Add an explicit diagnostic for the case where an expression is not a constant

expression because it uses 'this'. Inspired by PR20219 comment#2.

llvm-svn: 212433

clang/include/clang/Basic/DiagnosticASTKinds.td

@@ -89,6 +89,9 @@ def note_constexpr_call_limit_exceeded : Note<
   "constexpr evaluation hit maximum call limit">;
 def note_constexpr_step_limit_exceeded : Note<
   "constexpr evaluation hit maximum step limit; possible infinite loop?">;
+def note_constexpr_this : Note<
+  "%select{|implicit }0use of 'this' pointer is only allowed within the "
+  "evaluation of a call to a 'constexpr' member function">;
 def note_constexpr_lifetime_ended : Note<
   "%select{read of|assignment to|increment of|decrement of}0 "
   "%select{temporary|variable}1 whose lifetime has ended">;
@@ -138,6 +141,7 @@ def note_constexpr_calls_suppressed : Note<
   "(skipping %0 call%s0 in backtrace; use -fconstexpr-backtrace-limit=0 to "
   "see all)">;
 def note_constexpr_call_here : Note<"in call to '%0'">;
+
 def warn_integer_constant_overflow : Warning<
   "overflow in expression; result is %0 with type %1">,
   InGroup<DiagGroup<"integer-overflow">>;

clang/lib/AST/ExprConstant.cpp

@@ -4666,8 +4666,13 @@ public:
     // Can't look at 'this' when checking a potential constant expression.
     if (Info.checkingPotentialConstantExpression())
       return false;
-    if (!Info.CurrentCall->This)
-      return Error(E);
+    if (!Info.CurrentCall->This) {
+      if (Info.getLangOpts().CPlusPlus11)
+        Info.Diag(E, diag::note_constexpr_this) << E->isImplicit();
+      else
+        Info.Diag(E);
+      return false;
+    }
     Result = *Info.CurrentCall->This;
     return true;
   }

clang/test/SemaCXX/constant-expression-cxx11.cpp

@@ -823,6 +823,19 @@ static_assert(X() == 0, "");
 
 }
 
+struct This {
+  constexpr int f() const { return 0; }
+  static constexpr int g() { return 0; }
+  void h() {
+    constexpr int x = f(); // expected-error {{must be initialized by a constant}}
+    // expected-note@-1 {{implicit use of 'this' pointer is only allowed within the evaluation of a call to a 'constexpr' member function}}
+    constexpr int y = this->f(); // expected-error {{must be initialized by a constant}}
+    // expected-note-re@-1 {{{{^}}use of 'this' pointer}}
+    constexpr int z = g();
+    static_assert(z == 0, "");
+  }
+};
+
 }
 
 namespace Temporaries {