[C++ PATCH] Fix ICE with label difference as template non-type argument (PR c++/79650)

[Jakub Jelinek]

Hi!

reduced_constant_expression_p uses initializer_constant_valid_p
to determine what is a valid constant expression.  That accepts several
cases which aren't compile time INTEGER_CST, just something that the assembler
can finalize into a constant, e.g. difference of labels, difference of
STRING_CSTs, something plus ADDR_EXPR of a static var etc.
But for template non-type arguments my understanding is we really need
an INTEGER_CST, we cam hardly instantiate on something that only during
assembly will become a constant.

The following patch attempts to diagnose it.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

Or do you have better suggestions for the diagnostics wording?

2017-12-13  Jakub Jelinek  <ja...@redhat.com>

        PR c++/79650
        * pt.c (convert_nontype_argument): Diagnose
        reduced_constant_expression_p expressions that aren't INTEGER_CST.

        * g++.dg/template/pr79650.C: New test.

--- gcc/cp/pt.c.jj      2017-12-13 15:52:51.000000000 +0100
+++ gcc/cp/pt.c 2017-12-13 19:21:35.861825357 +0100
@@ -6577,7 +6577,19 @@ convert_nontype_argument (tree type, tre
                return NULL_TREE;
              /* else cxx_constant_value complained but gave us
                 a real constant, so go ahead.  */
-             gcc_assert (TREE_CODE (expr) == INTEGER_CST);
+             if (TREE_CODE (expr) != INTEGER_CST)
+               {
+                 /* Some assemble time constant expressions like
+                    (intptr_t)&&lab1 - (intptr_t)&&lab2 or
+                    4 + (intptr_t)&&var satisfy reduced_constant_expression_p
+                    as we can emit them into .rodata initializers of
+                    variables, yet they can't fold into an INTEGER_CST at
+                    compile time.  Refuse them here.  */
+                 gcc_checking_assert (reduced_constant_expression_p (expr));
+                 error_at (loc, "template argument %qE for type %qT not "
+                                "a constant integer", expr, type);
+                 return NULL_TREE;
+               }
            }
          else
            return NULL_TREE;
--- gcc/testsuite/g++.dg/template/pr79650.C.jj  2017-12-13 19:29:04.549196268 
+0100
+++ gcc/testsuite/g++.dg/template/pr79650.C     2017-12-13 19:34:15.202298913 
+0100
@@ -0,0 +1,20 @@
+// PR c++/79650
+// { dg-do compile { target c++11 } }
+// { dg-options "" }
+
+typedef __INTPTR_TYPE__ intptr_t;
+template<intptr_t> struct A {};
+
+void
+foo ()
+{
+  static int a, b;
+lab1:
+lab2:
+  A<(intptr_t)&&lab1 - (__INTPTR_TYPE__)&&lab2> c;     // { dg-error "not a 
constant integer" }
+  A<(intptr_t)&&lab1 - (__INTPTR_TYPE__)&&lab1> d;
+  A<(intptr_t)&a - (intptr_t)&b> e;                    // { dg-error "is not a 
constant expression" }
+  A<(intptr_t)&a - (intptr_t)&a> f;
+  A<(intptr_t)sizeof(a) + (intptr_t)&a> g;             // { dg-error "not a 
constant integer" }
+  A<(intptr_t)&a> h;                                   // { dg-error 
"conversion from pointer type" }
+}

        Jakub