[C++14][Sema] Disallow decltype(auto) deduction for lambdas

[Rakete1111]

Summary

decltype(auto) lambda = []{}; is currently allowed by clang, gcc and MSVC, but it is actually not valid, because decltype([]{}) (which is how the type of lambda is deduced) is ill-formed, I think. I honestly could argue both ways (and I did once), but the fact that decltype(auto) list = {0,1}; is already rejected by all three, it seems like the most sensible way is to disallow it for lambdas too.

[rsmith]

decltype(auto) lambda = []{}; and decltype(auto) list = {0,1}; seem like very different situations to me; in the latter case, the initializer is not even an expression.

The rule that bans decltype([]{}) is that a *lambda-expression* shall not appear in an unevaluated operand. But the initializer in decltype(auto) lambda = []{}; is not an unevaluated operand. The relevant rule is [dcl.type.auto.deduct]p5, which says: "The type deduced for T is determined as described in 10.1.7.2, as though e had been the operand of the decltype." -- so, while this is certainly not completely clear, this suggests that we only consider the expression as the operand of decltype for the purpose of determining the type, not for the purpose of determining whether the deduction is valid. And certainly seems to be the interpretation that is more friendly to users.

[Rakete1111]

Agreed. Closing this.

include/clang/Basic/DiagnosticSemaKinds.td

   "cannot form %select{pointer to|reference to|array of}0 'decltype(auto)'">;
 def err_decltype_auto_initializer_list : Error<
   "cannot deduce 'decltype(auto)' from initializer list">;
+def err_decltype_auto_lambda : Error<
+  "cannot deduce 'decltype(auto)' from lambda">;
 
 // C++17 deduced class template specialization types
 def err_deduced_class_template_compound_type : Error<

lib/Sema/SemaTemplateDeduction.cpp

       if (isa<InitListExpr>(Init)) {
         Diag(Init->getLocStart(), diag::err_decltype_auto_initializer_list);
         return DAR_FailedAlreadyDiagnosed;
+      } else if (isa<LambdaExpr>(Init)) {
+        Diag(Init->getLocStart(), diag::err_decltype_auto_lambda);
+        return DAR_FailedAlreadyDiagnosed;
       }
 
       QualType Deduced = BuildDecltypeType(Init, Init->getLocStart(), false);

test/CXX/dcl.dcl/dcl.spec/dcl.type/dcl.spec.auto/p7-1y.cpp

 
 auto init_list_1() { return { 1, 2, 3 }; } // expected-error {{cannot deduce return type from initializer list}}
 decltype(auto) init_list_2() { return { 1, 2, 3 }; } // expected-error {{cannot deduce return type from initializer list}}
+
+auto auto_lambda() { return []{}; }
+decltype(auto) decltype_lambda() { return []{}; } // expected-error {{cannot deduce 'decltype(auto)' from lambda}}
+
+auto AutoLambda = []{};
+decltype(auto) DecltypeLambda = []{}; // expected-error {{cannot deduce 'decltype(auto)' from lambda}}
+