Decl.cpp
index 8855d0107daca..f4cb5a026e25f 100644
--- a/clang/lib/AST/Decl.cpp
+++ b/clang/lib/AST/Decl.cpp
@@ -4677,11 +4677,14 @@ void FieldDecl::setLazyInClassInitializer(LazyDeclStmtPtr NewInit) {
     Init = NewInit;
 }
 
+bool FieldDecl::hasConstantIntegerBitWidth() const {
+  const auto *CE = dyn_cast_if_present<ConstantExpr>(getBitWidth());
+  return CE && CE->getAPValueResult().isInt();
+}
+
 unsigned FieldDecl::getBitWidthValue() const {
   assert(isBitField() && "not a bitfield");
-  assert(isa<ConstantExpr>(getBitWidth()));
-  assert(cast<ConstantExpr>(getBitWidth())->hasAPValueResult());
-  assert(cast<ConstantExpr>(getBitWidth())->getAPValueResult().isInt());
+  assert(hasConstantIntegerBitWidth());
   return cast<ConstantExpr>(getBitWidth())
       ->getAPValueResult()
       .getInt()