[gcc r10-9481] c++: Fix up [[nodiscard]] on ctors on targetm.cxx.cdtor_returns_this targets [PR99362]

[gcc r10-9481] c++: Fix up [[nodiscard]] on ctors on targetm.cxx.cdtor_returns_this targets [PR99362]

[Jakub Jelinek]

https://gcc.gnu.org/g:01edf2031461b558f630afea382a813e41b631e6

commit r10-9481-g01edf2031461b558f630afea382a813e41b631e6
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Thu Mar 4 16:04:48 2021 +0100

    c++: Fix up [[nodiscard]] on ctors on targetm.cxx.cdtor_returns_this targets [PR99362]
    
    In the P1771R1 changes JeanHeyd reverted part of Alex' PR88146 fix,
    but that seems to be incorrect to me.
    Where P1771R1 suggests warnings for [[nodiscard]] on constructors is
    handled in a different place - in particular the TARGET_EXPR handling
    of convert_to_void.  When we have CALL_EXPR of a ctor, on most arches
    that call has void return type and so returns early, and on arm where
    the ctor returns the this pointer it is undesirable to warn as it warns
    about all ctor calls, not just the ones where it should warn.
    
    The P1771R1 changes added a test for this, but as it was given *.c
    extension rather than *.C, the test was never run and so this didn't get
    spotted immediately.  The test also had a bug, (?n) can't be used
    in dg-warning/dg-error because those are implemented by prepending
    some regexp before the user provided one and (?n) must come at the start
    of the regexp.  Furthermore, while -ftrack-macro-expansion=0 is useful
    in one nodiscard test which uses macros, I don't see how it would be
    relevant to all the other cpp2a/nodiscard* tests which don't use any
    macros.
    
    2021-03-04  Jakub Jelinek  <jakub@redhat.com>
    
            PR c++/88146
            PR c++/99362
    gcc/cp/
            * cvt.c (convert_to_void): Revert 2019-10-17 changes.  Clarify
            comment.
    gcc/testsuite/
            * g++.dg/cpp2a/nodiscard-constructor.c: Renamed to ...
            * g++.dg/cpp2a/nodiscard-constructor1.C: ... this.  Remove
            -ftrack-macro-expansion=0 from dg-options.  Don't use (?n) in
            dg-warning regexps, instead replace .* with \[^\n\r]*.
            * g++.dg/cpp2a/nodiscard-constructor2.C: New test.
            * g++.dg/cpp2a/nodiscard-reason-only-one.C: Remove
            -ftrack-macro-expansion=0 from dg-options.
            * g++.dg/cpp2a/nodiscard-reason-nonstring.C: Likewise.
            * g++.dg/cpp2a/nodiscard-once.C: Likewise.
    
    (cherry picked from commit c9816196328a4f4b927f08cf2f66cf255849da0b)

Diff:
---
 gcc/cp/cvt.c                                            | 12 +++++++-----
 ...nodiscard-constructor.c => nodiscard-constructor1.C} |  6 +++---
 gcc/testsuite/g++.dg/cpp2a/nodiscard-constructor2.C     | 17 +++++++++++++++++
 gcc/testsuite/g++.dg/cpp2a/nodiscard-once.C             |  2 +-
 gcc/testsuite/g++.dg/cpp2a/nodiscard-reason-nonstring.C |  2 +-
 gcc/testsuite/g++.dg/cpp2a/nodiscard-reason-only-one.C  |  2 +-
 6 files changed, 30 insertions(+), 11 deletions(-)

diff --git a/gcc/cp/cvt.c b/gcc/cp/cvt.c
index 791661d8364..9f9f6b658ad 100644
--- a/gcc/cp/cvt.c
+++ b/gcc/cp/cvt.c
@@ -1222,12 +1222,14 @@ convert_to_void (tree expr, impl_conv_void implicit, tsubst_flags_t complain)
     case CALL_EXPR:   /* We have a special meaning for volatile void fn().  */
       /* cdtors may return this or void, depending on
 	 targetm.cxx.cdtor_returns_this, but this shouldn't affect our
-	 decisions here: neither nodiscard warnings (nodiscard cdtors
-	 are nonsensical), nor should any constexpr or template
-	 instantiations be affected by an ABI property that is, or at
-	 least ought to be transparent to the language.  */
+	 decisions here: neither nodiscard warnings (nodiscard dtors
+	 are nonsensical and ctors have a different behavior with that
+	 attribute that is handled in the TARGET_EXPR case), nor should
+	 any constexpr or template instantiations be affected by an ABI
+	 property that is, or at least ought to be transparent to the
+	 language.  */
       if (tree fn = cp_get_callee_fndecl_nofold (expr))
-	if (DECL_DESTRUCTOR_P (fn))
+	if (DECL_CONSTRUCTOR_P (fn) || DECL_DESTRUCTOR_P (fn))
 	  return expr;
 
       if (complain & tf_warning)
diff --git a/gcc/testsuite/g++.dg/cpp2a/nodiscard-constructor.c b/gcc/testsuite/g++.dg/cpp2a/nodiscard-constructor1.C
similarity index 54%
rename from gcc/testsuite/g++.dg/cpp2a/nodiscard-constructor.c
rename to gcc/testsuite/g++.dg/cpp2a/nodiscard-constructor1.C
index a5c2c6514cf..f6358c9115b 100644
--- a/gcc/testsuite/g++.dg/cpp2a/nodiscard-constructor.c
+++ b/gcc/testsuite/g++.dg/cpp2a/nodiscard-constructor1.C
@@ -1,6 +1,6 @@
 /* nodiscard attribute tests  */
 /* { dg-do compile { target c++2a } } */
-/* { dg-options "-O -ftrack-macro-expansion=0" } */
+/* { dg-options "-O" } */
 
 struct A { [[nodiscard("bad constructor")]] A() {} };
 struct B { [[nodiscard]] B() {} };
@@ -8,6 +8,6 @@ struct B { [[nodiscard]] B() {} };
 void
 test (void)
 {
-  A{}; /* { dg-warning "(?n)nodiscard.*bad constructor" } */
-  B{}; /* { dg-warning "(?n)nodiscard" } */
+  A{}; /* { dg-warning "nodiscard\[^\n\r]*bad constructor" } */
+  B{}; /* { dg-warning "nodiscard" } */
 }
diff --git a/gcc/testsuite/g++.dg/cpp2a/nodiscard-constructor2.C b/gcc/testsuite/g++.dg/cpp2a/nodiscard-constructor2.C
new file mode 100644
index 00000000000..37506ffa9e2
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/nodiscard-constructor2.C
@@ -0,0 +1,17 @@
+// PR c++/99362
+// { dg-do compile { target c++2a } }
+
+struct S {
+  [[nodiscard]] S() {}
+  S(int) {}
+};
+
+int main()
+{
+  S s;
+  S();		// { dg-warning "ignoring return value" }
+  (void)(S());
+  S t = 1;
+  S(1);
+  (void)(S(1));
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/nodiscard-once.C b/gcc/testsuite/g++.dg/cpp2a/nodiscard-once.C
index 78349715aae..9da8534ba42 100644
--- a/gcc/testsuite/g++.dg/cpp2a/nodiscard-once.C
+++ b/gcc/testsuite/g++.dg/cpp2a/nodiscard-once.C
@@ -1,6 +1,6 @@
 /* nodiscard attribute tests  */
 /* { dg-do compile { target c++2a } } */
-/* { dg-options "-O -ftrack-macro-expansion=0" } */
+/* { dg-options "-O" } */
 
 [[nodiscard, nodiscard]] int check1 (void); /* { dg-error "nodiscard\[^\n\r]*can appear at most once" } */
 
diff --git a/gcc/testsuite/g++.dg/cpp2a/nodiscard-reason-nonstring.C b/gcc/testsuite/g++.dg/cpp2a/nodiscard-reason-nonstring.C
index 76692e732c4..546c7d28021 100644
--- a/gcc/testsuite/g++.dg/cpp2a/nodiscard-reason-nonstring.C
+++ b/gcc/testsuite/g++.dg/cpp2a/nodiscard-reason-nonstring.C
@@ -1,6 +1,6 @@
 /* nodiscard attribute tests  */
 /* { dg-do compile { target c++2a } } */
-/* { dg-options "-O -ftrack-macro-expansion=0" } */
+/* { dg-options "-O" } */
 
 [[nodiscard(123)]] int check1 (void); /* { dg-error "nodiscard\[^\n\r]*must be a string constant" } */
 
diff --git a/gcc/testsuite/g++.dg/cpp2a/nodiscard-reason-only-one.C b/gcc/testsuite/g++.dg/cpp2a/nodiscard-reason-only-one.C
index d42f4dc7bbe..5f94ae8c086 100644
--- a/gcc/testsuite/g++.dg/cpp2a/nodiscard-reason-only-one.C
+++ b/gcc/testsuite/g++.dg/cpp2a/nodiscard-reason-only-one.C
@@ -1,6 +1,6 @@
 /* nodiscard attribute tests  */
 /* { dg-do compile { target c++2a } } */
-/* { dg-options "-O -ftrack-macro-expansion=0" } */
+/* { dg-options "-O" } */
 
 [[nodiscard("not", "allowed")]] int check1 (void); /* { dg-error "wrong number of arguments.\[^\n\r]*nodiscard" } */