semantics.c (cxx_eval_constant_expression): Explain unacceptable use of variable better.

	* semantics.c (cxx_eval_constant_expression): Explain
	unacceptable use of variable better.

From-SVN: r166168

gcc/cp/ChangeLog

+2010-11-01  Jason Merrill  <jason@redhat.com>
+
+	* semantics.c (cxx_eval_constant_expression): Explain
+	unacceptable use of variable better.
+
 2010-11-01  Gabriel Dos Reis  <gdr@cse.tamu.edu>
 	    Jason Merrill  <jason@redhat.com>
 

gcc/cp/semantics.c

@@ -6513,7 +6513,36 @@ cxx_eval_constant_expression (const constexpr_call *call, tree t,
       if (DECL_P (r))
 	{
 	  if (!allow_non_constant)
-	    error ("%qD cannot appear in a constant expression", r);
+	    {
+	      tree type = TREE_TYPE (r);
+	      error ("the value of %qD is not usable in a constant "
+		     "expression", r);
+	      if (INTEGRAL_OR_ENUMERATION_TYPE_P (type))
+		{
+		  if (!CP_TYPE_CONST_P (type))
+		    inform (DECL_SOURCE_LOCATION (r),
+			    "%q#D is not const", r);
+		  else if (CP_TYPE_VOLATILE_P (type))
+		    inform (DECL_SOURCE_LOCATION (r),
+			    "%q#D is volatile", r);
+		  else if (!DECL_INITIAL (r))
+		    inform (DECL_SOURCE_LOCATION (r),
+			    "%qD was not initialized with a constant "
+			    "expression", r);
+		  else
+		    gcc_unreachable ();
+		}
+	      else
+		{
+		  if (cxx_dialect >= cxx0x && !DECL_DECLARED_CONSTEXPR_P (r))
+		    inform (DECL_SOURCE_LOCATION (r),
+			    "%qD was not declared %<constexpr%>", r);
+		  else
+		    inform (DECL_SOURCE_LOCATION (r),
+			    "%qD does not have integral or enumeration type",
+			    r);
+		}
+	    }
 	  *non_constant_p = true;
 	}
       break;

gcc/testsuite/g++.dg/cpp0x/constexpr-ex1.C

@@ -55,7 +55,7 @@ constexpr complex I(0, 1);  // OK -- literal complex
 
 
 // 2 invoked with non-const args
-double x5 = 1.0;
+double x5 = 1.0;	       // { dg-message "not declared .constexpr" }
 constexpr complex unit(x5, 0);	// { dg-error "x5|argument" } error: x5 non-constant
 const complex one(x5, 0);   // OK, ‘‘ordinary const’’ -- dynamic
                            //   initialization

gcc/testsuite/g++.dg/cpp0x/constexpr-ex3.C

@@ -15,7 +15,7 @@ struct B
   constexpr B(T _t): t(_t) { }
 };
 
-B<int> b(1);
+B<int> b(1);		       // { dg-message "not declared .constexpr" }
 SA(b.t==1);			// { dg-error "non-constant condition|'b'" }
 constexpr B<int> b2(1);
 SA(b2.t==1);

gcc/testsuite/g++.dg/cpp0x/constexpr-function2.C

@@ -17,7 +17,7 @@ inline constexpr double
 squared(double x) { return x * x; }
 
 constexpr int squarei(int x) { return x * x; }
-extern const int side;
+extern const int side; // { dg-message "not initialized with a constant expression" }
 constexpr int area = squarei(side); // { dg-error "side|argument" }
 // error: squarei(side) is not a constant expression
 

gcc/testsuite/g++.dg/cpp0x/constexpr-non-const-arg.C

@@ -7,7 +7,7 @@ struct B {
   int i;
 };
 
-int global; // not constant
+int global;			// { dg-message "not const" }
 
 struct D : B {
   constexpr D() : B(global) { }   // { dg-error "global|argument" }

gcc/testsuite/g++.dg/parse/constant4.C

@@ -16,7 +16,7 @@ void Foo ()
   
   Y<I> i;
   
-  static const unsigned J = X<T>::J;
+  static const unsigned J = X<T>::J; // { dg-message "not initialized with a constant expression" }
   
   Y<J> j; // { dg-error "constant" "" }
 }

gcc/testsuite/g++.dg/template/qualified-id3.C

@@ -2,7 +2,7 @@
 
 template <const int N> struct A { };
 template <class T> struct B {
-  static const int c;
+  static const int c; // { dg-message "not initialized with a constant expression" }
   typedef A<B<T>::c> C;		// { dg-error "constant expression" }
 };
 template <class T> const int B<T>::c = sizeof (T);