Re: [PATCH] c-family: Tweak -Woverflow diagnostic

[Jason Merrill via Gcc-patches]

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