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

[via cfe-commits]

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

>From 668c02da2d472c6d13981cd7fdb258419d0ffb05 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/docs/ReleaseNotes.rst                         |  2 ++
 clang/include/clang/AST/Decl.h                      |  5 +++++
 clang/include/clang/ASTMatchers/ASTMatchers.h       |  3 ++-
 clang/lib/AST/Decl.cpp                              |  9 ++++++---
 .../ASTMatchers/ASTMatchersNarrowingTest.cpp        | 13 +++++++++++++
 5 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 57a94242c9e61..0171fb03505cc 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -1088,6 +1088,8 @@ Fixed Point Support in Clang
 AST Matchers
 ------------
 
+- Ensure ``hasBitWidth`` doesn't crash on bit widths that are dependent on template
+  parameters.
 - Ensure ``isDerivedFrom`` matches the correct base in case more than one alias exists.
 - Extend ``templateArgumentCountIs`` to support function and variable template
   specialization.
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()
diff --git a/clang/unittests/ASTMatchers/ASTMatchersNarrowingTest.cpp b/clang/unittests/ASTMatchers/ASTMatchersNarrowingTest.cpp
index 49abe881eeabb..287122393446d 100644
--- a/clang/unittests/ASTMatchers/ASTMatchersNarrowingTest.cpp
+++ b/clang/unittests/ASTMatchers/ASTMatchersNarrowingTest.cpp
@@ -2925,6 +2925,19 @@ TEST_P(ASTMatchersTest, IsBitField) {
                          fieldDecl(isBitField(), hasName("b"))));
   EXPECT_TRUE(matches("struct C { int a : 2; int b : 4; };",
                       fieldDecl(isBitField(), hasBitWidth(2), hasName("a"))));
+  if (GetParam().isCXX()) {
+    // This test verifies 2 things:
+    // (1) That templates work correctly.
+    // (2) That the matcher does not crash on template-dependent bit widths.
+    EXPECT_TRUE(matches("template<int N> "
+                        "struct C { "
+                        "explicit C(bool x) : a(x) { } "
+                        "int a : N; "
+                        "int b : 4; "
+                        "}; "
+                        "template struct C<2>;",
+                        fieldDecl(isBitField(), hasBitWidth(2), hasName("a"))));
+  }
 }
 
 TEST_P(ASTMatchersTest, HasInClassInitializer) {