[clang] Implement P0692R1 from C++20 (access checking on specializations and instantiations)

This patch implements paper P0692R1 from the C++20 standard. Disable usual access checking rules to template argument names in a declaration of partial specializations, explicit instantiation or explicit specialization (C++20 13.7.5/10, 13.9.1/6).
Fixes: https://llvm.org/PR37424
This patch also implements option *A* from this paper P0692R1 from the C++20 standard.
This patch follows the @rsmith suggestion from D78404.

Reviewed By: krisb

Differential Revision: https://reviews.llvm.org/D92024

1 files changed, 30 insertions, 1 deletions

diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index 9393235..2b19ade 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -3091,6 +3091,12 @@ void Parser::ParseDeclarationSpecifiers(DeclSpec &DS,
       return true;
     };
 
+    // Turn off usual access checking for template specializations and
+    // instantiations.
+    bool IsTemplateSpecOrInst =
+        (TemplateInfo.Kind == ParsedTemplateInfo::ExplicitInstantiation ||
+         TemplateInfo.Kind == ParsedTemplateInfo::ExplicitSpecialization);
+
     switch (Tok.getKind()) {
     default:
     DoneWithDeclSpec:
@@ -3261,6 +3267,12 @@ void Parser::ParseDeclarationSpecifiers(DeclSpec &DS,
           isConstructorDeclarator(/*Unqualified*/ false))
         goto DoneWithDeclSpec;
 
+      // C++20 [temp.spec] 13.9/6.
+      // This disables the access checking rules for function template explicit
+      // instantiation and explicit specialization:
+      // - `return type`.
+      SuppressAccessChecks SAC(*this, IsTemplateSpecOrInst);
+
       ParsedType TypeRep =
           Actions.getTypeName(*Next.getIdentifierInfo(), Next.getLocation(),
                               getCurScope(), &SS, false, false, nullptr,
@@ -3268,6 +3280,9 @@ void Parser::ParseDeclarationSpecifiers(DeclSpec &DS,
                               /*WantNontrivialTypeSourceInfo=*/true,
                               isClassTemplateDeductionContext(DSContext));
 
+      if (IsTemplateSpecOrInst)
+        SAC.done();
+
       // If the referenced identifier is not a type, then this declspec is
       // erroneous: We already checked about that it has no type specifier, and
       // C++ doesn't have implicit int.  Diagnose it as a typo w.r.t. to the
@@ -3377,10 +3392,24 @@ void Parser::ParseDeclarationSpecifiers(DeclSpec &DS,
       // In C++, check to see if this is a scope specifier like foo::bar::, if
       // so handle it as such.  This is important for ctor parsing.
       if (getLangOpts().CPlusPlus) {
-        if (TryAnnotateCXXScopeToken(EnteringContext)) {
+        // C++20 [temp.spec] 13.9/6.
+        // This disables the access checking rules for function template
+        // explicit instantiation and explicit specialization:
+        // - `return type`.
+        SuppressAccessChecks SAC(*this, IsTemplateSpecOrInst);
+
+        const bool Success = TryAnnotateCXXScopeToken(EnteringContext);
+
+        if (IsTemplateSpecOrInst)
+          SAC.done();
+
+        if (Success) {
+          if (IsTemplateSpecOrInst)
+            SAC.redelay();
           DS.SetTypeSpecError();
           goto DoneWithDeclSpec;
         }
+
         if (!Tok.is(tok::identifier))
           continue;
       }