[constexpr][c++2a] Try-catch blocks in constexpr functions

[bruno]

Summary

Implement support for try-catch blocks in constexpr functions, as
proposed in http://wg21.link/P1002 and voted in San Diego for c++20.

The idea is that we can still never throw inside constexpr, so the catch
block is never entered. A try-catch block like this:

try { f(); } catch (...) { }

is then morally equivalent to just

{ f(); }

Same idea should apply for function/constructor try blocks.

rdar://problem/45530773

[rsmith]

lib/AST/ExprConstant.cpp

4278–4287

The children of a try statement are the try body followed by the catch statements, in order. We only want to evaluate the normal body, so you should just recurse to evaluating cast<CXXTryStmt>(S)->getTryBlock()...

4290–4294

... and this should then be unreachable.

lib/Sema/SemaDeclCXX.cpp

1904–1905

Is there a reason to not allow this as an extension in earlier language modes?

1906–1907

Cxx1yLoc isn't right here; we should produce a -Wc++17-compat diagnostic, not a -Wc++14-compat diagnostic. We should probably prefer the Cxx1zLoc warning over the Cxx1yLoc warning if we have both.

1915–1922

We should just unconditionally allow CXXCatchStmts (that is, don't bother checking the language mode, just walk the children), since whenever we encounter one we must also have a CXXTryStmt, which we'll already have checked.

1959–1971

The repetition can be avoided here by walking the children of Body (regardless of what kind of statement it is), and checking them all. (We can't *quite* just pass Body directly to CheckConstexprFunctionStmt because (nested) compound statements aren't permitted until C++14.)

1970

Don't return here: we still need to do the other checks below.

[bruno]

Address @rsmith comments

[erik.pilkington]

Hi Bruno, thanks for working on this!

include/clang/Basic/DiagnosticSemaKinds.td

2370

I guess this should technically be C++2a

2431

(ditto)

lib/Sema/SemaDeclCXX.cpp

1913–1916

I think we still need to check out the catch stmt (even if it'll never be evaluated) to make sure that it doesn't have any prohibited statements. i.e., we should error here:

constexpr int f() { 
  try { return 0; }
  catch (...) {
    merp: goto merp;
  }
}

But I believe that this patch will accept this.

[bruno]

Update patch after @erik.pilkington review!

[erik.pilkington]

LGTM, but you should probably let @rsmith have the final word!

lib/Sema/SemaDeclCXX.cpp

1916–1919

Might be clearer to just write if (!CheckConstexprFunctionStmt(SemaRef, Dcl, cast<CXXCatchStmt>(SubStmt)->getHandlerBlock())), rather than looping over 1 statement. Could you also add that example I posted as a testcase for this?

[aaron.ballman]

Some minor nits from the peanut gallery.

lib/AST/ExprConstant.cpp

4278

Missing full stop.

lib/Sema/SemaDeclCXX.cpp

1906

I'd appreciate curly braces here, even though they're not strictly required by the coding standard. ;-)

1966

Likewise here.

[bruno]

Address @aaron.ballman and @erik.pilkington reviews.

[aaron.ballman]

LGTM aside from a few other small nits.

lib/Sema/SemaDeclCXX.cpp

1904

Cxx2aLoc.isInvalid()

1951

This restriction is lifted in C++2a, as long as inner statements also apply the general constexpr rules.

include/clang/Basic/DiagnosticSemaKinds.td

def err_constexpr_body_invalid_stmt : Error<
   "statement not allowed in constexpr %select{function|constructor}0">;
 def ext_constexpr_body_invalid_stmt : ExtWarn<
   "use of this statement in a constexpr %select{function|constructor}0 "
   "is a C++14 extension">, InGroup<CXX14>;
 def warn_cxx11_compat_constexpr_body_invalid_stmt : Warning<
   "use of this statement in a constexpr %select{function|constructor}0 "
   "is incompatible with C++ standards before C++14">,
   InGroup<CXXPre14Compat>, DefaultIgnore;
+def ext_constexpr_body_invalid_stmt_cxx2a : ExtWarn<
+  "use of this statement in a constexpr %select{function|constructor}0 "
+  "is a C++2a extension">, InGroup<CXX2a>;
+def warn_cxx17_compat_constexpr_body_invalid_stmt : Warning<
+  "use of this statement in a constexpr %select{function|constructor}0 "
+  "is incompatible with C++ standards before C++20">,
+  InGroup<CXXPre2aCompat>, DefaultIgnore;
 def ext_constexpr_type_definition : ExtWarn<
   "type definition in a constexpr %select{function|constructor}0 "
   "is a C++14 extension">, InGroup<CXX14>;
 def warn_cxx11_compat_constexpr_type_definition : Warning<
   "type definition in a constexpr %select{function|constructor}0 "
   "is incompatible with C++ standards before C++14">,
   InGroup<CXXPre14Compat>, DefaultIgnore;
 def err_constexpr_vla : Error<

 def warn_cxx11_compat_constexpr_body_multiple_return : Warning<
   "multiple return statements in constexpr function "
   "is incompatible with C++ standards before C++14">,
   InGroup<CXXPre14Compat>, DefaultIgnore;
 def note_constexpr_body_previous_return : Note<
   "previous return statement is here">;
 def err_constexpr_function_try_block : Error<
   "function try block not allowed in constexpr %select{function|constructor}0">;
+
+// c++2a function try blocks in constexpr
+def ext_constexpr_function_try_block_cxx2a : ExtWarn<
+  "function try block in constexpr %select{function|constructor}0 is "
+  "a C++2a extension">, InGroup<CXX2a>;
+def warn_cxx17_compat_constexpr_function_try_block : Warning<
+  "function try block in constexpr %select{function|constructor}0 is "
+  "incompatible with C++ standards before C++20">,
+  InGroup<CXXPre2aCompat>, DefaultIgnore;
+
 def err_constexpr_union_ctor_no_init : Error<
   "constexpr union constructor does not initialize any member">;
 def err_constexpr_ctor_missing_init : Error<
   "constexpr constructor must initialize all members">;
 def note_constexpr_ctor_missing_init : Note<
   "member not initialized by constructor">;
 def note_non_literal_no_constexpr_ctors : Note<
   "%0 is not literal because it is not an aggregate and has no constexpr "

lib/AST/ExprConstant.cpp

case Stmt::AttributedStmtClass:
     // As a general principle, C++11 attributes can be ignored without
     // any semantic impact.
     return EvaluateStmt(Result, Info, cast<AttributedStmt>(S)->getSubStmt(),
                         Case);
 
   case Stmt::CaseStmtClass:
   case Stmt::DefaultStmtClass:
     return EvaluateStmt(Result, Info, cast<SwitchCase>(S)->getSubStmt(), Case);
+  case Stmt::CXXTryStmtClass:
+    // Evaluate try blocks by evaluating all sub statements
+    return EvaluateStmt(Result, Info, cast<CXXTryStmt>(S)->getTryBlock(), Case);
   }
 }
 
 /// CheckTrivialDefaultConstructor - Check whether a constructor is a trivial
 /// default constructor. If so, we'll fold it whether or not it's marked as
 /// constexpr. If it is marked as constexpr, we will never implicitly define it,
 /// so we need special handling.
 static bool CheckTrivialDefaultConstructor(EvalInfo &Info, SourceLocation Loc,
                                            const CXXConstructorDecl *CD,
                                            bool IsValueInitialization) {
   if (!CD->isTrivial() || !CD->isDefaultConstructor())
     return false;
 
   // Value-initialization does not call a trivial default constructor, so such a
   // call is a core constant expression whether or not the constructor is
   // constexpr.
   if (!CD->isConstexpr() && !IsValueInitialization) {
     if (Info.getLangOpts().CPlusPlus11) {
       // FIXME: If DiagDecl is an implicitly-declared special member function,
       // we should be much more explicit about why it's not constexpr.
       Info.CCEDiag(Loc, diag::note_constexpr_invalid_function, 1)
         << /*IsConstexpr*/0 << /*IsConstructor*/1 << CD;
       Info.Note(CD->getLocation(), diag::note_declared_at);

lib/Sema/SemaDeclCXX.cpp

static void CheckConstexprCtorInitializer(Sema &SemaRef,
   }
 }
 
 /// Check the provided statement is allowed in a constexpr function
 /// definition.
 static bool
 CheckConstexprFunctionStmt(Sema &SemaRef, const FunctionDecl *Dcl, Stmt *S,
                            SmallVectorImpl<SourceLocation> &ReturnStmts,
-                           SourceLocation &Cxx1yLoc) {
+                           SourceLocation &Cxx1yLoc, SourceLocation &Cxx2aLoc) {
   // - its function-body shall be [...] a compound-statement that contains only
   switch (S->getStmtClass()) {
   case Stmt::NullStmtClass:
     //   - null statements,
     return true;
 
   case Stmt::DeclStmtClass:
     //   - static_assert-declarations
CheckConstexprFunctionStmt(Sema &SemaRef, const FunctionDecl *Dcl, Stmt *S,
   case Stmt::CompoundStmtClass: {
     // C++1y allows compound-statements.
     if (!Cxx1yLoc.isValid())
       Cxx1yLoc = S->getBeginLoc();
 
     CompoundStmt *CompStmt = cast<CompoundStmt>(S);
     for (auto *BodyIt : CompStmt->body()) {
       if (!CheckConstexprFunctionStmt(SemaRef, Dcl, BodyIt, ReturnStmts,
-                                      Cxx1yLoc))
+                                      Cxx1yLoc, Cxx2aLoc))
         return false;
     }
     return true;
   }
 
   case Stmt::AttributedStmtClass:
     if (!Cxx1yLoc.isValid())
       Cxx1yLoc = S->getBeginLoc();
     return true;
 
   case Stmt::IfStmtClass: {
     // C++1y allows if-statements.
     if (!Cxx1yLoc.isValid())
       Cxx1yLoc = S->getBeginLoc();
 
     IfStmt *If = cast<IfStmt>(S);
     if (!CheckConstexprFunctionStmt(SemaRef, Dcl, If->getThen(), ReturnStmts,
-                                    Cxx1yLoc))
+                                    Cxx1yLoc, Cxx2aLoc))
       return false;
     if (If->getElse() &&
         !CheckConstexprFunctionStmt(SemaRef, Dcl, If->getElse(), ReturnStmts,
-                                    Cxx1yLoc))
+                                    Cxx1yLoc, Cxx2aLoc))
       return false;
     return true;
   }
 
   case Stmt::WhileStmtClass:
   case Stmt::DoStmtClass:
   case Stmt::ForStmtClass:
   case Stmt::CXXForRangeStmtClass:
   case Stmt::ContinueStmtClass:
     // C++1y allows all of these. We don't allow them as extensions in C++11,
     // because they don't make sense without variable mutation.
     if (!SemaRef.getLangOpts().CPlusPlus14)
       break;
     if (!Cxx1yLoc.isValid())
       Cxx1yLoc = S->getBeginLoc();
     for (Stmt *SubStmt : S->children())
       if (SubStmt &&
           !CheckConstexprFunctionStmt(SemaRef, Dcl, SubStmt, ReturnStmts,
-                                      Cxx1yLoc))
+                                      Cxx1yLoc, Cxx2aLoc))
         return false;
     return true;
 
   case Stmt::SwitchStmtClass:
   case Stmt::CaseStmtClass:
   case Stmt::DefaultStmtClass:
   case Stmt::BreakStmtClass:
     // C++1y allows switch-statements, and since they don't need variable
     // mutation, we can reasonably allow them in C++11 as an extension.
     if (!Cxx1yLoc.isValid())
       Cxx1yLoc = S->getBeginLoc();
     for (Stmt *SubStmt : S->children())
       if (SubStmt &&
           !CheckConstexprFunctionStmt(SemaRef, Dcl, SubStmt, ReturnStmts,
-                                      Cxx1yLoc))
+                                      Cxx1yLoc, Cxx2aLoc))
         return false;
     return true;
 
+  case Stmt::CXXTryStmtClass:
+    if (!Cxx2aLoc.isValid())
+      Cxx2aLoc = S->getBeginLoc();
+    for (Stmt *SubStmt : S->children())
+      if (SubStmt &&
+          !CheckConstexprFunctionStmt(SemaRef, Dcl, SubStmt, ReturnStmts,
+                                      Cxx1yLoc, Cxx2aLoc))
+        return false;
+    return true;
+
+  case Stmt::CXXCatchStmtClass:
+    // In case we got a valid constexpr try block, the catch block can be
+    // ignored since it will never be evaluated in a constexpr context.
+    return true;
+
   default:
     if (!isa<Expr>(S))
       break;
 
     // C++1y allows expression-statements.
     if (!Cxx1yLoc.isValid())
       Cxx1yLoc = S->getBeginLoc();
     return true;
   }
 
   SemaRef.Diag(S->getBeginLoc(), diag::err_constexpr_body_invalid_stmt)
       << isa<CXXConstructorDecl>(Dcl);
   return false;
 }
 
 /// Check the body for the given constexpr function declaration only contains
 /// the permitted types of statement. C++11 [dcl.constexpr]p3,p4.
 ///
 /// \return true if the body is OK, false if we have diagnosed a problem.
 bool Sema::CheckConstexprFunctionBody(const FunctionDecl *Dcl, Stmt *Body) {
+  SmallVector<SourceLocation, 4> ReturnStmts;
+
   if (isa<CXXTryStmt>(Body)) {
     // C++11 [dcl.constexpr]p3:
     //  The definition of a constexpr function shall satisfy the following
     //  constraints: [...]
     // - its function-body shall be = delete, = default, or a
     //   compound-statement
     //
     // C++11 [dcl.constexpr]p4:
     //  In the definition of a constexpr constructor, [...]
     // - its function-body shall not be a function-try-block;
-    Diag(Body->getBeginLoc(), diag::err_constexpr_function_try_block)
+    //
+    // In C++2a lifts this restriction, as long as inner statements do also
+    // apply to general constexpr rules.
+    Diag(Body->getBeginLoc(),
+         !getLangOpts().CPlusPlus2a
+             ? diag::ext_constexpr_function_try_block_cxx2a
+             : diag::warn_cxx17_compat_constexpr_function_try_block)
         << isa<CXXConstructorDecl>(Dcl);
-    return false;
   }
 
-  SmallVector<SourceLocation, 4> ReturnStmts;
-
   // - its function-body shall be [...] a compound-statement that contains only
   //   [... list of cases ...]
-  CompoundStmt *CompBody = cast<CompoundStmt>(Body);
-  SourceLocation Cxx1yLoc;
-  for (auto *BodyIt : CompBody->body()) {
-    if (!CheckConstexprFunctionStmt(*this, Dcl, BodyIt, ReturnStmts, Cxx1yLoc))
+  //
+  // Note that walking the children here is enough to properly check for
+  // CompoundStmt and CXXTryStmt body.
+  SourceLocation Cxx1yLoc, Cxx2aLoc;
+  for (Stmt *SubStmt : Body->children())
+    if (SubStmt &&
+        !CheckConstexprFunctionStmt(*this, Dcl, SubStmt, ReturnStmts,
+                                    Cxx1yLoc, Cxx2aLoc))
       return false;
-  }
 
+  if (Cxx2aLoc.isValid())
+    Diag(Cxx2aLoc,
+         getLangOpts().CPlusPlus2a
+           ? diag::warn_cxx17_compat_constexpr_body_invalid_stmt
+           : diag::ext_constexpr_body_invalid_stmt_cxx2a)
+      << isa<CXXConstructorDecl>(Dcl);
   if (Cxx1yLoc.isValid())
     Diag(Cxx1yLoc,
          getLangOpts().CPlusPlus14
            ? diag::warn_cxx11_compat_constexpr_body_invalid_stmt
            : diag::ext_constexpr_body_invalid_stmt)
       << isa<CXXConstructorDecl>(Dcl);
 
   if (const CXXConstructorDecl *Constructor

test/CXX/dcl.dcl/dcl.spec/dcl.constexpr/p3.cpp

-// RUN: %clang_cc1 -verify -fcxx-exceptions -triple=x86_64-linux-gnu -std=c++11 -Werror=c++1y-extensions %s
-// RUN: %clang_cc1 -verify -fcxx-exceptions -triple=x86_64-linux-gnu -std=c++1y -DCXX1Y %s
+// RUN: %clang_cc1 -verify -fcxx-exceptions -triple=x86_64-linux-gnu -std=c++11 -Werror=c++1y-extensions -Werror=c++2a-extensions %s
+// RUN: %clang_cc1 -verify -fcxx-exceptions -triple=x86_64-linux-gnu -std=c++1y -DCXX1Y -Werror=c++2a-extensions %s
+// RUN: %clang_cc1 -verify -fcxx-exceptions -triple=x86_64-linux-gnu -std=c++2a -DCXX1Y -DCXX2A %s
 
 namespace N {
   typedef char C;
 }
 
 namespace M {
   typedef double D;
 }

 constexpr int T::OutOfLineVirtual() const { return 0; }
 #ifdef CXX1Y
 struct T2 {
   int n = 0;
   constexpr T2 &operator=(const T2&) = default; // ok
 };
 struct T3 {
   constexpr T3 &operator=(const T3&) const = default;
-  // expected-error@-1 {{an explicitly-defaulted copy assignment operator may not have 'const' or 'volatile' qualifiers}}
+#ifndef CXX2A
+  // expected-error@-2 {{an explicitly-defaulted copy assignment operator may not have 'const' or 'volatile' qualifiers}}
+#else
+  // expected-warning@-4 {{explicitly defaulted copy assignment operator is implicitly deleted}}
+  // expected-note@-5 {{function is implicitly deleted because its declared type does not match the type of an implicit copy assignment operator}}
+#endif
 };
 #endif
 struct U {
   constexpr U SelfReturn() const;
   constexpr int SelfParam(U) const;
 };
 
 struct V : virtual U { // expected-note {{here}}

 constexpr int DisallowedStmtsCXX1Y_2() {
   //  - a goto statement
   goto x; // expected-error {{statement not allowed in constexpr function}}
 x:
   return 0;
 }
 constexpr int DisallowedStmtsCXX1Y_3() {
   //  - a try-block,
-  try {} catch (...) {} // expected-error {{statement not allowed in constexpr function}}
+  try {} catch (...) {}
+#ifndef CXX2A
+  // expected-error@-2 {{use of this statement in a constexpr function is a C++2a extension}}
+#ifndef CXX1Y
+  // expected-error@-4 {{use of this statement in a constexpr function is a C++14 extension}}
+#endif
+#endif
   return 0;
 }
 constexpr int DisallowedStmtsCXX1Y_4() {
   //  - a definition of a variable of non-literal type
   NonLiteral nl; // expected-error {{variable of non-literal type 'NonLiteral' cannot be defined in a constexpr function}}
   return 0;
 }
 constexpr int DisallowedStmtsCXX1Y_5() {

test/CXX/dcl.dcl/dcl.spec/dcl.constexpr/p4.cpp

-// RUN: %clang_cc1 -verify -std=c++11 -fcxx-exceptions -Werror=c++1y-extensions %s
-// RUN: %clang_cc1 -verify -std=c++1y -fcxx-exceptions -DCXX1Y %s
+// RUN: %clang_cc1 -verify -std=c++11 -fcxx-exceptions -Werror=c++1y-extensions -Werror=c++2a-extensions %s
+// RUN: %clang_cc1 -verify -std=c++1y -fcxx-exceptions -DCXX1Y -Werror=c++2a-extensions %s
+// RUN: %clang_cc1 -verify -std=c++2a -fcxx-exceptions -DCXX1Y -DCXX2A %s
 
 namespace N {
   typedef char C;
 }
 
 namespace M {
   typedef double D;
 }
namespace IndirectVBase {
   public:
     constexpr C() {} // expected-error {{constexpr constructor not allowed in class with virtual base class}}
   };
 }
 
 // - its function-body shall not be a function-try-block;
 struct U {
   constexpr U()
-    try // expected-error {{function try block not allowed in constexpr constructor}}
+    try
+#ifndef CXX2A
+  // expected-error@-2 {{function try block in constexpr constructor is a C++2a extension}}
+#endif
     : u() {
+#ifndef CXX1Y
+  // expected-error@-2 {{use of this statement in a constexpr constructor is a C++14 extension}}
+#endif
   } catch (...) {
     throw;
   }
   int u;
 };
 
 // - the compound-statememt of its function-body shall contain only
 struct V {

test/CXX/drs/dr6xx.cpp

struct B : X {
     constexpr void f() const {} // expected-error {{virtual function cannot be constexpr}}
   };
 
   struct NonLiteral { NonLiteral() {} }; // expected-note {{not an aggregate and has no constexpr constructors}}
 
   struct C {
     constexpr C(NonLiteral);
     constexpr C(NonLiteral, int) {} // expected-error {{not a literal type}}
-    constexpr C() try {} catch (...) {} // expected-error {{function try block}}
+    constexpr C() try {} catch (...) {}
+#if __cplusplus <= 201703L
+    // expected-error@-2 {{function try block in constexpr constructor is a C++2a extension}}
+#endif
+#if __cplusplus < 201402L
+    // expected-error@-5 {{use of this statement in a constexpr constructor is a C++14 extension}}
+#endif
   };
 
   struct D {
     operator int() const;
     constexpr D(int) {}
     D(float); // expected-note 2{{declared here}}
   };
   constexpr int get();

www/cxx_status.html

</tr>
     <tr>
       <td>Destroying operator delete</td>
       <td><a href="http://wg21.link/p0722r3">P0722R3</a></td>
       <td class="full" align="center">Clang 6</td>
     </tr>
     <tr>
       <td rowspan=4>Relaxations of <tt>constexpr</tt> restrictions</td>
       <td><a href="http://wg21.link/p1064r0">P1064R0</a></td>
-      <td rowspan=4 class="none" align="center">No</td>
+      <td class="none" align="center">No</td>
     </tr>
       <tr> <!-- from San Diego -->
         <td><a href="http://wg21.link/p1002r1">P1002R1</a></td>
+        <td class="full" align="center">SVN</td>
       </tr>
       <tr>
         <td><a href="http://wg21.link/p1327r1">P1327R1</a></td>
+        <td rowspan=2 class="none" align="center">No</td>
       </tr>
       <tr>
         <td><a href="http://wg21.link/p1330r0">P1330R0</a></td>
       </tr>
     <tr>
       <td>Prohibit aggregates with user-declared constructors</td>
       <td><a href="http://wg21.link/p1008r1">P1008R1</a></td>
       <td class="svn" align="center">SVN</td>