c++: Fix -Wtype-limits in templates.

When instantiating a template tsubst_copy_and_build suppresses -Wtype-limits
warnings about e.g. == always being false because it might not always be
false for an instantiation with other template arguments.  But we should
warn if the operands don't depend on template arguments.

	PR c++/82521
	* pt.c (tsubst_copy_and_build) [EQ_EXPR]: Only suppress warnings if
	the expression was dependent before substitution.

gcc/cp/ChangeLog

+2020-01-29  Jason Merrill  <jason@redhat.com>
+
+	PR c++/82521
+	* pt.c (tsubst_copy_and_build) [EQ_EXPR]: Only suppress warnings if
+	the expression was dependent before substitution.
+
 2020-01-30  Bin Cheng  <bin.cheng@linux.alibaba.com>
 
 	* coroutines.cc (act_des_fn): New.

gcc/cp/pt.c

@@ -19279,10 +19279,14 @@ tsubst_copy_and_build (tree t,
     case MEMBER_REF:
     case DOTSTAR_EXPR:
       {
-	warning_sentinel s1(warn_type_limits);
-	warning_sentinel s2(warn_div_by_zero);
-	warning_sentinel s3(warn_logical_op);
-	warning_sentinel s4(warn_tautological_compare);
+	/* If T was type-dependent, suppress warnings that depend on the range
+	   of the types involved.  */
+	bool was_dep = uses_template_parms (t);
+	warning_sentinel s1(warn_type_limits, was_dep);
+	warning_sentinel s2(warn_div_by_zero, was_dep);
+	warning_sentinel s3(warn_logical_op, was_dep);
+	warning_sentinel s4(warn_tautological_compare, was_dep);
+
 	tree op0 = RECUR (TREE_OPERAND (t, 0));
 	tree op1 = RECUR (TREE_OPERAND (t, 1));
 	tree r = build_x_binary_op

gcc/testsuite/g++.dg/warn/Wtype-limits3.C

+// PR c++/82521
+// { dg-additional-options "-Wtype-limits" }
+
+template <typename T>
+const char * g(const unsigned char value)
+{
+  return value == -1 ? "-1" : "no"; // { dg-warning "always false" }
+}
+
+int main()
+{
+  g<int>(-1);
+}