c++: Strengthen checks on 'main'

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?

Bootstrapped and regtested on x86_64-pc-linux-gnu, and lightly checked
with a MinGW32 cross.  OK for trunk?

-- >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: Rework to avoid declaring main with
	language linkage.
	* 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 <nathanieloshead@gmail.com>
---
 gcc/cp/decl.cc                               | 19 ++++++++++---
 gcc/testsuite/g++.dg/abi/main.C              | 29 ++++++++------------
 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, 63 insertions(+), 32 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

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.

> Bootstrapped and regtested on x86_64-pc-linux-gnu, and lightly checked
> with a MinGW32 cross.  OK for trunk?
> 
> -- >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: Rework to avoid declaring main with
> 	language linkage.
> 	* 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 <nathanieloshead@gmail.com>
> ---
>   gcc/cp/decl.cc                               | 19 ++++++++++---
>   gcc/testsuite/g++.dg/abi/main.C              | 29 ++++++++------------
>   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, 63 insertions(+), 32 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 e02562466a7..5ab3b787d6f 100644
> --- a/gcc/cp/decl.cc
> +++ b/gcc/cp/decl.cc
> @@ -10781,6 +10781,11 @@ 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)
> +	error_at (location, "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;
>       }
> @@ -11280,10 +11285,16 @@ 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..f3882d70612 100644
> --- a/gcc/testsuite/g++.dg/abi/main.C
> +++ b/gcc/testsuite/g++.dg/abi/main.C
> @@ -1,24 +1,19 @@
>   /* { dg-do compile } */
>   
> -/* Check if entry points get implicit C linkage. If they don't, compiler will
> - * error on incompatible declarations */
> +/* Check if entry points get implicit C linkage. Determined by checking that
> +   the names are unmangled */
>   
> -int main();
> -extern "C" int main();
> +int main() {}
>   
>   #ifdef __MINGW32__
> -
> -int wmain();
> -extern "C" int wmain();
> -
> -int DllMain();
> -extern "C" int DllMain();
> -
> -int WinMain();
> -extern "C" int WinMain();
> -
> -int wWinMain();
> -extern "C" int wWinMain();
> -
> +int wmain() {}
> +int DllMain() {}
> +int WinMain() {}
> +int wWinMain() {}
>   #endif
>   
> +// { dg-final { scan-assembler-not "_Z4mainv" } }
> +// { dg-final { scan-assembler-not "_Z5wmainv" { target *-*-mingw32 } } }
> +// { dg-final { scan-assembler-not "_Z7DllMainv" { target *-*-mingw32 } } }
> +// { dg-final { scan-assembler-not "_Z7WinMainv" { target *-*-mingw32 } } }
> +// { dg-final { scan-assembler-not "_Z8wWinMainv" { target *-*-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
> +  };
> +}