c-family: Tweak -Woverflow diagnostic

When g++ emits

warning: overflow in conversion from 'int' to 'char' changes value from '300' to '',''

for code like "char c = 300;" it might raise a few eyebrows.  With this
warning we're not interested in the ASCII representation of the char, only
the numerical value, so convert constants of type char to int.  It looks
like this conversion only needs to be done for char_type_node.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?  I'm also happy
to defer this to GCC 13.

gcc/c-family/ChangeLog:

	* c-warn.cc (warnings_for_convert_and_check): Convert constants of type
	char to int.

gcc/testsuite/ChangeLog:

	* c-c++-common/Wconversion-1.c: New test.
---
 gcc/c-family/c-warn.cc                     | 16 +++++++++++-----
 gcc/testsuite/c-c++-common/Wconversion-1.c | 14 ++++++++++++++
 2 files changed, 25 insertions(+), 5 deletions(-)
 create mode 100644 gcc/testsuite/c-c++-common/Wconversion-1.c


base-commit: b4e4b35f4ebe561826489bed971324efc99c5423

On 3/30/22 18:28, Marek Polacek wrote:
> When g++ emits
> 
> warning: overflow in conversion from 'int' to 'char' changes value from '300' to '',''
> 
> for code like "char c = 300;" it might raise a few eyebrows.  With this
> warning we're not interested in the ASCII representation of the char, only
> the numerical value, so convert constants of type char to int.  It looks
> like this conversion only needs to be done for char_type_node.
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?  I'm also happy
> to defer this to GCC 13.

OK for stage 1.

> gcc/c-family/ChangeLog:
> 
> 	* c-warn.cc (warnings_for_convert_and_check): Convert constants of type
> 	char to int.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* c-c++-common/Wconversion-1.c: New test.
> ---
>   gcc/c-family/c-warn.cc                     | 16 +++++++++++-----
>   gcc/testsuite/c-c++-common/Wconversion-1.c | 14 ++++++++++++++
>   2 files changed, 25 insertions(+), 5 deletions(-)
>   create mode 100644 gcc/testsuite/c-c++-common/Wconversion-1.c
> 
> diff --git a/gcc/c-family/c-warn.cc b/gcc/c-family/c-warn.cc
> index f24ac5d0539..cae89294aea 100644
> --- a/gcc/c-family/c-warn.cc
> +++ b/gcc/c-family/c-warn.cc
> @@ -1404,8 +1404,14 @@ warnings_for_convert_and_check (location_t loc, tree type, tree expr,
>       result = TREE_OPERAND (result, 1);
>   
>     bool cst = TREE_CODE_CLASS (TREE_CODE (result)) == tcc_constant;
> -
>     tree exprtype = TREE_TYPE (expr);
> +  tree result_diag;
> +  /* We're interested in the actual numerical value here, not its ASCII
> +     representation.  */
> +  if (cst && TYPE_MAIN_VARIANT (TREE_TYPE (result)) == char_type_node)
> +    result_diag = fold_convert (integer_type_node, result);
> +  else
> +    result_diag = result;
>   
>     if (TREE_CODE (expr) == INTEGER_CST
>         && (TREE_CODE (type) == INTEGER_TYPE
> @@ -1430,7 +1436,7 @@ warnings_for_convert_and_check (location_t loc, tree type, tree expr,
>   				  "changes value from %qE to %qE")
>   			     : G_("unsigned conversion from %qT to %qT "
>   				  "changes value from %qE to %qE")),
> -			    exprtype, type, expr, result);
> +			    exprtype, type, expr, result_diag);
>   	      else
>   		warning_at (loc, OPT_Woverflow,
>   			    (TYPE_UNSIGNED (exprtype)
> @@ -1449,7 +1455,7 @@ warnings_for_convert_and_check (location_t loc, tree type, tree expr,
>   	    warning_at (loc, OPT_Woverflow,
>   			"overflow in conversion from %qT to %qT "
>   			"changes value from %qE to %qE",
> -			exprtype, type, expr, result);
> +			exprtype, type, expr, result_diag);
>   	  else
>   	    warning_at (loc, OPT_Woverflow,
>   			"overflow in conversion from %qT to %qT "
> @@ -1466,7 +1472,7 @@ warnings_for_convert_and_check (location_t loc, tree type, tree expr,
>   	    warning_at (loc, OPT_Woverflow,
>   			"overflow in conversion from %qT to %qT "
>   			"changes value from %qE to %qE",
> -			exprtype, type, expr, result);
> +			exprtype, type, expr, result_diag);
>   	  else
>   	    warning_at (loc, OPT_Woverflow,
>   			"overflow in conversion from %qT to %qT "
> @@ -1483,7 +1489,7 @@ warnings_for_convert_and_check (location_t loc, tree type, tree expr,
>   	warning_at (loc, OPT_Woverflow,
>   		    "overflow in conversion from %qT to %qT "
>   		    "changes value from %qE to %qE",
> -		    exprtype, type, expr, result);
> +		    exprtype, type, expr, result_diag);
>         else
>   	warning_at (loc, OPT_Woverflow,
>   		    "overflow in conversion from %qT to %qT "
> diff --git a/gcc/testsuite/c-c++-common/Wconversion-1.c b/gcc/testsuite/c-c++-common/Wconversion-1.c
> new file mode 100644
> index 00000000000..ed65918c70f
> --- /dev/null
> +++ b/gcc/testsuite/c-c++-common/Wconversion-1.c
> @@ -0,0 +1,14 @@
> +/* { dg-do compile } */
> +/* { dg-options "-Wconversion" } */
> +
> +typedef char T;
> +
> +void g()
> +{
> +  char c = 300; /* { dg-warning "conversion from .int. to .char. changes value from .300. to .44." } */
> +  T t = 300; /* { dg-warning "conversion from .int. to .T. {aka .char.} changes value from .300. to .44." } */
> +  signed char sc = 300; /* { dg-warning "conversion from .int. to .signed char. changes value from .300. to .44." } */
> +  unsigned char uc = 300; /* { dg-warning "conversion from .int. to .unsigned char. changes value from .300. to .44." } */
> +  unsigned char uc2 = 300u; /* { dg-warning "conversion from .unsigned int. to .unsigned char. changes value from .300. to .44." } */
> +  char c2 = (double)1.0 + 200; /* { dg-warning "overflow in conversion from .double. to .char. changes value from .2.01e\\+2. to .127." } */
> +}
> 
> base-commit: b4e4b35f4ebe561826489bed971324efc99c5423