[clang] Accept lambdas in C++03 as an extensions

This is a fairly simple extension, but makes the life for people who
have to support C++03 a lot easier. As a nice bonus, this also improves
diagnostics, since lambdas are now properly recognized when parsing
C++03 code.

Author: philnik777

clang/docs/LanguageExtensions.rst

@@ -1458,40 +1458,45 @@ More information could be found `here <https://clang.llvm.org/docs/Modules.html>
 Language Extensions Back-ported to Previous Standards
 =====================================================
 
-====================================== ================================ ============= =============
-Feature                                Feature Test Macro               Introduced In Backported To
-====================================== ================================ ============= =============
-variadic templates                     __cpp_variadic_templates         C++11         C++03
-Alias templates                        __cpp_alias_templates            C++11         C++03
-Non-static data member initializers    __cpp_nsdmi                      C++11         C++03
-Range-based ``for`` loop               __cpp_range_based_for            C++11         C++03
-RValue references                      __cpp_rvalue_references          C++11         C++03
-Attributes                             __cpp_attributes                 C++11         C++03
-variable templates                     __cpp_variable_templates         C++14         C++03
-Binary literals                        __cpp_binary_literals            C++14         C++03
-Relaxed constexpr                      __cpp_constexpr                  C++14         C++11
-``if constexpr``                       __cpp_if_constexpr               C++17         C++11
-fold expressions                       __cpp_fold_expressions           C++17         C++03
-Lambda capture of \*this by value      __cpp_capture_star_this          C++17         C++11
-Attributes on enums                    __cpp_enumerator_attributes      C++17         C++03
-Guaranteed copy elision                __cpp_guaranteed_copy_elision    C++17         C++03
-Hexadecimal floating literals          __cpp_hex_float                  C++17         C++03
-``inline`` variables                   __cpp_inline_variables           C++17         C++03
-Attributes on namespaces               __cpp_namespace_attributes       C++17         C++11
-Structured bindings                    __cpp_structured_bindings        C++17         C++03
-template template arguments            __cpp_template_template_args     C++17         C++03
-``static operator[]``                  __cpp_multidimensional_subscript C++20         C++03
-Designated initializers                __cpp_designated_initializers    C++20         C++03
-Conditional ``explicit``               __cpp_conditional_explicit       C++20         C++03
-``using enum``                         __cpp_using_enum                 C++20         C++03
-``if consteval``                       __cpp_if_consteval               C++23         C++20
-``static operator()``                  __cpp_static_call_operator       C++23         C++03
-Attributes on Lambda-Expressions                                        C++23         C++11
--------------------------------------- -------------------------------- ------------- -------------
-Designated initializers (N494)                                          C99           C89
-Array & element qualification (N2607)                                   C23           C89
-Attributes (N2335)                                                      C23           C89
-====================================== ================================ ============= =============
+============================================ ================================ ============= =============
+Feature                                      Feature Test Macro               Introduced In Backported To
+============================================ ================================ ============= =============
+variadic templates                           __cpp_variadic_templates         C++11         C++03
+Alias templates                              __cpp_alias_templates            C++11         C++03
+Non-static data member initializers          __cpp_nsdmi                      C++11         C++03
+Range-based ``for`` loop                     __cpp_range_based_for            C++11         C++03
+RValue references                            __cpp_rvalue_references          C++11         C++03
+Attributes                                   __cpp_attributes                 C++11         C++03
+Lambdas                                      __cpp_lambdas                    C++11         C++03
+Generalized lambda captures                  __cpp_init_captures              C++14         C++03
+Generic lambda expressions                   __cpp_generic_lambdas            C++14         C++03
+variable templates                           __cpp_variable_templates         C++14         C++03
+Binary literals                              __cpp_binary_literals            C++14         C++03
+Relaxed constexpr                            __cpp_constexpr                  C++14         C++11
+Pack expansion in generalized lambda-capture __cpp_init_captures              C++17         C++03
+``if constexpr``                             __cpp_if_constexpr               C++17         C++11
+fold expressions                             __cpp_fold_expressions           C++17         C++03
+Lambda capture of \*this by value            __cpp_capture_star_this          C++17         C++03
+Attributes on enums                          __cpp_enumerator_attributes      C++17         C++03
+Guaranteed copy elision                      __cpp_guaranteed_copy_elision    C++17         C++03
+Hexadecimal floating literals                __cpp_hex_float                  C++17         C++03
+``inline`` variables                         __cpp_inline_variables           C++17         C++03
+Attributes on namespaces                     __cpp_namespace_attributes       C++17         C++11
+Structured bindings                          __cpp_structured_bindings        C++17         C++03
+template template arguments                  __cpp_template_template_args     C++17         C++03
+Familiar template syntax for generic lambdas __cpp_generic_lambdas            C++20         C++03
+``static operator[]``                        __cpp_multidimensional_subscript C++20         C++03
+Designated initializers                      __cpp_designated_initializers    C++20         C++03
+Conditional ``explicit``                     __cpp_conditional_explicit       C++20         C++03
+``using enum``                               __cpp_using_enum                 C++20         C++03
+``if consteval``                             __cpp_if_consteval               C++23         C++20
+``static operator()``                        __cpp_static_call_operator       C++23         C++03
+Attributes on Lambda-Expressions                                              C++23         C++11
+-------------------------------------------- -------------------------------- ------------- -------------
+Designated initializers (N494)                                                C99           C89
+Array & element qualification (N2607)                                         C23           C89
+Attributes (N2335)                                                            C23           C89
+============================================ ================================ ============= =============
 
 Type Trait Primitives
 =====================

clang/include/clang/Basic/DiagnosticParseKinds.td

@@ -1026,6 +1026,7 @@ def err_expected_lambda_body : Error<"expected body of lambda expression">;
 def warn_cxx98_compat_lambda : Warning<
   "lambda expressions are incompatible with C++98">,
   InGroup<CXX98Compat>, DefaultIgnore;
+def ext_lambda : ExtWarn<"lambdas are a C++11 extension">, InGroup<CXX11>;
 def err_lambda_decl_specifier_repeated : Error<
   "%select{'mutable'|'static'|'constexpr'|'consteval'}0 cannot "
   "appear multiple times in a lambda declarator">;

clang/include/clang/Basic/Features.def

@@ -257,6 +257,7 @@ EXTENSION(cxx_defaulted_functions, LangOpts.CPlusPlus)
 EXTENSION(cxx_deleted_functions, LangOpts.CPlusPlus)
 EXTENSION(cxx_explicit_conversions, LangOpts.CPlusPlus)
 EXTENSION(cxx_inline_namespaces, LangOpts.CPlusPlus)
+EXTENSION(cxx_lambdas, LangOpts.CPlusPlus)
 EXTENSION(cxx_local_type_template_args, LangOpts.CPlusPlus)
 EXTENSION(cxx_nonstatic_member_init, LangOpts.CPlusPlus)
 EXTENSION(cxx_override_control, LangOpts.CPlusPlus)

clang/lib/Parse/ParseExpr.cpp

@@ -1810,7 +1810,7 @@ ExprResult Parser::ParseCastExpression(CastParseKind ParseKind,
     }
     goto ExpectedExpression;
   case tok::l_square:
-    if (getLangOpts().CPlusPlus11) {
+    if (getLangOpts().CPlusPlus) {
       if (getLangOpts().ObjC) {
         // C++11 lambda expressions and Objective-C message sends both start with a
         // square bracket.  There are three possibilities here:

clang/lib/Parse/ParseExprCXX.cpp

@@ -806,9 +806,8 @@ ExprResult Parser::ParseLambdaExpression() {
 ///
 /// If we are not looking at a lambda expression, returns ExprError().
 ExprResult Parser::TryParseLambdaExpression() {
-  assert(getLangOpts().CPlusPlus11
-         && Tok.is(tok::l_square)
-         && "Not at the start of a possible lambda expression.");
+  assert(getLangOpts().CPlusPlus && Tok.is(tok::l_square) &&
+         "Not at the start of a possible lambda expression.");
 
   const Token Next = NextToken();
   if (Next.is(tok::eof)) // Nothing else to lookup here...
@@ -1326,7 +1325,9 @@ static void DiagnoseStaticSpecifierRestrictions(Parser &P,
 ExprResult Parser::ParseLambdaExpressionAfterIntroducer(
                      LambdaIntroducer &Intro) {
   SourceLocation LambdaBeginLoc = Intro.Range.getBegin();
-  Diag(LambdaBeginLoc, diag::warn_cxx98_compat_lambda);
+  Diag(LambdaBeginLoc, getLangOpts().CPlusPlus11
+                           ? diag::warn_cxx98_compat_lambda
+                           : diag::ext_lambda);
 
   PrettyStackTraceLoc CrashInfo(PP.getSourceManager(), LambdaBeginLoc,
                                 "lambda expression parsing");

clang/lib/Parse/ParseInit.cpp

@@ -35,7 +35,7 @@ bool Parser::MayBeDesignationStart() {
     return true;
 
   case tok::l_square: {  // designator: array-designator
-    if (!PP.getLangOpts().CPlusPlus11)
+    if (!PP.getLangOpts().CPlusPlus)
       return true;
 
     // C++11 lambda expressions and C99 designators can be ambiguous all the

clang/lib/Sema/SemaDecl.cpp

@@ -16111,7 +16111,7 @@ Decl *Sema::ActOnFinishFunctionBody(Decl *dcl, Stmt *Body,
               FD->setInvalidDecl();
           }
         }
-      } else if (getLangOpts().CPlusPlus11 && isLambdaCallOperator(FD)) {
+      } else if (getLangOpts().CPlusPlus && isLambdaCallOperator(FD)) {
         // In C++11, we don't use 'auto' deduction rules for lambda call
         // operators because we don't support return type deduction.
         auto *LSI = getCurLambda();

clang/lib/Sema/SemaDeclCXX.cpp

@@ -9738,7 +9738,8 @@ bool Sema::ShouldDeleteSpecialMember(CXXMethodDecl *MD, CXXSpecialMember CSM,
     return false;
   CXXRecordDecl *RD = MD->getParent();
   assert(!RD->isDependentType() && "do deletion after instantiation");
-  if (!LangOpts.CPlusPlus11 || RD->isInvalidDecl())
+  if (!LangOpts.CPlusPlus || (!LangOpts.CPlusPlus11 && !RD->isLambda()) ||
+      RD->isInvalidDecl())
     return false;
 
   // C++11 [expr.lambda.prim]p19:

clang/test/Lexer/has_extension_cxx.cpp

@@ -33,6 +33,11 @@ int has_deleted_functions();
 int has_inline_namespaces();
 #endif
 
+// CHECK: has_lambdas
+#if __has_extension(cxx_lambdas)
+int has_lambdas();
+#endif
+
 // CHECK: has_override_control
 #if __has_extension(cxx_override_control)
 int has_override_control();

clang/test/OpenMP/declare_reduction_messages.cpp

@@ -58,16 +58,10 @@ class Class2 : public Class1<T> {
 #pragma omp declare reduction(fun1 : long : omp_out += omp_in) initializer                              // expected-error {{expected '(' after 'initializer'}}
 #pragma omp declare reduction(fun2 : long : omp_out += omp_in) initializer {                            // expected-error {{expected '(' after 'initializer'}} expected-error {{expected expression}} expected-warning {{extra tokens at the end of '#pragma omp declare reduction' are ignored}}
 #pragma omp declare reduction(fun3 : long : omp_out += omp_in) initializer[
-#if __cplusplus <= 199711L
-// expected-error@-2 {{expected '(' after 'initializer'}}
-// expected-error@-3 {{expected expression}}
-// expected-warning@-4 {{extra tokens at the end of '#pragma omp declare reduction' are ignored}}
-#else
-// expected-error@-6 {{expected '(' after 'initializer'}}
-// expected-error@-7 {{expected variable name or 'this' in lambda capture list}}
-// expected-error@-8 {{expected ')'}}
-// expected-note@-9 {{to match this '('}}
-#endif
+// expected-error@-1 {{expected '(' after 'initializer'}}
+// expected-error@-2 {{expected variable name or 'this' in lambda capture list}}
+// expected-error@-3 {{expected ')'}}
+// expected-note@-4 {{to match this '('}}
 #pragma omp declare reduction(fun4 : long : omp_out += omp_in) initializer()                            // expected-error {{expected expression}}
 #pragma omp declare reduction(fun5 : long : omp_out += omp_in) initializer(temp)                        // expected-error {{only 'omp_priv' or 'omp_orig' variables are allowed in initializer expression}}
 #pragma omp declare reduction(fun6 : long : omp_out += omp_in) initializer(omp_orig                     // expected-error {{expected ')'}} expected-note {{to match this '('}}