[COMMITTED] c++: Fix -Wtype-limits in templates.

[Jason Merrill]

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.

I also tried giving these warnings during parsing of the template, but that had
issues with NON_DEPENDENT_EXPR that I don't feel comfortable messing with in
stage 4.

Tested x86_64-pc-linux-gnu, applying to trunk.

	PR c++/82521
	* pt.c (tsubst_copy_and_build) [EQ_EXPR]: Only suppress warnings if
	the expression was dependent before substitution.
---
 gcc/cp/pt.c                               | 12 ++++++++----
 gcc/testsuite/g++.dg/warn/Wtype-limits3.C | 13 +++++++++++++
 2 files changed, 21 insertions(+), 4 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/warn/Wtype-limits3.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 416ff63ca3c..40ff3c3a089 100644
--- a/gcc/cp/pt.c
+++ b/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
diff --git a/gcc/testsuite/g++.dg/warn/Wtype-limits3.C b/gcc/testsuite/g++.dg/warn/Wtype-limits3.C
new file mode 100644
index 00000000000..b9059ac488e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wtype-limits3.C
@@ -0,0 +1,13 @@
+// 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);
+}

base-commit: b0e9b18ed432c4b7cb9a4b963b65911b4c103cbe
-- 
2.18.1