c++: -Wconversion vs value-dependent expressions [PR99331]

This PR complains that we issue a -Wconversion warning in

  template <int N> struct X {};
  template <class T> X<sizeof(T)> foo();

saying "conversion from 'long unsigned int' to 'int' may change value".
While it's not technically wrong, I suspect -Wconversion warnings aren't
all that useful for value-dependent expressions.  So this patch disables
them.  This is a regression that started with r241425:

@@ -7278,7 +7306,7 @@ convert_template_argument (tree parm,
          val = error_mark_node;
        }
    }
-      else if (!dependent_template_arg_p (orig_arg)
+      else if (!type_dependent_expression_p (orig_arg)
           && !uses_template_parms (t))
    /* We used to call digest_init here.  However, digest_init
       will report errors, which we don't want when complain

Here orig_arg is SIZEOF_EXPR<T>; dependent_template_arg_p (orig_arg) was
true, but type_dependent_expression_p (orig_arg) is false so we warn in
convert_nontype_argument.

gcc/cp/ChangeLog:

	PR c++/99331
	* call.c (build_converted_constant_expr_internal): Don't emit
	-Wconversion warnings.

gcc/testsuite/ChangeLog:

	PR c++/99331
	* g++.dg/warn/Wconversion5.C: New test.

2 files changed, 22 insertions, 0 deletions

diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index bab0c89..e757e18 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -4484,6 +4484,9 @@ build_converted_constant_expr_internal (tree type, tree expr,
 	  && processing_template_decl)
 	conv = next_conversion (conv);
 
+      /* Issuing conversion warnings for value-dependent expressions is
+	 likely too noisy.  */
+      warning_sentinel w (warn_conversion);
       conv->check_narrowing = true;
       conv->check_narrowing_const_only = true;
       expr = convert_like (conv, expr, complain);
diff --git a/gcc/testsuite/g++.dg/warn/Wconversion5.C b/gcc/testsuite/g++.dg/warn/Wconversion5.C
new file mode 100644
index 0000000..f5ae631
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wconversion5.C
@@ -0,0 +1,19 @@
+// PR c++/99331
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wconversion" }
+// Don't issue -Wconversion warnings for value-dependent expressions.
+
+template <int> struct X {};
+template <signed char> struct Y {};
+template <typename T> X<sizeof(T)> foo();
+template <typename T> X<alignof(T)> foo2();
+template<int I> Y<I> foo3();
+template<int> Y<1024> foo4(); // { dg-error "narrowing conversion" }
+template<int> Y<1u> foo5();
+template<int> X<__INT_MAX__ + 1U> foo6(); // { dg-error "narrowing conversion" }
+
+template <typename T>
+struct S {
+  using t = X<sizeof(T)>;
+  using u = X<alignof(T)>;
+};