From fd740165e54151ea794fca34904f5c2e2ea1dcda Mon Sep 17 00:00:00 2001
From: Patrick Palka <ppalka@redhat.com>
Date: Fri, 19 Nov 2021 08:54:25 -0500
Subject: [PATCH] c++: nested lambda capturing a capture proxy [PR94376]

Here when determining the type of the FIELD_DECL for the by-value capture
of 'i' in the inner lambda, we incorrectly give it the type const int
instead of int since the effective initializer is the proxy for the outer
capture, and this proxy is const since the outer lambda is non-mutable.

This patch fixes this by making lambda_capture_field_type handle
by-value capturing of capture proxies specially, namely we instead
consider the type of their FIELD_DECL which unlike the proxy has the
true cv-quals of the captured entity.

	PR c++/94376

gcc/cp/ChangeLog:

	* lambda.c (lambda_capture_field_type): Simplify by handling the
	is_this case first.  When capturing by-value a capture proxy,
	consider the type of the corresponding field instead.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/lambda/lambda-nested9.C: New test.
---
 gcc/cp/lambda.c                                    | 19 +++++++---
 gcc/testsuite/g++.dg/cpp0x/lambda/lambda-nested9.C | 41 ++++++++++++++++++++++
 2 files changed, 56 insertions(+), 4 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/lambda/lambda-nested9.C

diff --git a/gcc/cp/lambda.c b/gcc/cp/lambda.c
index f68c68c..c39a2bc 100644
--- a/gcc/cp/lambda.c
+++ b/gcc/cp/lambda.c
@@ -195,7 +195,9 @@ lambda_capture_field_type (tree expr, bool explicit_init_p,
   tree type;
   bool is_this = is_this_parameter (tree_strip_nop_conversions (expr));
 
-  if (!is_this && explicit_init_p)
+  if (is_this)
+    type = TREE_TYPE (expr);
+  else if (explicit_init_p)
     {
       tree auto_node = make_auto ();
       
@@ -209,7 +211,7 @@ lambda_capture_field_type (tree expr, bool explicit_init_p,
       else
 	type = do_auto_deduction (type, expr, auto_node);
     }
-  else if (!is_this && type_dependent_expression_p (expr))
+  else if (type_dependent_expression_p (expr))
     {
       type = cxx_make_type (DECLTYPE_TYPE);
       DECLTYPE_TYPE_EXPR (type) = expr;
@@ -219,10 +221,19 @@ lambda_capture_field_type (tree expr, bool explicit_init_p,
     }
   else
     {
+      if (!by_reference_p && is_capture_proxy (expr))
+	{
+	  /* When capturing by-value another capture proxy from an enclosing
+	     lambda, consider the type of the corresponding field instead,
+	     as the proxy may be additionally const-qualifed if the enclosing
+	     lambda is non-mutable (PR94376).  */
+	  gcc_assert (TREE_CODE (DECL_VALUE_EXPR (expr)) == COMPONENT_REF);
+	  expr = TREE_OPERAND (DECL_VALUE_EXPR (expr), 1);
+	}
+
       type = non_reference (unlowered_expr_type (expr));
 
-      if (!is_this
-	  && (by_reference_p || TREE_CODE (type) == FUNCTION_TYPE))
+      if (by_reference_p || TREE_CODE (type) == FUNCTION_TYPE)
 	type = build_reference_type (type);
     }
 
diff --git a/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-nested9.C b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-nested9.C
new file mode 100644
index 0000000..ff7da3b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-nested9.C
@@ -0,0 +1,41 @@
+// PR c++/94376
+// { dg-do compile { target c++11 } }
+
+int main() {
+  // We used to incorrectly reject the first two cases.
+  int i = 0;
+  [=] () {
+    [=] () mutable {
+      ++i;
+    };
+  };
+
+#if __cpp_init_captures
+  [j=0] () {
+    [=] () mutable {
+      ++j;
+    };
+  };
+#endif
+
+  [=] () {
+    [&] () mutable {
+      ++i; // { dg-error "read-only" }
+    };
+  };
+
+  const int j = 0;
+  [=] () {
+    [=] () mutable {
+      ++j; // { dg-error "read-only" }
+    };
+  };
+
+#if __cpp_init_captures
+  [j=0] () {
+    [&] () mutable {
+      ++j; // { dg-error "read-only" }
+    };
+  };
+#endif
+}
-- 
2.7.4