Implement mangling rules for C++20 concepts and requires-expressions.

This implements proposals from:

- itanium-cxx-abi/cxx-abi#24: mangling for
  constraints, requires-clauses, requires-expressions.
- itanium-cxx-abi/cxx-abi#31: requires-clauses and
  template parameters in a lambda expression are mangled into the <lambda-sig>.
- itanium-cxx-abi/cxx-abi#47 (STEP 3): mangling for
  template argument is prefixed by mangling of template parameter declaration
  if it's not "obvious", for example because the template parameter is
  constrained (we already implemented STEP 1 and STEP 2).

This changes the manglings for a few cases:

- Functions and function templates with constraints.
- Function templates with template parameters with deduced types:
  `typename<auto N> void f();`
- Function templates with template template parameters where the argument has a
  different template-head:
  `template<template<typename...T>> void f(); f<std::vector>();`

In each case where a mangling changed, the change fixes a mangling collision.

Note that only function templates are affected, not class templates or variable
templates, and only new constructs (template parameters with deduced types,
constrained templates) and esoteric constructs (templates with template
template parameters with non-matching template template arguments, most of
which Clang still does not accept by default due to
`-frelaxed-template-template-args` not being enabled by default), so the risk
to ABI stability from this change is relatively low. Nonetheless,
`-fclang-abi-compat=17` can be used to restore the old manglings for cases
which we could successfully but incorrectly mangle before.

Fixes #48216, #49884, #61273

Reviewed By: erichkeane, #libc_abi

Differential Revision: https://reviews.llvm.org/D147655

clang/docs/ReleaseNotes.rst

@@ -46,6 +46,24 @@ C/C++ Language Potentially Breaking Changes
 
 C++ Specific Potentially Breaking Changes
 -----------------------------------------
+- The name mangling rules for function templates has been changed to take into
+  account the possibility that functions could be overloaded on their template
+  parameter lists or requires-clauses. This causes mangled names to change for
+  function templates in the following cases:
+    - When the function has any constraints, whether from constrained template
+      parameters or requires-clauses.
+    - When the template parameter list includes a deduced type -- either
+      ``auto``, ``decltype(auto)``, or a deduced class template specialization
+      type.
+    - When a template template parameter is given a template template argument
+      that has a different template parameter list.
+  This fixes a number of issues where valid programs would be rejected due to
+  mangling collisions, or would in some cases be silently miscompiled. Clang
+  will use the old manglings if ``-fclang-abi-compat=17`` or lower is
+  specified.
+  (`#48216 <https://github.com/llvm/llvm-project/issues/48216>`_),
+  (`#49884 <https://github.com/llvm/llvm-project/issues/49884>`_), and
+  (`#61273 <https://github.com/llvm/llvm-project/issues/61273>`_)
 
 ABI Changes in This Version
 ---------------------------

clang/include/clang/AST/ExprConcepts.h

@@ -511,6 +511,8 @@ class RequiresExpr final : public Expr,
   unsigned NumLocalParameters;
   unsigned NumRequirements;
   RequiresExprBodyDecl *Body;
+  SourceLocation LParenLoc;
+  SourceLocation RParenLoc;
   SourceLocation RBraceLoc;
 
   unsigned numTrailingObjects(OverloadToken<ParmVarDecl *>) const {
@@ -522,19 +524,22 @@ class RequiresExpr final : public Expr,
   }
 
   RequiresExpr(ASTContext &C, SourceLocation RequiresKWLoc,
-               RequiresExprBodyDecl *Body,
+               RequiresExprBodyDecl *Body, SourceLocation LParenLoc,
                ArrayRef<ParmVarDecl *> LocalParameters,
+               SourceLocation RParenLoc,
                ArrayRef<concepts::Requirement *> Requirements,
                SourceLocation RBraceLoc);
   RequiresExpr(ASTContext &C, EmptyShell Empty, unsigned NumLocalParameters,
                unsigned NumRequirements);
 
 public:
-  static RequiresExpr *
-  Create(ASTContext &C, SourceLocation RequiresKWLoc,
-         RequiresExprBodyDecl *Body, ArrayRef<ParmVarDecl *> LocalParameters,
-         ArrayRef<concepts::Requirement *> Requirements,
-         SourceLocation RBraceLoc);
+  static RequiresExpr *Create(ASTContext &C, SourceLocation RequiresKWLoc,
+                              RequiresExprBodyDecl *Body,
+                              SourceLocation LParenLoc,
+                              ArrayRef<ParmVarDecl *> LocalParameters,
+                              SourceLocation RParenLoc,
+                              ArrayRef<concepts::Requirement *> Requirements,
+                              SourceLocation RBraceLoc);
   static RequiresExpr *
   Create(ASTContext &C, EmptyShell Empty, unsigned NumLocalParameters,
          unsigned NumRequirements);
@@ -567,6 +572,8 @@ class RequiresExpr final : public Expr,
     return RequiresExprBits.RequiresKWLoc;
   }
 
+  SourceLocation getLParenLoc() const { return LParenLoc; }
+  SourceLocation getRParenLoc() const { return RParenLoc; }
   SourceLocation getRBraceLoc() const { return RBraceLoc; }
 
   static bool classof(const Stmt *T) {

clang/include/clang/Basic/LangOptions.h

@@ -231,6 +231,15 @@ class LangOptions : public LangOptionsBase {
     ///   - consider classes with defaulted special member functions non-pod.
     Ver15,
 
+    /// Attempt to be ABI-compatible with code generated by Clang 17.0.x.
+    /// This causes clang to revert some fixes to its implementation of the
+    /// Itanium name mangling scheme, with the consequence that overloaded
+    /// function templates are mangled the same if they differ only by:
+    ///   - constraints
+    ///   - whether a non-type template parameter has a deduced type
+    ///   - the parameter list of a template template parameter
+    Ver17,
+
     /// Conform to the underlying platform's C and C++ ABIs as closely
     /// as we can.
     Latest

clang/include/clang/Sema/Sema.h

@@ -8724,7 +8724,9 @@ class Sema final {
                          const ASTConstraintSatisfaction &Satisfaction);
   ExprResult ActOnRequiresExpr(SourceLocation RequiresKWLoc,
                                RequiresExprBodyDecl *Body,
+                               SourceLocation LParenLoc,
                                ArrayRef<ParmVarDecl *> LocalParameters,
+                               SourceLocation RParenLoc,
                                ArrayRef<concepts::Requirement *> Requirements,
                                SourceLocation ClosingBraceLoc);
 

clang/lib/AST/ExprConcepts.cpp

@@ -117,13 +117,15 @@ static bool RequirementContainsError(concepts::Requirement *R) {
 }
 
 RequiresExpr::RequiresExpr(ASTContext &C, SourceLocation RequiresKWLoc,
-                           RequiresExprBodyDecl *Body,
+                           RequiresExprBodyDecl *Body, SourceLocation LParenLoc,
                            ArrayRef<ParmVarDecl *> LocalParameters,
+                           SourceLocation RParenLoc,
                            ArrayRef<concepts::Requirement *> Requirements,
                            SourceLocation RBraceLoc)
     : Expr(RequiresExprClass, C.BoolTy, VK_PRValue, OK_Ordinary),
       NumLocalParameters(LocalParameters.size()),
-      NumRequirements(Requirements.size()), Body(Body), RBraceLoc(RBraceLoc) {
+      NumRequirements(Requirements.size()), Body(Body), LParenLoc(LParenLoc),
+      RParenLoc(RParenLoc), RBraceLoc(RBraceLoc) {
   RequiresExprBits.IsSatisfied = false;
   RequiresExprBits.RequiresKWLoc = RequiresKWLoc;
   bool Dependent = false;
@@ -168,18 +170,18 @@ RequiresExpr::RequiresExpr(ASTContext &C, EmptyShell Empty,
   : Expr(RequiresExprClass, Empty), NumLocalParameters(NumLocalParameters),
     NumRequirements(NumRequirements) { }
 
-RequiresExpr *
-RequiresExpr::Create(ASTContext &C, SourceLocation RequiresKWLoc,
-                     RequiresExprBodyDecl *Body,
-                     ArrayRef<ParmVarDecl *> LocalParameters,
-                     ArrayRef<concepts::Requirement *> Requirements,
-                     SourceLocation RBraceLoc) {
+RequiresExpr *RequiresExpr::Create(
+    ASTContext &C, SourceLocation RequiresKWLoc, RequiresExprBodyDecl *Body,
+    SourceLocation LParenLoc, ArrayRef<ParmVarDecl *> LocalParameters,
+    SourceLocation RParenLoc, ArrayRef<concepts::Requirement *> Requirements,
+    SourceLocation RBraceLoc) {
   void *Mem =
       C.Allocate(totalSizeToAlloc<ParmVarDecl *, concepts::Requirement *>(
                      LocalParameters.size(), Requirements.size()),
                  alignof(RequiresExpr));
-  return new (Mem) RequiresExpr(C, RequiresKWLoc, Body, LocalParameters,
-                                Requirements, RBraceLoc);
+  return new (Mem)
+      RequiresExpr(C, RequiresKWLoc, Body, LParenLoc, LocalParameters,
+                   RParenLoc, Requirements, RBraceLoc);
 }
 
 RequiresExpr *