[libclang] No longer attempt to get a dependent bit-width

[aaron.ballman]

Summary

Handle template parameter-dependent bit field widths in libclang

In a class template, a bit field's width may depend on a template parameter. In this case the width expression cannot be evaluated.

Previously clang_getFieldDeclBitWidth() would assert, or cause memory unsafety and return an invalid result if assertions are disabled.

This adds a check for this case which returns an error code.

This work was largely taken from an earlier patch which was reverted due to an accidental API duplication (https://reviews.llvm.org/D130303).

Fixes: https://github.com/llvm/llvm-project/issues/56644

[vedgy]

clang/include/clang-c/Index.h

3043

Surround the example with \code and \endcode commands.

3046

"non-dependent" is unclear to me. How about rewording to:

*   int Width = clang_getFieldDeclBitWidth(Cursor);
*   if (Width == -1) {
*     // The bit-field width depends on a template parameter.

or alternatively more specific standardese:

*     // The bit-field width is [not] value-dependent.

[aaron.ballman]

Added \code and \endcode markers, reworded a comment slightly.

[vedgy]

LGTM!

[aaron.ballman]

In D146039#4192923, @vedgy wrote:

LGTM!

Thank you for the quick reviews! I've landed in 48fb6659610a3177e8606681046dfa0d19f67203

clang/docs/ReleaseNotes.rst

 - Introduced the new ``CXIndex`` constructor function
   ``clang_createIndexWithOptions``, which allows overriding precompiled preamble
   storage path.
 
 - Deprecated two functions ``clang_CXIndex_setGlobalOptions`` and
   ``clang_CXIndex_setInvocationEmissionPathOption`` in favor of the new
   function ``clang_createIndexWithOptions`` in order to improve thread safety.
 
+- Added check in ``clang_getFieldDeclBitWidth`` for whether a bit-field
+  has an evaluable bit width. Fixes undefined behavior when called on a
+  bit-field whose width depends on a template paramter.
+
 Static Analyzer
 ---------------
 - Fix incorrect alignment attribute on the this parameter of certain
   non-complete destructors when using the Microsoft ABI.
   (`#60465 <https://github.com/llvm/llvm-project/issues/60465>`_)
 
 .. _release-notes-sanitizers:
 

clang/include/clang-c/Index.h

  * If the cursor does not reference an enum constant declaration, ULLONG_MAX is
  * returned. Since this is also potentially a valid constant value, the kind of
  * the cursor must be verified before calling this function.
  */
 CINDEX_LINKAGE unsigned long long
 clang_getEnumConstantDeclUnsignedValue(CXCursor C);
 
 /**
- * Retrieve the bit width of a bit field declaration as an integer.
+ * Returns non-zero if the cursor specifies a Record member that is a bit-field.
+ */
+CINDEX_LINKAGE unsigned clang_Cursor_isBitField(CXCursor C);
+
+/**
+ * Retrieve the bit width of a bit-field declaration as an integer.
  *
- * If a cursor that is not a bit field declaration is passed in, -1 is returned.
+ * If the cursor does not reference a bit-field, or if the bit-field's width
+ * expression cannot be evaluated, -1 is returned.
+ *
+ * For example:
+ * \code
+ * if (clang_Cursor_isBitField(Cursor)) {
+ *   int Width = clang_getFieldDeclBitWidth(Cursor);
+ *   if (Width != -1) {
+ *     // The bit-field width is not value-dependent.
+ *   }
+ * }
+ * \endcode
  */
 CINDEX_LINKAGE int clang_getFieldDeclBitWidth(CXCursor C);
 
 /**
  * Retrieve the number of non-variadic arguments associated with a given
  * cursor.
  *
  * The number of arguments can be determined for calls as well as for

  * Retrieve the ref-qualifier kind of a function or method.
  *
  * The ref-qualifier is returned for C++ functions or methods. For other types
  * or non-C++ declarations, CXRefQualifier_None is returned.
  */
 CINDEX_LINKAGE enum CXRefQualifierKind clang_Type_getCXXRefQualifier(CXType T);
 
 /**
- * Returns non-zero if the cursor specifies a Record member that is a
- *   bitfield.
- */
-CINDEX_LINKAGE unsigned clang_Cursor_isBitField(CXCursor C);
-
-/**
  * Returns 1 if the base class specified by the cursor with kind
  *   CX_CXXBaseSpecifier is virtual.
  */
 CINDEX_LINKAGE unsigned clang_isVirtualBase(CXCursor);
 
 /**
  * Represents the C++ access control level to a base class for a
  * cursor with kind CX_CXXBaseSpecifier.

clang/tools/libclang/CXType.cpp

 //===- CXType.cpp - Implements 'CXTypes' aspect of libclang ---------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===--------------------------------------------------------------------===//
 //
 // This file implements the 'CXTypes' API hooks in the Clang-C library.
 //
 //===--------------------------------------------------------------------===//
 
+#include "CXType.h"
 #include "CIndexer.h"
 #include "CXCursor.h"
 #include "CXString.h"
 #include "CXTranslationUnit.h"
-#include "CXType.h"
 #include "clang/AST/Decl.h"
 #include "clang/AST/DeclObjC.h"
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/Expr.h"
 #include "clang/AST/Type.h"
 #include "clang/Basic/AddressSpaces.h"
 #include "clang/Frontend/ASTUnit.h"
 #include <optional>

 
 int clang_getFieldDeclBitWidth(CXCursor C) {
   using namespace cxcursor;
 
   if (clang_isDeclaration(C.kind)) {
     const Decl *D = getCursorDecl(C);
 
     if (const FieldDecl *FD = dyn_cast_or_null<FieldDecl>(D)) {
-      if (FD->isBitField())
+      if (FD->isBitField() && !FD->getBitWidth()->isValueDependent())
         return FD->getBitWidthValue(getCursorContext(C));
     }
   }
 
   return -1;
 }
 
 CXType clang_getCanonicalType(CXType CT) {