[clang] Mark lambda-to-function-pointer conversion as noexcept

[loskutov]

Summary

Starting from C++17, [expr.prim.lambda.closure]p9 requires that "The [lambda to function pointer] conversion function [...] has a non-throwing exception specification".
This commit adds such a specification to the generated conversion functions.
Pre-C++17 standards don't have this requirement; however, it was reflected in DR 1722.
Due to that, the specification is added unconditionally, no matter what standard revision is used.

Issue related: https://bugs.llvm.org/show_bug.cgi?id=40309

[aaron.ballman]

Pre-C++17 standards don't have this requirement; however, they don't forbid the conversion functions to have this specification either.

This change happened as a result of DR 1722 (http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1722), so the correct behavior is to treat this as a requirement since the feature was introduced (aka, what you're doing here is correct). You should also update clang/www/cxx_dr_status.html with the change for the DR.

clang/lib/Sema/SemaLambda.cpp

1231–1232

I would update the previous comment rather than add this one. `The conversion function is always const with a non-throwing exception specification." or some such.

[loskutov]

Fixed the comment.

[loskutov]

Uploaded an updated diff instead of a diff-from-previous-diff

[aaron.ballman]

Aside from the DR status page and a nit with the test's RUN line, this LGTM. Thank you for working on this -- do you need me to commit on your behalf? If so, are you aware of the recent relicensing of the project and intending to contribute under the new license?

clang/test/CXX/expr/expr.prim/expr.prim.lambda/p9.cpp

2

The test can use -std=c++17

[loskutov]

Reflected the change in cxx_dr_status.html and specified -std=c++17 for the test.

[loskutov]

Yes, I'm not able to commit by myself, so it would be nice if you did that. Apache 2.0 with LLVM Exceptions is fine.

[aaron.ballman]

I needed to make some minor alterations to your patch. The CXX DR status page is automatically generated and I didn't remember that. I went and added an additional test case and did the automated DR regeneration. I've commit in r351750, thank you for the patch!

clang/lib/Sema/SemaLambda.cpp

QualType InvokerFunctionTy = S.getLambdaConversionFunctionResultType(
       CallOperator->getType()->castAs<FunctionProtoType>());
   QualType PtrToFunctionTy = S.Context.getPointerType(InvokerFunctionTy);
 
   // Create the type of the conversion function.
   FunctionProtoType::ExtProtoInfo ConvExtInfo(
       S.Context.getDefaultCallingConvention(
       /*IsVariadic=*/false, /*IsCXXMethod=*/true));
   // The conversion function is always const.
+  // C++17 also obliges it to be noexcept (which it in fact is),
+  // and previous standards don't forbid that either.
   ConvExtInfo.TypeQuals = Qualifiers();
   ConvExtInfo.TypeQuals.addConst();
+  ConvExtInfo.ExceptionSpec.Type = EST_BasicNoexcept;
   QualType ConvTy =
       S.Context.getFunctionType(PtrToFunctionTy, None, ConvExtInfo);
 
   SourceLocation Loc = IntroducerRange.getBegin();
   DeclarationName ConversionName
     = S.Context.DeclarationNames.getCXXConversionFunctionName(
         S.Context.getCanonicalType(PtrToFunctionTy));
   DeclarationNameLoc ConvNameLoc;

clang/test/AST/ast-dump-expr.cpp

void PrimaryExpressions(Ts... a) {
   // CHECK-NEXT: DefaultConstructor
   // CHECK-NEXT: CopyConstructor
   // CHECK-NEXT: MoveConstructor
   // CHECK-NEXT: CopyAssignment
   // CHECK-NEXT: MoveAssignment
   // CHECK-NEXT: Destructor
   // CHECK-NEXT: CXXMethodDecl 0x{{[^ ]*}} <col:6, col:8> col:3 operator() 'auto () const' inline
   // CHECK-NEXT: CompoundStmt
-  // CHECK-NEXT: CXXConversionDecl 0x{{[^ ]*}} <col:3, col:8> col:3 implicit constexpr operator auto (*)() 'auto (*() const)()' inline
+  // CHECK-NEXT: CXXConversionDecl 0x{{[^ ]*}} <col:3, col:8> col:3 implicit constexpr operator auto (*)() 'auto (*() const noexcept)()' inline
   // CHECK-NEXT: CXXMethodDecl 0x{{[^ ]*}} <col:3, col:8> col:3 implicit __invoke 'auto ()' static inline
   // CHECK-NEXT: CompoundStmt 0x{{[^ ]*}} <col:7, col:8>
 
   [](int a, ...){};
   // CHECK: LambdaExpr 0x{{[^ ]*}} <line:[[@LINE-1]]:3, col:18> '(lambda at {{.*}}:[[@LINE-1]]:3)'
   // CHECK-NEXT: CXXRecordDecl 0x{{[^ ]*}} <col:3> col:3 implicit class definition
   // CHECK-NEXT: DefinitionData lambda
   // CHECK-NEXT: DefaultConstructor
   // CHECK-NEXT: CopyConstructor
   // CHECK-NEXT: MoveConstructor
   // CHECK-NEXT: CopyAssignment
   // CHECK-NEXT: MoveAssignment
   // CHECK-NEXT: Destructor
   // CHECK-NEXT: CXXMethodDecl 0x{{[^ ]*}} <col:16, col:18> col:3 operator() 'auto (int, ...) const' inline
   // CHECK-NEXT: ParmVarDecl 0x{{[^ ]*}} <col:6, col:10> col:10 a 'int'
   // CHECK-NEXT: CompoundStmt
-  // CHECK-NEXT: CXXConversionDecl 0x{{[^ ]*}} <col:3, col:18> col:3 implicit constexpr operator auto (*)(int, ...) 'auto (*() const)(int, ...)' inline
+  // CHECK-NEXT: CXXConversionDecl 0x{{[^ ]*}} <col:3, col:18> col:3 implicit constexpr operator auto (*)(int, ...) 'auto (*() const noexcept)(int, ...)' inline
   // CHECK-NEXT: CXXMethodDecl 0x{{[^ ]*}} <col:3, col:18> col:3 implicit __invoke 'auto (int, ...)' static inline
   // CHECK-NEXT: ParmVarDecl 0x{{[^ ]*}} <col:6, col:10> col:10 a 'int'
   // CHECK-NEXT: CompoundStmt 0x{{[^ ]*}} <col:17, col:18>
 
   [a...]{};
   // CHECK: LambdaExpr 0x{{[^ ]*}} <line:[[@LINE-1]]:3, col:10> '(lambda at {{.*}}:[[@LINE-1]]:3)'
   // CHECK-NEXT: CXXRecordDecl 0x{{[^ ]*}} <col:3> col:3 implicit class definition
   // CHECK-NEXT: DefinitionData lambda
void PrimaryExpressions(Ts... a) {
   // CHECK-NEXT: DefaultConstructor
   // CHECK-NEXT: CopyConstructor
   // CHECK-NEXT: MoveConstructor
   // CHECK-NEXT: CopyAssignment
   // CHECK-NEXT: MoveAssignment
   // CHECK-NEXT: Destructor
   // CHECK-NEXT: CXXMethodDecl 0x{{[^ ]*}} <col:8, col:19> col:3 constexpr operator() 'auto () const' inline
   // CHECK-NEXT: CompoundStmt
-  // CHECK-NEXT: CXXConversionDecl 0x{{[^ ]*}} <col:3, col:19> col:3 implicit constexpr operator auto (*)() 'auto (*() const)()' inline
+  // CHECK-NEXT: CXXConversionDecl 0x{{[^ ]*}} <col:3, col:19> col:3 implicit constexpr operator auto (*)() 'auto (*() const noexcept)()' inline
   // CHECK-NEXT: CXXMethodDecl 0x{{[^ ]*}} <col:3, col:19> col:3 implicit __invoke 'auto ()' static inline
   // CHECK-NEXT: CompoundStmt 0x{{[^ ]*}} <col:18, col:19>
 
   []() mutable {};
   // CHECK: LambdaExpr 0x{{[^ ]*}} <line:[[@LINE-1]]:3, col:17> '(lambda at {{.*}}:[[@LINE-1]]:3)'
   // CHECK-NEXT: CXXRecordDecl 0x{{[^ ]*}} <col:3> col:3 implicit class definition
   // CHECK-NEXT: DefinitionData lambda
   // CHECK-NEXT: DefaultConstructor
   // CHECK-NEXT: CopyConstructor
   // CHECK-NEXT: MoveConstructor
   // CHECK-NEXT: CopyAssignment
   // CHECK-NEXT: MoveAssignment
   // CHECK-NEXT: Destructor
   // CHECK-NEXT: CXXMethodDecl 0x{{[^ ]*}} <col:8, col:17> col:3 operator() 'auto ()' inline
   // CHECK-NEXT: CompoundStmt
-  // CHECK-NEXT: CXXConversionDecl 0x{{[^ ]*}} <col:3, col:17> col:3 implicit constexpr operator auto (*)() 'auto (*() const)()' inline
+  // CHECK-NEXT: CXXConversionDecl 0x{{[^ ]*}} <col:3, col:17> col:3 implicit constexpr operator auto (*)() 'auto (*() const noexcept)()' inline
   // CHECK-NEXT: CXXMethodDecl 0x{{[^ ]*}} <col:3, col:17> col:3 implicit __invoke 'auto ()' static inline
   // CHECK-NEXT: CompoundStmt 0x{{[^ ]*}} <col:16, col:17>
 
   []() noexcept {};
   // CHECK: LambdaExpr 0x{{[^ ]*}} <line:[[@LINE-1]]:3, col:18> '(lambda at {{.*}}:[[@LINE-1]]:3)'
   // CHECK-NEXT: CXXRecordDecl 0x{{[^ ]*}} <col:3> col:3 implicit class definition
   // CHECK-NEXT: DefinitionData lambda
   // CHECK-NEXT: DefaultConstructor
   // CHECK-NEXT: CopyConstructor
   // CHECK-NEXT: MoveConstructor
   // CHECK-NEXT: CopyAssignment
   // CHECK-NEXT: MoveAssignment
   // CHECK-NEXT: Destructor
   // CHECK-NEXT: CXXMethodDecl 0x{{[^ ]*}} <col:8, col:18> col:3 operator() 'auto () const noexcept' inline
   // CHECK-NEXT: CompoundStmt
-  // CHECK-NEXT: CXXConversionDecl 0x{{[^ ]*}} <col:3, col:18> col:3 implicit constexpr operator auto (*)() noexcept 'auto (*() const)() noexcept' inline
+  // CHECK-NEXT: CXXConversionDecl 0x{{[^ ]*}} <col:3, col:18> col:3 implicit constexpr operator auto (*)() noexcept 'auto (*() const noexcept)() noexcept' inline
   // CHECK-NEXT: CXXMethodDecl 0x{{[^ ]*}} <col:3, col:18> col:3 implicit __invoke 'auto () noexcept' static inline
   // CHECK-NEXT: CompoundStmt 0x{{[^ ]*}} <col:17, col:18>
 
   []() -> int { return 0; };
   // CHECK: LambdaExpr 0x{{[^ ]*}} <line:[[@LINE-1]]:3, col:27> '(lambda at {{.*}}:[[@LINE-1]]:3)'
   // CHECK-NEXT: CXXRecordDecl 0x{{[^ ]*}} <col:3> col:3 implicit class definition
   // CHECK-NEXT: DefinitionData lambda
   // CHECK-NEXT: DefaultConstructor
   // CHECK-NEXT: CopyConstructor
   // CHECK-NEXT: MoveConstructor
   // CHECK-NEXT: CopyAssignment
   // CHECK-NEXT: MoveAssignment
   // CHECK-NEXT: Destructor
   // CHECK-NEXT: CXXMethodDecl 0x{{[^ ]*}} <col:11, col:27> col:3 operator() 'auto () const -> int' inline
   // CHECK-NEXT: CompoundStmt
   // CHECK-NEXT: ReturnStmt 0x{{[^ ]*}} <col:17, col:24>
   // CHECK-NEXT: IntegerLiteral 0x{{[^ ]*}} <col:24> 'int' 0
-  // CHECK-NEXT: CXXConversionDecl 0x{{[^ ]*}} <col:3, col:27> col:3 implicit constexpr operator int (*)() 'auto (*() const)() -> int' inline
+  // CHECK-NEXT: CXXConversionDecl 0x{{[^ ]*}} <col:3, col:27> col:3 implicit constexpr operator int (*)() 'auto (*() const noexcept)() -> int' inline
   // CHECK-NEXT: CXXMethodDecl 0x{{[^ ]*}} <col:3, col:27> col:3 implicit __invoke 'auto () -> int' static inline
   // CHECK-NEXT: CompoundStmt 0x{{[^ ]*}} <col:15, col:27>
   // CHECK-NEXT: ReturnStmt 0x{{[^ ]*}} <col:17, col:24>
   // CHECK-NEXT: IntegerLiteral 0x{{[^ ]*}} <col:24> 'int' 0
 
   (a + ...);
   // CHECK: CXXFoldExpr 0x{{[^ ]*}} <line:[[@LINE-1]]:3, col:11> '<dependent type>'
   // CHECK-NEXT: DeclRefExpr 0x{{[^ ]*}} <col:4> 'Ts...' lvalue ParmVar 0x{{[^ ]*}} 'a' 'Ts...'

 namespace test_adl_call_three {
 using namespace NS;
 // CHECK-LABEL: FunctionDecl 0x{{[^ ]*}} {{.*}}NonADLCall3 'void ()'
 void NonADLCall3() {
   X x;
   // CHECK: CallExpr 0x{{[^ ]*}} <line:[[@LINE+1]]:{{[^>]+}}> 'void'{{$}}
   f(x);
 }
-} // namespace test_adl_call_three
+} // namespace test_adl_call_three
 

clang/test/CXX/expr/expr.prim/expr.prim.lambda/p9.cpp

+// RUN: %clang_cc1 -fsyntax-only -std=c++1z %s -verify
+// expected-no-diagnostics
+
+void test_noexcept() {
+  const auto lambda = [](int x) { return x + 1; };
+  static_assert(noexcept((int (*)(int))(lambda)),
+                "Lambda-to-function-pointer conversion is expected to be noexcept");
+}