[C++] Implement "Deducing this" (P0847R7)

This patch implements P0847R7 (partially),
CWG2561 and CWG2653.

Reviewed By: aaron.ballman, #clang-language-wg

Differential Revision: https://reviews.llvm.org/D140828

1 files changed, 25 insertions, 13 deletions

diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index 9bda4ec..a149159 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -5835,8 +5835,7 @@ bool Parser::isDeclarationSpecifier(
 bool Parser::isConstructorDeclarator(bool IsUnqualified, bool DeductionGuide,
                                      DeclSpec::FriendSpecified IsFriend,
                                      const ParsedTemplateInfo *TemplateInfo) {
-  TentativeParsingAction TPA(*this);
-
+  RevertingTentativeParsingAction TPA(*this);
   // Parse the C++ scope specifier.
   CXXScopeSpec SS;
   if (TemplateInfo && TemplateInfo->TemplateParams)
@@ -5845,7 +5844,6 @@ bool Parser::isConstructorDeclarator(bool IsUnqualified, bool DeductionGuide,
   if (ParseOptionalCXXScopeSpecifier(SS, /*ObjectType=*/nullptr,
                                      /*ObjectHasErrors=*/false,
                                      /*EnteringContext=*/true)) {
-    TPA.Revert();
     return false;
   }
 
@@ -5857,7 +5855,6 @@ bool Parser::isConstructorDeclarator(bool IsUnqualified, bool DeductionGuide,
   } else if (Tok.is(tok::annot_template_id)) {
     ConsumeAnnotationToken();
   } else {
-    TPA.Revert();
     return false;
   }
 
@@ -5867,7 +5864,6 @@ bool Parser::isConstructorDeclarator(bool IsUnqualified, bool DeductionGuide,
 
   // Current class name must be followed by a left parenthesis.
   if (Tok.isNot(tok::l_paren)) {
-    TPA.Revert();
     return false;
   }
   ConsumeParen();
@@ -5876,7 +5872,6 @@ bool Parser::isConstructorDeclarator(bool IsUnqualified, bool DeductionGuide,
   // that we have a constructor.
   if (Tok.is(tok::r_paren) ||
       (Tok.is(tok::ellipsis) && NextToken().is(tok::r_paren))) {
-    TPA.Revert();
     return true;
   }
 
@@ -5885,7 +5880,6 @@ bool Parser::isConstructorDeclarator(bool IsUnqualified, bool DeductionGuide,
   if (getLangOpts().CPlusPlus11 &&
       isCXX11AttributeSpecifier(/*Disambiguate*/ false,
                                 /*OuterMightBeMessageSend*/ true)) {
-    TPA.Revert();
     return true;
   }
 
@@ -5906,9 +5900,17 @@ bool Parser::isConstructorDeclarator(bool IsUnqualified, bool DeductionGuide,
   // If we parsed a scope specifier as well as friend,
   // we might be parsing a friend constructor.
   bool IsConstructor = false;
-  if (isDeclarationSpecifier(IsFriend && !SS.isSet()
-                                 ? ImplicitTypenameContext::No
-                                 : ImplicitTypenameContext::Yes))
+  ImplicitTypenameContext ITC = IsFriend && !SS.isSet()
+                                    ? ImplicitTypenameContext::No
+                                    : ImplicitTypenameContext::Yes;
+  // Constructors cannot have this parameters, but we support that scenario here
+  // to improve diagnostic.
+  if (Tok.is(tok::kw_this)) {
+    ConsumeToken();
+    return isDeclarationSpecifier(ITC);
+  }
+
+  if (isDeclarationSpecifier(ITC))
     IsConstructor = true;
   else if (Tok.is(tok::identifier) ||
            (Tok.is(tok::annot_cxxscope) && NextToken().is(tok::identifier))) {
@@ -5977,8 +5979,6 @@ bool Parser::isConstructorDeclarator(bool IsUnqualified, bool DeductionGuide,
       break;
     }
   }
-
-  TPA.Revert();
   return IsConstructor;
 }
 
@@ -7356,6 +7356,7 @@ void Parser::ParseFunctionDeclaratorIdentifierList(
 ///           '=' assignment-expression
 /// [GNU]   declaration-specifiers abstract-declarator[opt] attributes
 /// [C++11] attribute-specifier-seq parameter-declaration
+/// [C++2b] attribute-specifier-seq 'this' parameter-declaration
 ///
 void Parser::ParseParameterDeclarationClause(
     DeclaratorContext DeclaratorCtx, ParsedAttributes &FirstArgAttrs,
@@ -7420,9 +7421,16 @@ void Parser::ParseParameterDeclarationClause(
 
     SourceLocation DSStart = Tok.getLocation();
 
+    // Parse a C++23 Explicit Object Parameter
+    // We do that in all language modes to produce a better diagnostic.
+    SourceLocation ThisLoc;
+    if (getLangOpts().CPlusPlus && Tok.is(tok::kw_this))
+      ThisLoc = ConsumeToken();
+
     ParseDeclarationSpecifiers(DS, /*TemplateInfo=*/ParsedTemplateInfo(),
                                AS_none, DeclSpecContext::DSC_normal,
                                /*LateAttrs=*/nullptr, AllowImplicitTypename);
+
     DS.takeAttributesFrom(ArgDeclSpecAttrs);
 
     // Parse the declarator.  This is "PrototypeContext" or
@@ -7436,6 +7444,9 @@ void Parser::ParseParameterDeclarationClause(
                                   : DeclaratorContext::Prototype);
     ParseDeclarator(ParmDeclarator);
 
+    if (ThisLoc.isValid())
+      ParmDeclarator.SetRangeBegin(ThisLoc);
+
     // Parse GNU attributes, if present.
     MaybeParseGNUAttributes(ParmDeclarator);
     if (getLangOpts().HLSL)
@@ -7505,7 +7516,8 @@ void Parser::ParseParameterDeclarationClause(
       }
       // Inform the actions module about the parameter declarator, so it gets
       // added to the current scope.
-      Decl *Param = Actions.ActOnParamDeclarator(getCurScope(), ParmDeclarator);
+      Decl *Param =
+          Actions.ActOnParamDeclarator(getCurScope(), ParmDeclarator, ThisLoc);
       // Parse the default argument, if any. We parse the default
       // arguments in all dialects; the semantic analysis in
       // ActOnParamDefaultArgument will reject the default argument in