Fix ICE with constexpr and friend functions

[ogoffart]

Summary

Fix a crash while parsing this code:

    
struct X  {
  friend constexpr int foo(X*) { return 12; }
  static constexpr int j = foo(static_cast<X*>(nullptr));
};

I also added a test for the static function case because i don't think this was tested before.

[ogoffart]

Optimized by avoiding repeated call to getBody

[ogoffart]

ping?

[rsmith]

LGTM, thanks!

lib/AST/ExprConstant.cpp

 /// expression.
 static bool CheckConstexprFunction(EvalInfo &Info, SourceLocation CallLoc,
                                    const FunctionDecl *Declaration,
-                                   const FunctionDecl *Definition) {
+                                   const FunctionDecl *Definition,
+                                   const Stmt *Body) {
   // Potential constant expressions can contain calls to declared, but not yet
   // defined, constexpr functions.
   if (Info.checkingPotentialConstantExpression() && !Definition &&

     return false;
 
   // Can we evaluate this function call?
-  if (Definition && Definition->isConstexpr() && !Definition->isInvalidDecl())
+  if (Definition && Definition->isConstexpr() &&
+      !Definition->isInvalidDecl() && Body)
     return true;
 
   if (Info.getLangOpts().CPlusPlus11) {

     const FunctionDecl *Definition = nullptr;
     Stmt *Body = FD->getBody(Definition);
 
-    if (!CheckConstexprFunction(Info, E->getExprLoc(), FD, Definition) ||
+    if (!CheckConstexprFunction(Info, E->getExprLoc(), FD, Definition, Body) ||
         !HandleFunctionCall(E->getExprLoc(), Definition, This, Args, Body, Info,
                             Result, ResultSlot))
       return false;

   }
 
   const FunctionDecl *Definition = nullptr;
-  FD->getBody(Definition);
+  auto Body = FD->getBody(Definition);
 
-  if (!CheckConstexprFunction(Info, E->getExprLoc(), FD, Definition))
+  if (!CheckConstexprFunction(Info, E->getExprLoc(), FD, Definition, Body))
     return false;
 
   // Avoid materializing a temporary for an elidable copy/move constructor.

   }
 
   const FunctionDecl *Definition = nullptr;
-  FD->getBody(Definition);
+  auto Body = FD->getBody(Definition);
 
-  if (!CheckConstexprFunction(Info, E->getExprLoc(), FD, Definition))
+  if (!CheckConstexprFunction(Info, E->getExprLoc(), FD, Definition, Body))
     return false;
 
   if (ZeroInit && !HadZeroInit) {

test/SemaCXX/constant-expression-cxx11.cpp

   constexpr int a = *f().p;
   constexpr int b = *g().p;
 }
+
+namespace IncompleteClass {
+  struct XX {
+    static constexpr int f(XX*) { return 1; } // expected-note {{here}}
+    friend constexpr int g(XX*) { return 2; } // expected-note {{here}}
+
+    static constexpr int i = f(static_cast<XX*>(nullptr)); // expected-error {{constexpr variable 'i' must be initialized by a constant expression}}  expected-note {{undefined function 'f' cannot be used in a constant expression}}
+    static constexpr int j = g(static_cast<XX*>(nullptr)); // expected-error {{constexpr variable 'j' must be initialized by a constant expression}}  expected-note {{undefined function 'g' cannot be used in a constant expression}}
+  };
+}