Extend init-statement to allow alias-declaration

Implement P2360R0 in C++23 mode and as an extension in older languages mode.
2021-10-08 07:12:46 -04:00 · 2021-10-08 07:12:46 -04:00 · ff013b6100
parent d68b59f3eb
commit ff013b6100
10 changed files with 146 additions and 35 deletions
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@ -134,6 +134,8 @@ C++20 Feature Support
 C++2b Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 - Implemented `P1938R3: if consteval <https://wg21.link/P1938R3>`_.
+- Implemented `P2360R0: Extend init-statement to allow alias-declaration <https://wg21.link/P2360R0>`_.
+

 CUDA Language Changes in Clang
 ------------------------------
--- a/clang/include/clang/Basic/DiagnosticParseKinds.td
+++ b/clang/include/clang/Basic/DiagnosticParseKinds.td
@ -549,6 +549,12 @@ def err_expected_init_in_condition_lparen : Error<
  "variable declaration in condition cannot have a parenthesized initializer">;
 def err_extraneous_rparen_in_condition : Error<
  "extraneous ')' after condition, expected a statement">;
+def ext_alias_in_init_statement : ExtWarn<
+  "alias declaration in this context is a C++2b extension">,
+  InGroup<CXX2b>;
+def warn_cxx20_alias_in_init_statement  : Warning<
+  "alias declaration in this context is incompatible with C++ standards before C++2b">,
+  DefaultIgnore, InGroup<CXXPre2bCompat>;
 def warn_dangling_else : Warning<
  "add explicit braces to avoid dangling else">,
  InGroup<DanglingElse>;
--- a/clang/include/clang/Parse/Parser.h
+++ b/clang/include/clang/Parse/Parser.h
@ -1977,6 +1977,9 @@ private:
                                          Sema::ConditionKind CK,
                                          ForRangeInfo *FRI = nullptr,
                                          bool EnterForConditionScope = false);
+  DeclGroupPtrTy
+  ParseAliasDeclarationInInitStatement(DeclaratorContext Context,
+                                       ParsedAttributesWithRange &Attrs);

  //===--------------------------------------------------------------------===//
  // C++ Coroutines
@ -2396,7 +2399,8 @@ private:
    if (getLangOpts().OpenMP)
      Actions.startOpenMPLoop();
    if (getLangOpts().CPlusPlus)
-      return isCXXSimpleDeclaration(/*AllowForRangeDecl=*/true);
+      return Tok.is(tok::kw_using) ||
+             isCXXSimpleDeclaration(/*AllowForRangeDecl=*/true);
    return isDeclarationSpecifier(true);
  }

--- a/clang/lib/Parse/ParseDeclCXX.cpp
+++ b/clang/lib/Parse/ParseDeclCXX.cpp
@ -678,7 +678,10 @@ Parser::ParseUsingDeclaration(
    SourceLocation UsingLoc, SourceLocation &DeclEnd,
    ParsedAttributesWithRange &PrefixAttrs, AccessSpecifier AS) {
  SourceLocation UELoc;
-  if (TryConsumeToken(tok::kw_enum, UELoc)) {
+  bool InInitStatement = Context == DeclaratorContext::SelectionInit ||
+                         Context == DeclaratorContext::ForInit;
+
+  if (TryConsumeToken(tok::kw_enum, UELoc) && !InInitStatement) {
    // C++20 using-enum
    Diag(UELoc, getLangOpts().CPlusPlus20
                    ? diag::warn_cxx17_compat_using_enum_declaration
@ -714,6 +717,9 @@ Parser::ParseUsingDeclaration(
  ParsedAttributesWithRange MisplacedAttrs(AttrFactory);
  MaybeParseCXX11Attributes(MisplacedAttrs);

+  if (InInitStatement && Tok.isNot(tok::identifier))
+    return nullptr;
+
  UsingDeclarator D;
  bool InvalidDeclarator = ParseUsingDeclarator(Context, D);

@ -732,7 +738,7 @@ Parser::ParseUsingDeclaration(
  }

  // Maybe this is an alias-declaration.
-  if (Tok.is(tok::equal)) {
+  if (Tok.is(tok::equal) || InInitStatement) {
    if (InvalidDeclarator) {
      SkipUntil(tok::semi);
      return nullptr;
--- a/clang/lib/Parse/ParseExprCXX.cpp
+++ b/clang/lib/Parse/ParseExprCXX.cpp
@ -1910,6 +1910,28 @@ Parser::ParseCXXTypeConstructExpression(const DeclSpec &DS) {
  }
 }

+Parser::DeclGroupPtrTy
+Parser::ParseAliasDeclarationInInitStatement(DeclaratorContext Context,
+                                             ParsedAttributesWithRange &Attrs) {
+  assert(Tok.is(tok::kw_using) && "Expected using");
+  assert((Context == DeclaratorContext::ForInit ||
+          Context == DeclaratorContext::SelectionInit) &&
+         "Unexpected Declarator Context");
+  DeclGroupPtrTy DG;
+  SourceLocation DeclStart = ConsumeToken(), DeclEnd;
+
+  DG = ParseUsingDeclaration(Context, {}, DeclStart, DeclEnd, Attrs, AS_none);
+  if (!DG)
+    return DG;
+
+  Diag(DeclStart, !getLangOpts().CPlusPlus2b
+                      ? diag::ext_alias_in_init_statement
+                      : diag::warn_cxx20_alias_in_init_statement)
+      << SourceRange(DeclStart, DeclEnd);
+
+  return DG;
+}
+
 /// ParseCXXCondition - if/switch/while condition expression.
 ///
 ///       condition:
@ -2017,9 +2039,14 @@ Sema::ConditionResult Parser::ParseCXXCondition(StmtResult *InitStmt,

  case ConditionOrInitStatement::InitStmtDecl: {
    WarnOnInit();
+    DeclGroupPtrTy DG;
    SourceLocation DeclStart = Tok.getLocation(), DeclEnd;
-    DeclGroupPtrTy DG = ParseSimpleDeclaration(
-        DeclaratorContext::SelectionInit, DeclEnd, attrs, /*RequireSemi=*/true);
+    if (Tok.is(tok::kw_using))
+      DG = ParseAliasDeclarationInInitStatement(
+          DeclaratorContext::SelectionInit, attrs);
+    else
+      DG = ParseSimpleDeclaration(DeclaratorContext::SelectionInit, DeclEnd,
+                                  attrs, /*RequireSemi=*/true);
    *InitStmt = Actions.ActOnDeclStmt(DG, DeclStart, DeclEnd);
    return ParseCXXCondition(nullptr, Loc, CK);
  }
--- a/clang/lib/Parse/ParseStmt.cpp
+++ b/clang/lib/Parse/ParseStmt.cpp
@ -1823,6 +1823,7 @@ bool Parser::isForRangeIdentifier() {
 /// [C++] for-init-statement:
 /// [C++]   expression-statement
 /// [C++]   simple-declaration
+/// [C++2b] alias-declaration
 ///
 /// [C++0x] for-range-declaration:
 /// [C++0x]   attribute-specifier-seq[opt] type-specifier-seq declarator
@ -1928,36 +1929,42 @@ StmtResult Parser::ParseForStatement(SourceLocation *TrailingElseLoc) {
      Diag(Tok, diag::ext_c99_variable_decl_in_for_loop);
      Diag(Tok, diag::warn_gcc_variable_decl_in_for_loop);
    }
-
-    // In C++0x, "for (T NS:a" might not be a typo for ::
-    bool MightBeForRangeStmt = getLangOpts().CPlusPlus;
-    ColonProtectionRAIIObject ColonProtection(*this, MightBeForRangeStmt);
-
-    SourceLocation DeclStart = Tok.getLocation(), DeclEnd;
-    DeclGroupPtrTy DG = ParseSimpleDeclaration(
-        DeclaratorContext::ForInit, DeclEnd, attrs, false,
-        MightBeForRangeStmt ? &ForRangeInfo : nullptr);
-    FirstPart = Actions.ActOnDeclStmt(DG, DeclStart, Tok.getLocation());
-    if (ForRangeInfo.ParsedForRangeDecl()) {
-      Diag(ForRangeInfo.ColonLoc, getLangOpts().CPlusPlus11 ?
-           diag::warn_cxx98_compat_for_range : diag::ext_for_range);
-      ForRangeInfo.LoopVar = FirstPart;
-      FirstPart = StmtResult();
-    } else if (Tok.is(tok::semi)) {  // for (int x = 4;
-      ConsumeToken();
-    } else if ((ForEach = isTokIdentifier_in())) {
-      Actions.ActOnForEachDeclStmt(DG);
-      // ObjC: for (id x in expr)
-      ConsumeToken(); // consume 'in'
-
-      if (Tok.is(tok::code_completion)) {
-        cutOffParsing();
-        Actions.CodeCompleteObjCForCollection(getCurScope(), DG);
-        return StmtError();
-      }
-      Collection = ParseExpression();
+    DeclGroupPtrTy DG;
+    if (Tok.is(tok::kw_using)) {
+      DG = ParseAliasDeclarationInInitStatement(DeclaratorContext::ForInit,
+                                                attrs);
    } else {
-      Diag(Tok, diag::err_expected_semi_for);
+      // In C++0x, "for (T NS:a" might not be a typo for ::
+      bool MightBeForRangeStmt = getLangOpts().CPlusPlus;
+      ColonProtectionRAIIObject ColonProtection(*this, MightBeForRangeStmt);
+
+      SourceLocation DeclStart = Tok.getLocation(), DeclEnd;
+      DG = ParseSimpleDeclaration(
+          DeclaratorContext::ForInit, DeclEnd, attrs, false,
+          MightBeForRangeStmt ? &ForRangeInfo : nullptr);
+      FirstPart = Actions.ActOnDeclStmt(DG, DeclStart, Tok.getLocation());
+      if (ForRangeInfo.ParsedForRangeDecl()) {
+        Diag(ForRangeInfo.ColonLoc, getLangOpts().CPlusPlus11
+                                        ? diag::warn_cxx98_compat_for_range
+                                        : diag::ext_for_range);
+        ForRangeInfo.LoopVar = FirstPart;
+        FirstPart = StmtResult();
+      } else if (Tok.is(tok::semi)) { // for (int x = 4;
+        ConsumeToken();
+      } else if ((ForEach = isTokIdentifier_in())) {
+        Actions.ActOnForEachDeclStmt(DG);
+        // ObjC: for (id x in expr)
+        ConsumeToken(); // consume 'in'
+
+        if (Tok.is(tok::code_completion)) {
+          cutOffParsing();
+          Actions.CodeCompleteObjCForCollection(getCurScope(), DG);
+          return StmtError();
+        }
+        Collection = ParseExpression();
+      } else {
+        Diag(Tok, diag::err_expected_semi_for);
+      }
    }
  } else {
    ProhibitAttributes(attrs);
--- a/clang/lib/Parse/ParseTentative.cpp
+++ b/clang/lib/Parse/ParseTentative.cpp
@ -483,6 +483,8 @@ Parser::isCXXConditionDeclarationOrInitStatement(bool CanBeInitStatement,
  ConditionDeclarationOrInitStatementState State(*this, CanBeInitStatement,
                                                 CanBeForRangeDecl);

+  if (CanBeInitStatement && Tok.is(tok::kw_using))
+    return ConditionOrInitStatement::InitStmtDecl;
  if (State.update(isCXXDeclarationSpecifier()))
    return State.result();

--- a/clang/test/Parser/cxx2b-init-statement.cpp
+++ b/clang/test/Parser/cxx2b-init-statement.cpp
@ -0,0 +1,29 @@
+// RUN: %clang_cc1 -fsyntax-only -verify=expected -std=c++2b -Wall %s
+// RUN: %clang_cc1 -fsyntax-only -verify=expected,expected-cxx20 -std=c++20 -Wall %s
+
+namespace ns {
+    int i;
+    enum class e {};
+}
+void f() {
+
+    for (using foo = int;true;); //expected-cxx20-warning {{alias declaration in this context is a C++2b extension}}
+
+    switch(using foo = int; 0) { //expected-cxx20-warning {{alias declaration in this context is a C++2b extension}}
+        case 0: break;
+    }
+
+    if(using foo = int; false) {} //expected-cxx20-warning {{alias declaration in this context is a C++2b extension}}
+
+
+    if (using enum ns::e; false){}  // expected-error {{expected '='}}
+
+    for (using ns::i; true;);  // expected-error {{expected '='}}
+
+    if (using ns::i; false){}  // expected-error {{expected '='}}
+
+    switch(using ns::i; 0) {   // expected-error {{expected '='}}
+        case 0: break;
+    }
+
+}
--- a/clang/test/SemaCXX/cxx2b-init-statement.cpp
+++ b/clang/test/SemaCXX/cxx2b-init-statement.cpp
@ -0,0 +1,28 @@
+// RUN: %clang_cc1 -verify -std=c++2b -Wall -Wshadow %s
+
+void f() {
+
+    for (using foo = int;true;) {} //expected-warning {{unused type alias 'foo'}}
+
+    switch(using foo = int; 0) { //expected-warning {{unused type alias 'foo'}}
+        case 0: break;
+    }
+
+    if(using foo = int; false) {} // expected-warning {{unused type alias 'foo'}}
+
+    int x; // expected-warning {{unused variable 'x'}}
+    if(using x = int; true) {}  // expected-warning {{unused type alias 'x'}}
+
+    using y = int; // expected-warning {{unused type alias 'y'}} \
+                   // expected-note 2{{previous declaration is here}}
+
+    if(using y = double; true) {}  // expected-warning {{unused type alias 'y'}} \
+                                   // expected-warning {{declaration shadows a type alias in function 'f'}}
+
+    for(using y = double; true;) { // expected-warning {{declaration shadows a type alias in function 'f'}}
+        y foo = 0;
+        (void)foo;
+        constexpr y var = 0;
+        static_assert(var == 0);
+    }
+}
--- a/clang/www/cxx_status.html
+++ b/clang/www/cxx_status.html
@ -1381,7 +1381,7 @@ C++20, informally referred to as C++2b.</p>
    <tr>
      <td>Extend init-statement to allow alias-declaration</td>
      <td><a href="https://wg21.link/P2360R0">P2360R0</a></td>
-      <td class="none" align="center">No</td>
+      <td class="unreleased" align="center">Clang 14</td>
    </tr>

 </table>