Re: [PATCH v2] c++: Strengthen checks on 'main'

Jason Merrill Mon, 20 May 2024 15:03:58 -0700

On 5/17/24 02:47, Nathaniel Shead wrote:

On Tue, May 14, 2024 at 06:25:29PM -0400, Jason Merrill wrote:

On 5/11/24 08:32, Nathaniel Shead wrote:

I wasn't entirely sure what to do with the 'abi/main.C' testcase here;
is this OK, or should I e.g. lower the linkage error to a pedwarn for
the purposes of this test?


I think it should be a pedwarn anyway, since it's harmless.  The others can
still be errors.


Fair enough; how about this?  OK for trunk if bootstrap + regtest
succeeds?

OK.

-- >8 --

This patch adds some missing requirements for legal main declarations,
as according to [basic.start.main] p2.

gcc/cp/ChangeLog:

        * decl.cc (grokfndecl): Check for main functions with language
        linkage or module attachment.
        (grokvardecl): Check for extern 'C' entities named main.

gcc/testsuite/ChangeLog:

        * g++.dg/abi/main.C: Check pedwarn for main with linkage-spec.
        * g++.dg/modules/contracts-1_b.C: Don't declare main in named
        module.
        * g++.dg/modules/contracts-3_b.C: Likewise.
        * g++.dg/modules/contracts-4_d.C: Likewise.
        * g++.dg/modules/horcrux-1_a.C: Export declarations, so that...
        * g++.dg/modules/horcrux-1_b.C: Don't declare main in named
        module.
        * g++.dg/modules/main-1.C: New test.
        * g++.dg/parse/linkage5.C: New test.
        * g++.dg/parse/linkage6.C: New test.

Signed-off-by: Nathaniel Shead <nathanielosh...@gmail.com>
---
  gcc/cp/decl.cc                               | 19 +++++++++++++++----
  gcc/testsuite/g++.dg/abi/main.C              |  3 ++-
  gcc/testsuite/g++.dg/modules/contracts-1_b.C |  4 ----
  gcc/testsuite/g++.dg/modules/contracts-3_b.C |  4 ----
  gcc/testsuite/g++.dg/modules/contracts-4_d.C |  2 --
  gcc/testsuite/g++.dg/modules/horcrux-1_a.C   |  3 +++
  gcc/testsuite/g++.dg/modules/horcrux-1_b.C   |  2 +-
  gcc/testsuite/g++.dg/modules/main-1.C        |  5 +++++
  gcc/testsuite/g++.dg/parse/linkage5.C        | 14 ++++++++++++++
  gcc/testsuite/g++.dg/parse/linkage6.C        | 13 +++++++++++++
  10 files changed, 53 insertions(+), 16 deletions(-)
  create mode 100644 gcc/testsuite/g++.dg/modules/main-1.C
  create mode 100644 gcc/testsuite/g++.dg/parse/linkage5.C
  create mode 100644 gcc/testsuite/g++.dg/parse/linkage6.C

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 6fcab615d55..a992d54dc8f 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -10788,6 +10788,11 @@ grokfndecl (tree ctype,
                  "cannot declare %<::main%> to be %qs", "consteval");
        if (!publicp)
        error_at (location, "cannot declare %<::main%> to be static");
+      if (current_lang_depth () != 0)
+       pedwarn (location, OPT_Wpedantic, "cannot declare %<::main%> with a"
+                " linkage specification");
+      if (module_attach_p ())
+       error_at (location, "cannot attach %<::main%> to a named module");
        inlinep = 0;
        publicp = 1;
      }
@@ -11287,10 +11292,16 @@ grokvardecl (tree type,
      DECL_INTERFACE_KNOWN (decl) = 1;

if (DECL_NAME (decl)

-      && MAIN_NAME_P (DECL_NAME (decl))
-      && scope == global_namespace)
-    error_at (DECL_SOURCE_LOCATION (decl),
-             "cannot declare %<::main%> to be a global variable");
+      && MAIN_NAME_P (DECL_NAME (decl)))
+    {
+      if (scope == global_namespace)
+       error_at (DECL_SOURCE_LOCATION (decl),
+                 "cannot declare %<::main%> to be a global variable");
+      else if (DECL_EXTERN_C_P (decl))
+       error_at (DECL_SOURCE_LOCATION (decl),
+                 "an entity named %<main%> cannot be declared with "
+                 "C language linkage");
+    }

/* Check that the variable can be safely declared as a concept.

       Note that this also forbids explicit specializations.  */
diff --git a/gcc/testsuite/g++.dg/abi/main.C b/gcc/testsuite/g++.dg/abi/main.C
index 4c5f1ea213c..2797a16df5b 100644
--- a/gcc/testsuite/g++.dg/abi/main.C
+++ b/gcc/testsuite/g++.dg/abi/main.C
@@ -1,10 +1,11 @@
  /* { dg-do compile } */
+/* { dg-additional-options "-Wno-error=pedantic" }

/* Check if entry points get implicit C linkage. If they don't, compiler will

   * error on incompatible declarations */

int main();

-extern "C" int main();
+extern "C" int main();  // { dg-warning "linkage specification" }

#ifdef __MINGW32__diff --git a/gcc/testsuite/g++.dg/modules/contracts-1_b.C b/gcc/testsuite/g++.dg/modules/contracts-1_b.C

index 30c15f6928b..aa36c8d6b1b 100644
--- a/gcc/testsuite/g++.dg/modules/contracts-1_b.C
+++ b/gcc/testsuite/g++.dg/modules/contracts-1_b.C
@@ -1,15 +1,11 @@
  // { dg-module-do run }
  // { dg-additional-options "-fmodules-ts -fcontracts 
-fcontract-continuation-mode=on" }
-module;
  #include <cstdio>
-export module bar;
-// { dg-module-cmi bar }
  import foo;

template<typename T>

  bool bar_fn_pre(T n) { printf("bar fn pre(%d)\n", n); return true; }

-export

  template<typename T>
  T bar_fn(T n)
    [[ pre: bar_fn_pre(n) && n > 0 ]]
diff --git a/gcc/testsuite/g++.dg/modules/contracts-3_b.C 
b/gcc/testsuite/g++.dg/modules/contracts-3_b.C
index b1d6375391b..1a29e2c3e5d 100644
--- a/gcc/testsuite/g++.dg/modules/contracts-3_b.C
+++ b/gcc/testsuite/g++.dg/modules/contracts-3_b.C
@@ -1,15 +1,11 @@
  // { dg-module-do run }
  // { dg-additional-options "-fmodules-ts -fcontracts 
-fcontract-role=default:ignore,ignore,ignore" }
-module;
  #include <cstdio>
-export module bar;
-// { dg-module-cmi bar }
  import foo;

template<typename T>

  bool bar_fn_pre(T n) { printf("bar fn pre(%d)\n", n); return true; }

-export

  template<typename T>
  T bar_fn(T n)
    [[ pre: bar_fn_pre(n) && n > 0 ]]
diff --git a/gcc/testsuite/g++.dg/modules/contracts-4_d.C 
b/gcc/testsuite/g++.dg/modules/contracts-4_d.C
index dc56251d1d8..9e6b7c3d4de 100644
--- a/gcc/testsuite/g++.dg/modules/contracts-4_d.C
+++ b/gcc/testsuite/g++.dg/modules/contracts-4_d.C
@@ -1,8 +1,6 @@
  // { dg-module-do run }
  // { dg-additional-options "-fmodules-ts -fcontracts" }
-module;
  #include <cstdio>
-export module baz;
  import foo;
  import bar;

diff --git a/gcc/testsuite/g++.dg/modules/horcrux-1_a.C b/gcc/testsuite/g++.dg/modules/horcrux-1_a.C

index ff548d039ea..c115bd2965b 100644
--- a/gcc/testsuite/g++.dg/modules/horcrux-1_a.C
+++ b/gcc/testsuite/g++.dg/modules/horcrux-1_a.C
@@ -3,13 +3,16 @@
  export module foo;
  // { dg-module-cmi foo }

+export

  template<typename _Tp, _Tp __v>
  struct integral_constant
  {};

+export

  template<bool __v>
  using __bool_constant = integral_constant<bool, __v>;

+export

  template<typename _Tp, typename... _Args>
  struct __is_constructible_impl
    : public __bool_constant<__is_constructible(_Tp, _Args...)>
diff --git a/gcc/testsuite/g++.dg/modules/horcrux-1_b.C 
b/gcc/testsuite/g++.dg/modules/horcrux-1_b.C
index 842bf413585..54aa069436a 100644
--- a/gcc/testsuite/g++.dg/modules/horcrux-1_b.C
+++ b/gcc/testsuite/g++.dg/modules/horcrux-1_b.C
@@ -1,6 +1,6 @@
  // { dg-additional-options -fmodules-ts }

-module foo;

+import foo;

int main ()

  {
diff --git a/gcc/testsuite/g++.dg/modules/main-1.C 
b/gcc/testsuite/g++.dg/modules/main-1.C
new file mode 100644
index 00000000000..0d79edddb75
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/main-1.C
@@ -0,0 +1,5 @@
+// { dg-additional-options "-fmodules-ts" }
+// { dg-prune-output "not writing module" }
+
+export module M;
+int main() {}  // { dg-error "attach" }
diff --git a/gcc/testsuite/g++.dg/parse/linkage5.C 
b/gcc/testsuite/g++.dg/parse/linkage5.C
new file mode 100644
index 00000000000..451406de69b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/parse/linkage5.C
@@ -0,0 +1,14 @@
+// { dg-do compile }
+// The main function shall not be declared with a linkage-specification.
+
+extern "C" {
+  int main();  // { dg-error "linkage" }
+}
+
+namespace foo {
+  extern "C" int main();  // { dg-error "linkage" }
+}
+
+extern "C++" int main(); // { dg-error "linkage" }
+
+extern "C" struct S { int main(); };  // OK
diff --git a/gcc/testsuite/g++.dg/parse/linkage6.C 
b/gcc/testsuite/g++.dg/parse/linkage6.C
new file mode 100644
index 00000000000..44081c1544c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/parse/linkage6.C
@@ -0,0 +1,13 @@
+// { dg-do compile }
+// A program that declares an entity named main with C language linkage
+// (in any namespace) is ill-formed.
+
+namespace foo {
+  extern "C" int main;  // { dg-error "linkage" }
+  extern "C" struct A {
+    int main;  // OK
+  };
+  extern "C" struct B {
+    int main();  // OK
+  };
+}

Re: [PATCH v2] c++: Strengthen checks on 'main'

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