Stop modifying trailing return types.

[rsmith]

Summary

This change reverts the functional change from D144626 but retains its
test. Instead of dealing with the possibility that a trailing requires
clause might have been rewritten into some other incorrect form, just
stop rewriting it.

No functionality changes intended.

[erichkeane]

I think this makes sense to me, so LGTM. But please give @ChuanqiXu a chance to look/ask questions.

[ChuanqiXu]

Thanks! It looks much better indeed.

[thakis]

Looks like this caused a change in behavior: https://github.com/llvm/llvm-project/issues/62361

clang/lib/AST/ASTContext.cpp

if (const auto *FuncX = dyn_cast<FunctionDecl>(X)) {
     // not the same entity if they are constrained.
     if ((FuncX->isMemberLikeConstrainedFriend() ||
          FuncY->isMemberLikeConstrainedFriend()) &&
         !FuncX->getLexicalDeclContext()->Equals(
             FuncY->getLexicalDeclContext())) {
       return false;
     }
 
-    // The trailing require clause of instantiated function may change during
-    // the semantic analysis. Trying to get the primary template function (if
-    // exists) to compare the primary trailing require clause.
-    auto TryToGetPrimaryTemplatedFunction =
-        [](const FunctionDecl *FD) -> const FunctionDecl * {
-      switch (FD->getTemplatedKind()) {
-      case FunctionDecl::TK_DependentNonTemplate:
-        return FD->getInstantiatedFromDecl();
-      case FunctionDecl::TK_FunctionTemplate:
-        return FD->getDescribedFunctionTemplate()->getTemplatedDecl();
-      case FunctionDecl::TK_MemberSpecialization:
-        return FD->getInstantiatedFromMemberFunction();
-      case FunctionDecl::TK_FunctionTemplateSpecialization:
-        return FD->getPrimaryTemplate()->getTemplatedDecl();
-      default:
-        return FD;
-      }
-    };
-    const FunctionDecl *PrimaryX = TryToGetPrimaryTemplatedFunction(FuncX);
-    const FunctionDecl *PrimaryY = TryToGetPrimaryTemplatedFunction(FuncY);
-    if (!isSameConstraintExpr(PrimaryX->getTrailingRequiresClause(),
-                              PrimaryY->getTrailingRequiresClause()))
+    if (!isSameConstraintExpr(FuncX->getTrailingRequiresClause(),
+                              FuncY->getTrailingRequiresClause()))
       return false;
 
     auto GetTypeAsWritten = [](const FunctionDecl *FD) {
       // Map to the first declaration that we've already merged into this one.
       // The TSI of redeclarations might not match (due to calling conventions
       // being inherited onto the type but not the TSI), but the TSI type of
       // the first declaration of the function should match across modules.
       FD = FD->getCanonicalDecl();

clang/lib/Sema/SemaConcept.cpp

bool Sema::CheckFunctionConstraints(const FunctionDecl *FD,
 
   Qualifiers ThisQuals;
   CXXRecordDecl *Record = nullptr;
   if (auto *Method = dyn_cast<CXXMethodDecl>(FD)) {
     ThisQuals = Method->getMethodQualifiers();
     Record = const_cast<CXXRecordDecl *>(Method->getParent());
   }
   CXXThisScopeRAII ThisScope(*this, Record, ThisQuals, Record != nullptr);
-  // We substitute with empty arguments in order to rebuild the atomic
-  // constraint in a constant-evaluated context.
-  // FIXME: Should this be a dedicated TreeTransform?
-  const Expr *RC = FD->getTrailingRequiresClause();
-  llvm::SmallVector<Expr *, 1> Converted;
-
-  if (CheckConstraintSatisfaction(
-          FD, {RC}, Converted, *MLTAL,
-          SourceRange(UsageLoc.isValid() ? UsageLoc : FD->getLocation()),
-          Satisfaction))
-    return true;
-
-  // FIXME: we need to do this for the function constraints for
-  // comparison of constraints to work, but do we also need to do it for
-  // CheckInstantiatedFunctionConstraints?  That one is more difficult, but we
-  // seem to always just pick up the constraints from the primary template.
-  assert(Converted.size() <= 1 && "Got more expressions converted?");
-  if (!Converted.empty() && Converted[0] != nullptr)
-    const_cast<FunctionDecl *>(FD)->setTrailingRequiresClause(Converted[0]);
-  return false;
+  return CheckConstraintSatisfaction(
+      FD, {FD->getTrailingRequiresClause()}, *MLTAL,
+      Satisfaction);
 }
 
 
 // Figure out the to-translation-unit depth for this function declaration for
 // the purpose of seeing if they differ by constraints. This isn't the same as
 // getTemplateDepth, because it includes already instantiated parents.
 static unsigned
 CalculateTemplateDepthForConstraints(Sema &S, const NamedDecl *ND,