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

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

Differential Revision: https://reviews.llvm.org/D55097

llvm-svn: 348789

Author: bcardosolopes

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -2357,6 +2357,13 @@ 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++2a">,
+  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>;
@@ -2409,6 +2416,16 @@ 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++2a">,
+  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<

clang/lib/AST/ExprConstant.cpp

@@ -4279,6 +4279,9 @@ static EvalStmtResult EvaluateStmt(StmtResult &Result, EvalInfo &Info,
   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);
   }
 }
 

clang/lib/Sema/SemaDeclCXX.cpp

@@ -1803,7 +1803,7 @@ static void CheckConstexprCtorInitializer(Sema &SemaRef,
 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:
@@ -1840,7 +1840,7 @@ CheckConstexprFunctionStmt(Sema &SemaRef, const FunctionDecl *Dcl, Stmt *S,
     CompoundStmt *CompStmt = cast<CompoundStmt>(S);
     for (auto *BodyIt : CompStmt->body()) {
       if (!CheckConstexprFunctionStmt(SemaRef, Dcl, BodyIt, ReturnStmts,
-                                      Cxx1yLoc))
+                                      Cxx1yLoc, Cxx2aLoc))
         return false;
     }
     return true;
@@ -1858,11 +1858,11 @@ CheckConstexprFunctionStmt(Sema &SemaRef, const FunctionDecl *Dcl, Stmt *S,
 
     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;
   }
@@ -1881,7 +1881,7 @@ CheckConstexprFunctionStmt(Sema &SemaRef, const FunctionDecl *Dcl, Stmt *S,
     for (Stmt *SubStmt : S->children())
       if (SubStmt &&
           !CheckConstexprFunctionStmt(SemaRef, Dcl, SubStmt, ReturnStmts,
-                                      Cxx1yLoc))
+                                      Cxx1yLoc, Cxx2aLoc))
         return false;
     return true;
 
@@ -1896,10 +1896,30 @@ CheckConstexprFunctionStmt(Sema &SemaRef, const FunctionDecl *Dcl, Stmt *S,
     for (Stmt *SubStmt : S->children())
       if (SubStmt &&
           !CheckConstexprFunctionStmt(SemaRef, Dcl, SubStmt, ReturnStmts,
-                                      Cxx1yLoc))
+                                      Cxx1yLoc, Cxx2aLoc))
         return false;
     return true;
 
+  case Stmt::CXXTryStmtClass:
+    if (Cxx2aLoc.isInvalid())
+      Cxx2aLoc = S->getBeginLoc();
+    for (Stmt *SubStmt : S->children()) {
+      if (SubStmt &&
+          !CheckConstexprFunctionStmt(SemaRef, Dcl, SubStmt, ReturnStmts,
+                                      Cxx1yLoc, Cxx2aLoc))
+        return false;
+    }
+    return true;
+
+  case Stmt::CXXCatchStmtClass:
+    // Do not bother checking the language mode (already covered by the
+    // try block check).
+    if (!CheckConstexprFunctionStmt(SemaRef, Dcl,
+                                    cast<CXXCatchStmt>(S)->getHandlerBlock(),
+                                    ReturnStmts, Cxx1yLoc, Cxx2aLoc))
+      return false;
+    return true;
+
   default:
     if (!isa<Expr>(S))
       break;
@@ -1920,6 +1940,8 @@ CheckConstexprFunctionStmt(Sema &SemaRef, const FunctionDecl *Dcl, Stmt *S,
 ///
 /// \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
@@ -1930,22 +1952,35 @@ bool Sema::CheckConstexprFunctionBody(const FunctionDecl *Dcl, Stmt *Body) {
     // 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)
+    //
+    // This restriction is lifted in C++2a, as long as inner statements also
+    // apply the 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

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

@@ -1,5 +1,6 @@
-// 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;
@@ -78,7 +79,12 @@ struct T2 {
 };
 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 {
@@ -129,9 +135,22 @@ constexpr int DisallowedStmtsCXX1Y_2() {
 x:
   return 0;
 }
+constexpr int DisallowedStmtsCXX1Y_2_1() {
+  try {
+    return 0;
+  } catch (...) {
+  merp: goto merp; // expected-error {{statement not allowed in constexpr function}}
+  }
+}
 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() {

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

@@ -1,5 +1,6 @@
-// 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;
@@ -49,8 +50,14 @@ namespace IndirectVBase {
 // - 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;
   }

clang/test/CXX/drs/dr6xx.cpp

@@ -492,7 +492,13 @@ namespace dr647 { // dr647: yes
   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 {

clang/www/cxx_status.html

@@ -953,13 +953,15 @@ <h2 id="cxx20">C++2a implementation status</h2>
     <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>