[clang] [clang] Represent array refs as `TemplateArgument::Declaration` (PR #80050)

[Andrey Ali Khan Bolshakov via cfe-commits]

https://github.com/bolshakov-a created https://github.com/llvm/llvm-project/pull/80050

This returns (probably temporarily) array-referring NTTP behavior to which was prior to #78041 because ~~I'm fed up~~ have no time to fix regressions.

>From 0ccf1c9eeec7641dfdca9f8a06499512f98ba6da Mon Sep 17 00:00:00 2001
From: Bolshakov <bolsh.andrey at yandex.ru>
Date: Tue, 30 Jan 2024 21:33:34 +0300
Subject: [PATCH] [clang] Represent array refs as
 `TemplateArgument::Declaration`

This returns (probably temporarily) array-referring NTTP behavior
to which was prior to #78041 to avoid regressions.
---
 clang/lib/Sema/SemaTemplate.cpp | 44 ++++++++++++++++++---------------
 1 file changed, 24 insertions(+), 20 deletions(-)

diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index 3f33ecb89502e..9e8d058041f04 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -7417,9 +7417,9 @@ ExprResult Sema::CheckTemplateArgument(NonTypeTemplateParmDecl *Param,
     if (ArgResult.isInvalid())
       return ExprError();
 
-    // Prior to C++20, enforce restrictions on possible template argument
-    // values.
-    if (!getLangOpts().CPlusPlus20 && Value.isLValue()) {
+    if (Value.isLValue()) {
+      APValue::LValueBase Base = Value.getLValueBase();
+      auto *VD = const_cast<ValueDecl *>(Base.dyn_cast<const ValueDecl *>());
       //   For a non-type template-parameter of pointer or reference type,
       //   the value of the constant expression shall not refer to
       assert(ParamType->isPointerType() || ParamType->isReferenceType() ||
@@ -7428,8 +7428,6 @@ ExprResult Sema::CheckTemplateArgument(NonTypeTemplateParmDecl *Param,
       // -- a string literal
       // -- the result of a typeid expression, or
       // -- a predefined __func__ variable
-      APValue::LValueBase Base = Value.getLValueBase();
-      auto *VD = const_cast<ValueDecl *>(Base.dyn_cast<const ValueDecl *>());
       if (Base &&
           (!VD ||
            isa<LifetimeExtendedTemporaryDecl, UnnamedGlobalConstantDecl>(VD))) {
@@ -7437,24 +7435,30 @@ ExprResult Sema::CheckTemplateArgument(NonTypeTemplateParmDecl *Param,
             << Arg->getSourceRange();
         return ExprError();
       }
-      // -- a subobject [until C++20]
-      if (Value.hasLValuePath() && Value.getLValuePath().size() == 1 &&
-          VD && VD->getType()->isArrayType() &&
+
+      if (Value.hasLValuePath() && Value.getLValuePath().size() == 1 && VD &&
+          VD->getType()->isArrayType() &&
           Value.getLValuePath()[0].getAsArrayIndex() == 0 &&
           !Value.isLValueOnePastTheEnd() && ParamType->isPointerType()) {
-        // Per defect report (no number yet):
-        //   ... other than a pointer to the first element of a complete array
-        //       object.
-      } else if (!Value.hasLValuePath() || Value.getLValuePath().size() ||
-                 Value.isLValueOnePastTheEnd()) {
-        Diag(StartLoc, diag::err_non_type_template_arg_subobject)
-          << Value.getAsString(Context, ParamType);
-        return ExprError();
+        SugaredConverted = TemplateArgument(VD, ParamType);
+        CanonicalConverted = TemplateArgument(
+            cast<ValueDecl>(VD->getCanonicalDecl()), CanonParamType);
+        return ArgResult.get();
+      }
+
+      // -- a subobject [until C++20]
+      if (!getLangOpts().CPlusPlus20) {
+        if (!Value.hasLValuePath() || Value.getLValuePath().size() ||
+            Value.isLValueOnePastTheEnd()) {
+          Diag(StartLoc, diag::err_non_type_template_arg_subobject)
+              << Value.getAsString(Context, ParamType);
+          return ExprError();
+        }
+        assert((VD || !ParamType->isReferenceType()) &&
+               "null reference should not be a constant expression");
+        assert((!VD || !ParamType->isNullPtrType()) &&
+               "non-null value of type nullptr_t?");
       }
-      assert((VD || !ParamType->isReferenceType()) &&
-             "null reference should not be a constant expression");
-      assert((!VD || !ParamType->isNullPtrType()) &&
-             "non-null value of type nullptr_t?");
     }
 
     if (Value.isAddrLabelDiff())