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

[Marek Polacek]

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.

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

2019-06-13  Marek Polacek  <pola...@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.

diff --git gcc/cp/decl.c gcc/cp/decl.c
index 0a3ef452536..655de1ea6af 100644
--- gcc/cp/decl.c
+++ gcc/cp/decl.c
@@ -3610,16 +3610,21 @@ case_conversion (tree type, tree value)
 
   value = mark_rvalue_use (value);
 
+  if (INTEGRAL_OR_UNSCOPED_ENUMERATION_TYPE_P (type))
+    type = type_promotes_to (type);
+
   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));
-    }
+    value = (perform_implicit_conversion_flags
+            (type, value, tf_warning_or_error,
+             LOOKUP_IMPLICIT | LOOKUP_NO_NON_INTEGRAL));
+
+  /* The constant-expression VALUE shall be a converted constant expression
+     of the adjusted type of the switch condition, which doesn't allow
+     narrowing conversions.  */
+  check_narrowing (type, value, tf_warning_or_error, /*const_only=*/true);
+
   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." }
+}