Fix fuzz bug that found 0-bit width issue. (Resulted in overshift.)

PiperOrigin-RevId: 586138891

Author: cdleary

xls/dslx/type_system/BUILD

@@ -93,6 +93,7 @@ cc_library(
         ":parametric_env",
         ":parametric_instantiator",
         ":type_and_parametric_env",
+        ":type_info",
         ":unwrap_meta_type",
         "@com_google_absl//absl/algorithm:container",
         "@com_google_absl//absl/cleanup",

xls/dslx/type_system/deduce.cc

@@ -60,6 +60,7 @@
 #include "xls/dslx/type_system/parametric_env.h"
 #include "xls/dslx/type_system/parametric_instantiator.h"
 #include "xls/dslx/type_system/type_and_parametric_env.h"
+#include "xls/dslx/type_system/type_info.h"
 #include "xls/dslx/type_system/unwrap_meta_type.h"
 #include "xls/dslx/warning_kind.h"
 #include "xls/ir/bits.h"

xls/dslx/type_system/deduce.h

@@ -20,6 +20,7 @@
 
 #include "absl/status/statusor.h"
 #include "absl/types/span.h"
+#include "xls/dslx/frontend/ast.h"
 #include "xls/dslx/frontend/ast_node.h"
 #include "xls/dslx/type_system/concrete_type.h"
 #include "xls/dslx/type_system/deduce_ctx.h"

xls/dslx/type_system/typecheck_test.cc

@@ -405,6 +405,16 @@ fn main(p: u32) -> () {
                        HasSubstr("const_assert! expression is not constexpr")));
 }
 
+TEST(TypecheckErrorTest, FitsInTypeSN0) {
+  EXPECT_THAT(Typecheck(R"(
+fn main() -> sN[0] {
+  sN[0]:0xffff_ffff_ffff_ffff_ffff
+})"),
+              StatusIs(absl::StatusCode::kInvalidArgument,
+                       HasSubstr("Value '0xffff_ffff_ffff_ffff_ffff' does not "
+                                 "fit in the bitwidth of a sN[0]")));
+}
+
 TEST(TypecheckTest, ForBuiltinInBody) {
   XLS_EXPECT_OK(Typecheck(R"(
 fn f() -> u32 {

xls/ir/bits.cc

@@ -75,11 +75,17 @@ Bits Bits::AllOnes(int64_t bit_count) {
 
 /* static */
 Bits Bits::MaxSigned(int64_t bit_count) {
+  if (bit_count == 0) {
+    return SBits(0, 0);
+  }
   return Bits::AllOnes(bit_count).UpdateWithSet(bit_count - 1, false);
 }
 
 /* static */
 Bits Bits::MinSigned(int64_t bit_count) {
+  if (bit_count == 0) {
+    return SBits(0, 0);
+  }
   return Bits::PowerOfTwo(bit_count - 1, bit_count);
 }
 

xls/ir/bits.h

@@ -21,6 +21,7 @@
 #include <vector>
 
 #include "absl/status/statusor.h"
+#include "absl/types/span.h"
 #include "xls/common/logging/logging.h"
 #include "xls/common/math_util.h"
 #include "xls/data_structures/inline_bitmap.h"