[pushed] c++: constexpr class access in generic lambda [PR126754]

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

-- 8< --

We don't want to see TARGET_EXPR in template trees because it's an
implementation detail, not a representation of the source.  But constant
evaluation normally represents a constant class value with a TARGET_EXPR.
Let's not do the latter in a template.

	PR c++/126754

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_outermost_constant_expr): Don't
	wrap in TARGET_EXPR when processing_template_decl.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/constexpr-lambda2.C: New test.
---
 gcc/cp/constexpr.cc                            |  3 +++
 gcc/testsuite/g++.dg/cpp1y/constexpr-lambda2.C | 11 +++++++++++
 2 files changed, 14 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-lambda2.C

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index e16c878d725..c9ebc0027b7 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -11288,6 +11288,9 @@ cxx_eval_outermost_constant_expr (tree t, bool allow_non_constant,
 	/* Don't add a TARGET_EXPR if our argument didn't have one.  */;
       else if (TREE_CODE (t) == TARGET_EXPR && TARGET_EXPR_CLEANUP (t))
 	r = get_target_expr (r);
+      else if (processing_template_decl)
+	/* Don't insert TARGET_EXPR in template trees.  */
+	return r;
       else
 	{
 	  r = get_target_expr (r, tf_warning_or_error | tf_no_cleanup);
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-lambda2.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-lambda2.C
new file mode 100644
index 00000000000..1ad31f83fc6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-lambda2.C
@@ -0,0 +1,11 @@
+// PR c++/126754
+// { dg-do compile { target c++14 } }
+
+struct S { int i; };
+int main() {
+  constexpr S a{1};
+  const auto f = [](auto b) {
+    return 1 - a.i;  // this expression must be independent of b in order to trigger the crash
+  };
+  return f(0);
+}

base-commit: 34fd4c49fbe6ff643fc6f5f2cccca03c86535ee0
-- 
2.55.0