In this testcase we were pulling out the constant value of the
variable without ever calling mark_rvalue_use, which is needed for
proper handling of lambda captures. This lack also lead to this false
positive warning.
Tested x86_64-pc-linux-gnu, applying to trunk and 8.
commit e3359de1378a74fd049bcb3e737feefaa359a8f6
Author: Jason Merrill <ja...@redhat.com>
Date: Mon Jun 11 16:54:24 2018 -0400
PR c++/85963 - -Wunused-but-set with ?: in template.
* pt.c (tsubst_copy_and_build) [COND_EXPR]: Call mark_rvalue_use.
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index d8880eb138d..ba78d2e34a5 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -18511,6 +18511,7 @@ tsubst_copy_and_build (tree t,
case COND_EXPR:
{
tree cond = RECUR (TREE_OPERAND (t, 0));
+ cond = mark_rvalue_use (cond);
tree folded_cond = fold_non_dependent_expr (cond);
tree exp1, exp2;
diff --git a/gcc/testsuite/g++.dg/warn/Wunused-var-34.C b/gcc/testsuite/g++.dg/warn/Wunused-var-34.C
new file mode 100644
index 00000000000..52c715121f5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wunused-var-34.C
@@ -0,0 +1,27 @@
+// PR c++/85963
+// { dg-additional-options -Wall }
+
+template<typename T>
+struct foo {
+ T val, alpha;
+ foo() : val(0), alpha(0) {}
+};
+
+template<typename T>
+inline void bar(const foo<T>& A, const foo<T>& B, foo<T>& C) {
+ const bool use_alpha = true;
+ const T alpha = use_alpha ? (A.alpha * B.alpha) : T(0);
+
+ C.val = A.val * B.val;
+ C.alpha = alpha;
+}
+
+
+int main() {
+ foo<double> A,B,C;
+
+ bar(A,B,C);
+
+ return 0;
+}
+
