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Re: C++ PATCH to detect narrowing in case values (PR c++/90805)


On Wed, Jul 03, 2019 at 01:57:06PM -0400, Jason Merrill wrote:
> On 7/3/19 10:13 AM, Marek Polacek wrote:
> > On Sat, Jun 22, 2019 at 11:28:36PM -0400, Jason Merrill wrote:
> > > On 6/13/19 5:03 PM, Marek Polacek wrote:
> > > > Case values are converted constant expressions, so narrowing conversion is not
> > > > permitted.  This patch adds detecting narrowing to case_conversion; it's a
> > > > handy spot because we have both the value and the (adjusted) type of the
> > > > condition.
> > > 
> > > Is there a reason not to use build_converted_constant_expr?
> > 
> > The function comment says "Note that if TYPE and VALUE are already integral
> > we don't really do the conversion because the language-independent
> > warning/optimization code will work better that way" so I avoided adding any
> > conversions.
> 
> > What I could do is to, instead of calling check_narrowing, call
> > build_converted_constant_expr (type, value, tf_warning_or_error);
> > and not use its result, but I'm not sure what the benefits would be.
> 
> I was thinking about using it instead of the current
> perform_implicit_conversion_flags, so we get the somewhat different
> constraints on the conversion.  And then it becomes simpler to use it
> unconditionally but throw the result away in the easy case.

Ah, I see.  So something like this?

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2019-07-17  Marek Polacek  <polacek@redhat.com>

	PR c++/90805 - detect narrowing in case values.
	* decl.c (case_conversion): Detect narrowing in case values.

	* c-c++-common/pr89888.c: Update expected dg-error.
	* g++.dg/cpp0x/Wnarrowing17.C: New test.
	* g++.dg/cpp0x/enum28.C: Update expected dg-error.

diff --git gcc/cp/decl.c gcc/cp/decl.c
index dbcf681c783..2d3ffdfbb54 100644
--- gcc/cp/decl.c
+++ gcc/cp/decl.c
@@ -3630,16 +3630,23 @@ case_conversion (tree type, tree value)
 
   value = mark_rvalue_use (value);
 
+  if (INTEGRAL_OR_UNSCOPED_ENUMERATION_TYPE_P (type))
+    type = type_promotes_to (type);
+
+  tree ovalue = value;
+  /* The constant-expression VALUE shall be a converted constant expression
+     of the adjusted type of the switch condition, which doesn't allow
+     narrowing conversions.  */
+  value = build_converted_constant_expr (type, value, tf_warning_or_error);
+
   if (cxx_dialect >= cxx11
       && (SCOPED_ENUM_P (type)
-	  || !INTEGRAL_OR_UNSCOPED_ENUMERATION_TYPE_P (TREE_TYPE (value))))
-    {
-      if (INTEGRAL_OR_UNSCOPED_ENUMERATION_TYPE_P (type))
-	type = type_promotes_to (type);
-      value = (perform_implicit_conversion_flags
-	       (type, value, tf_warning_or_error,
-		LOOKUP_IMPLICIT | LOOKUP_NO_NON_INTEGRAL));
-    }
+	  || !INTEGRAL_OR_UNSCOPED_ENUMERATION_TYPE_P (TREE_TYPE (ovalue))))
+    /* Use the converted value.  */;
+  else
+    /* The already integral case.  */
+    value = ovalue;
+
   return cxx_constant_value (value);
 }
 
diff --git gcc/testsuite/c-c++-common/pr89888.c gcc/testsuite/c-c++-common/pr89888.c
index d9e11d6f26a..f14881ca052 100644
--- gcc/testsuite/c-c++-common/pr89888.c
+++ gcc/testsuite/c-c++-common/pr89888.c
@@ -11,8 +11,8 @@ foo (unsigned char x)
     {
     case -1: y = -1; break;			/* { dg-message "previously used here" } */
 						/* { dg-warning "case label value is less than minimum value for type" "" { target *-*-* } .-1 } */
-    case 0xffffffff: y = 0xffffffff; break;	/* { dg-error "duplicate case value" } */
-    case ~0U: y = ~0U; break;			/* { dg-error "duplicate case value" } */
+    case 0xffffffff: y = 0xffffffff; break;	/* { dg-error "duplicate case value|narrowing" } */
+    case ~0U: y = ~0U; break;			/* { dg-error "duplicate case value|narrowing" } */
     }
 }
 
diff --git gcc/testsuite/g++.dg/cpp0x/Wnarrowing17.C gcc/testsuite/g++.dg/cpp0x/Wnarrowing17.C
new file mode 100644
index 00000000000..064de531cb3
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp0x/Wnarrowing17.C
@@ -0,0 +1,19 @@
+// PR c++/90805 - detect narrowing in case values.
+// { dg-do compile { target c++11 } }
+
+void f(int i, char c, unsigned u)
+{
+  switch (i)
+    {
+    case 2149056512u:; // { dg-error "narrowing conversion of .2149056512. from .unsigned int. to .int." }
+    case (long long int) 1e10:; // { dg-error "narrowing conversion of .10000000000. from .long long int. to .int." }
+    // { dg-warning "overflow in conversion" "overflow" { target *-*-* } .-1 }
+    }
+
+  switch (c)
+    // No narrowing, the adjusted type is int.
+    case 300:; // { dg-warning "exceeds maximum value for type" }
+
+  switch (u)
+    case -42:; // { dg-error "narrowing conversion of .-42. from .int. to .unsigned int." }
+}
diff --git gcc/testsuite/g++.dg/cpp0x/enum28.C gcc/testsuite/g++.dg/cpp0x/enum28.C
index 3967699dd03..bfebde57cb3 100644
--- gcc/testsuite/g++.dg/cpp0x/enum28.C
+++ gcc/testsuite/g++.dg/cpp0x/enum28.C
@@ -7,11 +7,11 @@ void f(int i)
 {
   switch (i)
     {
-    case 1.0:;    // { dg-error "could not convert" }
+    case 1.0:;    // { dg-error "could not convert|conversion from" }
     }
 
   switch (i)
     {
-    case g():;    // { dg-error "could not convert" }
+    case g():;    // { dg-error "could not convert|conversion from" }
     }
 }


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