r224189 - Sema: Constexpr functions must have return statements which have an expr

[David Majnemer]

Author: majnemer
Date: Sat Dec 13 02:12:56 2014
New Revision: 224189

URL: http://llvm.org/viewvc/llvm-project?rev=224189&view=rev
Log:
Sema: Constexpr functions must have return statements which have an expr

clang lets programmers be pretty cavalier when it comes to void return
statements in functions which have non-void return types.  However, we
cannot be so forgiving in constexpr functions: evaluation will go off
the rails very quickly.

Instead, keep the return statement in the AST but mark the function as
invalid.  Doing so gives us nice diagnostics while making constexpr
evaluation halt.

This fixes PR21859.

Modified:
    cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td
    cfe/trunk/lib/Sema/SemaStmt.cpp
    cfe/trunk/test/SemaCXX/constant-expression-cxx11.cpp

Modified: cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td?rev=224189&r1=224188&r2=224189&view=diff
==============================================================================
--- cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td (original)
+++ cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td Sat Dec 13 02:12:56 2014
@@ -1875,6 +1875,8 @@ def err_enable_if_never_constant_expr :
   "'enable_if' attribute expression never produces a constant expression">;
 def err_constexpr_body_no_return : Error<
   "no return statement in constexpr function">;
+def err_constexpr_return_missing_expr : Error<
+  "non-void constexpr function %0 should return a value">;
 def warn_cxx11_compat_constexpr_body_no_return : Warning<
   "constexpr function with no return statements is incompatible with C++ "
   "standards before C++14">, InGroup<CXXPre14Compat>, DefaultIgnore;

Modified: cfe/trunk/lib/Sema/SemaStmt.cpp
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Sema/SemaStmt.cpp?rev=224189&r1=224188&r2=224189&view=diff
==============================================================================
--- cfe/trunk/lib/Sema/SemaStmt.cpp (original)
+++ cfe/trunk/lib/Sema/SemaStmt.cpp Sat Dec 13 02:12:56 2014
@@ -3039,14 +3039,26 @@ StmtResult Sema::BuildReturnStmt(SourceL
 
     Result = new (Context) ReturnStmt(ReturnLoc, RetValExp, nullptr);
   } else if (!RetValExp && !HasDependentReturnType) {
-    unsigned DiagID = diag::warn_return_missing_expr;  // C90 6.6.6.4p4
-    // C99 6.8.6.4p1 (ext_ since GCC warns)
-    if (getLangOpts().C99) DiagID = diag::ext_return_missing_expr;
+    FunctionDecl *FD = getCurFunctionDecl();
 
-    if (FunctionDecl *FD = getCurFunctionDecl())
+    unsigned DiagID;
+    if (getLangOpts().CPlusPlus11 && FD && FD->isConstexpr()) {
+      // C++11 [stmt.return]p2
+      DiagID = diag::err_constexpr_return_missing_expr;
+      FD->setInvalidDecl();
+    } else if (getLangOpts().C99) {
+      // C99 6.8.6.4p1 (ext_ since GCC warns)
+      DiagID = diag::ext_return_missing_expr;
+    } else {
+      // C90 6.6.6.4p4
+      DiagID = diag::warn_return_missing_expr;
+    }
+
+    if (FD)
       Diag(ReturnLoc, DiagID) << FD->getIdentifier() << 0/*fn*/;
     else
       Diag(ReturnLoc, DiagID) << getCurMethodDecl()->getDeclName() << 1/*meth*/;
+
     Result = new (Context) ReturnStmt(ReturnLoc);
   } else {
     assert(RetValExp || HasDependentReturnType);

Modified: cfe/trunk/test/SemaCXX/constant-expression-cxx11.cpp
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/SemaCXX/constant-expression-cxx11.cpp?rev=224189&r1=224188&r2=224189&view=diff
==============================================================================
--- cfe/trunk/test/SemaCXX/constant-expression-cxx11.cpp (original)
+++ cfe/trunk/test/SemaCXX/constant-expression-cxx11.cpp Sat Dec 13 02:12:56 2014
@@ -1969,3 +1969,8 @@ namespace PR21786 {
   static_assert(&Bar::x != nullptr, "");
   static_assert(&Bar::x != &Bar::y, ""); // expected-error {{constant expression}}
 }
+
+namespace PR21859 {
+  constexpr int Fun() { return; } // expected-error {{non-void constexpr function 'Fun' should return a value}}
+  constexpr int Var = Fun(); // expected-error {{constexpr variable 'Var' must be initialized by a constant expression}}
+}