binpac: GH-14: Use larger temporary storage for evaluating case-expressions

For example:

    inum: uint32 = case (ed & 0x0f) of {
        0x00    -> n_8;  # n_8 is a uint8
        0x01    -> n_16; # n_16 is a uint16
        0x02    -> n_32; # n_32 is a uint32
        default -> 0;
    };

Previously, the temporary storage used for evaluating the
case-expression was based on whatever type the first case yields, which
is a uint8 in the above example.  That behavior can lead to a narrowing
conversion whenever the 0x01 or 0x02 cases occur.

The new behavior is to base the temporary storage's type on the largest
numeric type that the case-expression can yield, which is uint32 in the
above example.

tools/binpac/src/pac_expr.cc

@@ -528,6 +528,8 @@ Type *Expr::DataType(Env *env) const
 				{
 				Type *type1 = 
 					cases_->front()->value()->DataType(env);
+				Type* numeric_with_largest_width = 0;
+
 				foreach(i, CaseExprList, cases_)
 					{
 					Type *type2 = 
@@ -541,8 +543,34 @@ Type *Expr::DataType(Env *env) const
 						}
 					if ( type1 == extern_type_nullptr )
 						type1 = type2;
+
+					if ( type2 && type2->IsNumericType() )
+						{
+						if ( numeric_with_largest_width )
+							{
+							int largest;
+							int contender;
+
+							// External C++ types like "int", "bool", "enum" use "int"
+							// storage internally.
+							if ( numeric_with_largest_width->tot() == Type::EXTERN )
+								largest = sizeof(int);
+							else
+								largest = numeric_with_largest_width->StaticSize(env);
+
+							if ( type2->tot() == Type::EXTERN )
+								contender = sizeof(int);
+							else
+								contender = type2->StaticSize(env);
+
+							if ( contender > largest )
+								numeric_with_largest_width = type2;
+							}
+						else
+							numeric_with_largest_width = type2;
+						}
 					}
-				data_type = type1;
+				data_type = numeric_with_largest_width ? numeric_with_largest_width : type1;
 				}
 			else
 				data_type = 0;