d: Return the correct value for C++ constructor calls (PR101664)

C++ constructors return void, even though the front-end semantic treats
them as implicitly returning `this'.  To handle this correctly, the
object reference is cached and used as the result of the expression.

	PR d/101664

gcc/d/ChangeLog:

	* expr.cc (ExprVisitor::visit (CallExp *)): Use object expression as
	result for C++ constructor calls.

gcc/testsuite/ChangeLog:

	* gdc.dg/extern-c++/extern-c++.exp: New.
	* gdc.dg/extern-c++/pr101664.d: New test.
	* gdc.dg/extern-c++/pr101664_1.cc: New test.

4 files changed, 77 insertions, 0 deletions

diff --git a/gcc/d/expr.cc b/gcc/d/expr.cc
index 99ca958..85269c6 100644
--- a/gcc/d/expr.cc
+++ b/gcc/d/expr.cc
@@ -1751,6 +1751,7 @@ public:
     tree callee = NULL_TREE;
     tree object = NULL_TREE;
     tree cleanup = NULL_TREE;
+    tree returnvalue = NULL_TREE;
     TypeFunction *tf = NULL;
 
     /* Calls to delegates can sometimes look like this.  */
@@ -1819,6 +1820,15 @@ public:
 		else
 		  fndecl = build_address (fndecl);
 
+		/* C++ constructors return void, even though front-end semantic
+		   treats them as implicitly returning `this'.  Set returnvalue
+		   to override the result of this expression.  */
+		if (fd->isCtorDeclaration () && fd->linkage == LINKcpp)
+		  {
+		    thisexp = d_save_expr (thisexp);
+		    returnvalue = thisexp;
+		  }
+
 		callee = build_method_call (fndecl, thisexp, fd->type);
 	      }
 	  }
@@ -1885,6 +1895,9 @@ public:
        build the call expression.  */
     tree exp = d_build_call (tf, callee, object, e->arguments);
 
+    if (returnvalue != NULL_TREE)
+      exp = compound_expr (exp, returnvalue);
+
     if (tf->isref)
       exp = build_deref (exp);
 
diff --git a/gcc/testsuite/gdc.dg/extern-c++/extern-c++.exp b/gcc/testsuite/gdc.dg/extern-c++/extern-c++.exp
new file mode 100644
index 0000000..d38f993
--- /dev/null
+++ b/gcc/testsuite/gdc.dg/extern-c++/extern-c++.exp
@@ -0,0 +1,39 @@
+#   Copyright (C) 2021 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+# 
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+# 
+# You should have received a copy of the GNU General Public License
+# along with GCC; see the file COPYING3.  If not see
+# <http://www.gnu.org/licenses/>.
+
+# Load support procs.
+load_lib gdc-dg.exp
+
+# We are mixing D and C++ code, need to pull in libstdc++
+global GDC_INCLUDE_CXX_FLAGS
+set GDC_INCLUDE_CXX_FLAGS 1
+
+# Initialize `dg'.
+dg-init
+
+# Main loop.
+if [check_no_compiler_messages extern_c++_tests assembly {
+   // C++
+   int main() { return 0; }
+}] {
+    gdc-dg-runtest [lsort \
+	   [glob -nocomplain $srcdir/$subdir/*.d ] ] "" ""
+}
+
+set GDC_INCLUDE_CXX_FLAGS 0
+
+# All done.
+dg-finish
diff --git a/gcc/testsuite/gdc.dg/extern-c++/pr101664.d b/gcc/testsuite/gdc.dg/extern-c++/pr101664.d
new file mode 100644
index 0000000..57b3d90
--- /dev/null
+++ b/gcc/testsuite/gdc.dg/extern-c++/pr101664.d
@@ -0,0 +1,15 @@
+// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101664
+// { dg-do run }
+// { dg-options "-O2" }
+// { dg-additional-sources "pr101664_1.cc" }
+
+extern(C++) struct S101664
+{
+    int i;
+    this(int);
+}
+
+void main()
+{
+    assert(S101664(1).i == 1);
+}
diff --git a/gcc/testsuite/gdc.dg/extern-c++/pr101664_1.cc b/gcc/testsuite/gdc.dg/extern-c++/pr101664_1.cc
new file mode 100644
index 0000000..066e784
--- /dev/null
+++ b/gcc/testsuite/gdc.dg/extern-c++/pr101664_1.cc
@@ -0,0 +1,10 @@
+struct S101664
+{
+  int i;
+  S101664 (int n);
+};
+
+S101664::S101664 (int n)
+    : i(n)
+{
+}