[clang] Implement declcall (P2825) as a SYCL-gated extension

[koparasy]

This adds declcall, an overload-resolution hook from P2825R5. declcall(call-expression) yields a pointer, or pointer to member, to the function that overload resolution would select for the call-expression, which is an unevaluated operand.

The initial implementation is based on a prototype by Hana Dusíková (hanickadot/llvm-project); this PR ports it, fixes correctness issues, and adds test coverage.

Devirtualization. declcall of a qualified call to a virtual member produces a devirtualized pointer-to-member (dispatch skipped). This is represented by a Devirtualize bit on CXXDeclcallExpr, carried into the member-pointer APValue, and lowered via an AllowVirtual parameter in both the Itanium and Microsoft ABIs to a direct pointer rather than a vtable index. The flag is serialized (PCH/modules) and honored by both constant evaluators. Note this yields a pointer-to-member that bypasses virtual dispatch, which has no standard-C++ equivalent.

Feature gating. P2825 is not standardized. declcall is enabled as a native keyword under -fsycl; otherwise it is a Clang extension diagnosed by -Wdeclcall-extension (a warning by default, an error under -pedantic-errors). Outside those modes it remains an ordinary identifier.

Known limitations (design-level, from the paper). When the selected function relies on default arguments, or is an explicit-object-parameter ("deducing this") member function, the resulting pointer's parameter list does not match the argument list written at the call, so it cannot be invoked with those same arguments (e.g. declcall(f(1)) for void f(int, int = 1) yields void(*)(int,int)). These are limitations of the proposal itself and are documented rather than worked around.

Tests. Coverage spans Sema, SFINAE, AST dump, CodeGen (classic and -fexperimental-new-constant-interpreter), Itanium and Microsoft mangling, PCH, and C++20 modules.

[Copilot]

Pull request overview

Implements declcall(call-expression) (P2825R5) in Clang as a SYCL-gated keyword/extension, introducing a new AST node (CXXDeclcallExpr) that resolves a call-expression’s overload and yields the selected function (or member function) pointer, including support for “devirtualized” qualified virtual member calls.

Changes:

• Adds parsing, Sema, AST, serialization, ASTMatchers, and tooling support for CXXDeclcallExpr, including extension diagnostics and keyword gating.
• Extends constant evaluation / interpreter and CodeGen to carry and lower a “devirtualized member pointer” flag.
• Adds broad test coverage across Sema/SFINAE, AST dump, mangling, CodeGen, PCH, and C++20 modules.

Reviewed changes

Copilot reviewed 65 out of 65 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
clang/tools/libclang/CXCursor.cpp	Map CXXDeclcallExpr to libclang cursor kind.
clang/test/SemaCXX/declcall.cpp	Core semantic tests for declcall.
clang/test/SemaCXX/declcall-sfinae.cpp	SFINAE behavior tests.
clang/test/SemaCXX/declcall-extension.cpp	Extension vs SYCL keyword gating tests.
clang/test/PCH/declcall.cpp	PCH serialization test for devirtualize bit.
clang/test/CodeGenCXX/mangle-declcall.cpp	Itanium mangling coverage for dependent declcall.
clang/test/CodeGenCXX/declcall.cpp	CodeGen + constant-interpreter coverage.
clang/test/CodeGenCXX/declcall-modules.cpp	C++20 modules serialization coverage.
clang/test/AST/ast-dump-declcall.cpp	AST dump coverage for declcall + devirtualized marker.
clang/lib/StaticAnalyzer/Core/ExprEngine.cpp	Static analyzer visitor handling for declcall.
clang/lib/Serialization/ASTWriterStmt.cpp	Serialize CXXDeclcallExpr (+ devirtualize flag).
clang/lib/Serialization/ASTReaderStmt.cpp	Deserialize CXXDeclcallExpr (+ devirtualize flag).
clang/lib/Sema/TreeTransform.h	Template transformation support for CXXDeclcallExpr.
clang/lib/Sema/SemaFixItUtils.cpp	Parentheses heuristics updated for CXXDeclcallExpr.
clang/lib/Sema/SemaExprCXX.cpp	Sema build/diagnose logic for declcall.
clang/lib/Sema/SemaExpr.cpp	Address-of member-function operand checking touched.
clang/lib/Sema/SemaExceptionSpec.cpp	canThrow classification includes declcall.
clang/lib/Parse/ParseExpr.cpp	Parser support for declcall(...).
clang/lib/Edit/RewriteObjCFoundationAPI.cpp	Parentheses heuristics updated for CXXDeclcallExpr.
clang/lib/CodeGen/MicrosoftCXXABI.cpp	ABI interface extended with AllowVirtual knobs.
clang/lib/CodeGen/ItaniumCXXABI.cpp	Itanium member-pointer emission honors AllowVirtual.
clang/lib/CodeGen/CGExprScalar.cpp	Scalar emission support for CXXDeclcallExpr.
clang/lib/CodeGen/CGExprConstant.cpp	Member-pointer constant emission passes AllowVirtual.
clang/lib/CodeGen/CGCXXABI.h	ABI interface extended for devirtualization control.
clang/lib/CodeGen/CGCXXABI.cpp	Base ABI stubs updated for new parameters.
clang/lib/Basic/IdentifierTable.cpp	Add KEYCXX26 status + compat diagnostics.
clang/lib/ASTMatchers/Dynamic/Registry.cpp	Register cxxDeclcallExpr matcher.
clang/lib/ASTMatchers/ASTMatchersInternal.cpp	Define internal matcher object.
clang/lib/AST/TextNodeDumper.cpp	Dump member-pointer APValue “(devirtualized)” marker.
clang/lib/AST/StmtProfile.cpp	Stmt profiling visitor for CXXDeclcallExpr.
clang/lib/AST/StmtPrinter.cpp	Pretty-printer support for declcall(...).
clang/lib/AST/ItaniumMangle.cpp	Vendor-extended mangling for declcall expressions.
clang/lib/AST/ExprConstant.cpp	Constant-eval propagation for devirtualized member pointers.
clang/lib/AST/ExprClassification.cpp	Expression classification includes declcall.
clang/lib/AST/Expr.cpp	Side-effect classification includes declcall.
clang/lib/AST/ComputeDependence.cpp	Dependence computation for CXXDeclcallExpr.
clang/lib/AST/ByteCode/Opcodes.td	New opcode for member-pointer devirtualization.
clang/lib/AST/ByteCode/MemberPointer.h	Interpreter member-pointer tracks devirtualized bit.
clang/lib/AST/ByteCode/MemberPointer.cpp	APValue conversion preserves devirtualized bit.
clang/lib/AST/ByteCode/Interp.h	Declare interpreter opcode handler.
clang/lib/AST/ByteCode/Interp.cpp	Implement devirtualize-member-pointer opcode.
clang/lib/AST/ByteCode/Compiler.h	Add bytecode compiler visitor for declcall.
clang/lib/AST/ByteCode/Compiler.cpp	Emit bytecode for declcall + devirtualization.
clang/lib/AST/ASTImporter.cpp	Import CXXDeclcallExpr and APValue member-pointer flag.
clang/lib/AST/APValue.cpp	Extend member-pointer APValue representation with flag.
clang/lib/Analysis/UnsafeBufferUsage.cpp	Treat declcall as unevaluated context in traversal.
clang/include/clang/Serialization/ASTBitCodes.h	Add bitcode tag for CXXDeclcallExpr.
clang/include/clang/Sema/Sema.h	Add Sema entry points for declcall.
clang/include/clang/Basic/TokenKinds.h	Add KEYCXX26 and update keyword masks.
clang/include/clang/Basic/TokenKinds.def	Define declcall as a C++26 keyword (SYCL-gated).
clang/include/clang/Basic/StmtNodes.td	Register CXXDeclcallExpr AST node.
clang/include/clang/Basic/DiagnosticSemaKinds.td	Add declcall extension/error diagnostics.
clang/include/clang/Basic/DiagnosticLexKinds.td	Add C++26 keyword compatibility warning.
clang/include/clang/Basic/DiagnosticGroups.td	Add -Wdeclcall-extension group.
clang/include/clang/ASTMatchers/ASTMatchers.h	Expose cxxDeclcallExpr matcher in public API.
clang/include/clang/AST/TextNodeDumper.h	Update dumper visitor declarations.
clang/include/clang/AST/RecursiveASTVisitor.h	Add traversal macro for CXXDeclcallExpr.
clang/include/clang/AST/PropertiesBase.td	Add APValue member-pointer flag to properties layer.
clang/include/clang/AST/ExprCXX.h	Define CXXDeclcallExpr node.
clang/include/clang/AST/EvaluatedExprVisitor.h	Mark declcall as unevaluated for evaluated-expr visitor.
clang/include/clang/AST/ComputeDependence.h	Declare dependence computation for declcall.
clang/include/clang/AST/APValue.h	Public API for member-pointer devirtualized flag.
clang/docs/ReleaseNotes.md	Document declcall support as an extension.
clang/docs/LanguageExtensions.md	Add language extension documentation for declcall.
clang-tools-extra/clang-tidy/utils/Matchers.h	Treat declcall as an unevaluated context matcher.

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