Don't crash if a builtin fn is declared with positional parameters. (#5444)

Crash discovered by fuzzer.

toolchain/check/handle_function.cpp

@@ -681,6 +681,11 @@ static auto IsValidBuiltinDeclaration(Context& context,
                                       const SemIR::Function& function,
                                       SemIR::BuiltinFunctionKind builtin_kind)
     -> bool {
+  if (!function.call_params_id.has_value()) {
+    // For now, we have no builtins that support positional parameters.
+    return false;
+  }
+
   // Find the list of call parameters other than the implicit return slot.
   auto call_params = context.inst_blocks().Get(function.call_params_id);
   if (function.return_slot_pattern_id.has_value()) {

toolchain/check/testdata/function/builtin/positional.carbon

@@ -0,0 +1,78 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/function/builtin/positional.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/function/builtin/positional.carbon
+
+// --- fail_positional.carbon
+
+// CHECK:STDERR: fail_positional.carbon:[[@LINE+8]]:1: error: semantics TODO: `function with positional parameters` [SemanticsTodo]
+// CHECK:STDERR: fn Add -> i32 = "int.sadd";
+// CHECK:STDERR: ^~~~~~~~~~~~~~~
+// CHECK:STDERR:
+// CHECK:STDERR: fail_positional.carbon:[[@LINE+4]]:1: error: invalid signature for builtin function "int.sadd" [InvalidBuiltinSignature]
+// CHECK:STDERR: fn Add -> i32 = "int.sadd";
+// CHECK:STDERR: ^~~~~~~~~~~~~~~
+// CHECK:STDERR:
+fn Add -> i32 = "int.sadd";
+
+// CHECK:STDERR: fail_positional.carbon:[[@LINE+8]]:1: error: semantics TODO: `function with positional parameters` [SemanticsTodo]
+// CHECK:STDERR: fn Mul = "int.smul";
+// CHECK:STDERR: ^~~~~~~~
+// CHECK:STDERR:
+// CHECK:STDERR: fail_positional.carbon:[[@LINE+4]]:1: error: invalid signature for builtin function "int.smul" [InvalidBuiltinSignature]
+// CHECK:STDERR: fn Mul = "int.smul";
+// CHECK:STDERR: ^~~~~~~~
+// CHECK:STDERR:
+fn Mul = "int.smul";
+
+// CHECK:STDOUT: --- fail_positional.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %int_32: Core.IntLiteral = int_value 32 [concrete]
+// CHECK:STDOUT:   %Int.type: type = generic_class_type @Int [concrete]
+// CHECK:STDOUT:   %Int.generic: %Int.type = struct_value () [concrete]
+// CHECK:STDOUT:   %i32: type = class_type @Int, @Int(%int_32) [concrete]
+// CHECK:STDOUT:   %pattern_type.7ce: type = pattern_type %i32 [concrete]
+// CHECK:STDOUT:   %Add.type: type = fn_type @Add [concrete]
+// CHECK:STDOUT:   %Add: %Add.type = struct_value () [concrete]
+// CHECK:STDOUT:   %Mul.type: type = fn_type @Mul [concrete]
+// CHECK:STDOUT:   %Mul: %Mul.type = struct_value () [concrete]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT:   %Core: <namespace> = namespace file.%Core.import, [concrete] {
+// CHECK:STDOUT:     .Int = %Core.Int
+// CHECK:STDOUT:     import Core//prelude
+// CHECK:STDOUT:     import Core//prelude/...
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Core.Int: %Int.type = import_ref Core//prelude/types/int, Int, loaded [concrete = constants.%Int.generic]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [concrete] {
+// CHECK:STDOUT:     .Core = imports.%Core
+// CHECK:STDOUT:     .Add = %Add.decl
+// CHECK:STDOUT:     .Mul = %Mul.decl
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Core.import = import Core
+// CHECK:STDOUT:   %Add.decl: %Add.type = fn_decl @Add [concrete = constants.%Add] {
+// CHECK:STDOUT:     %return.patt: %pattern_type.7ce = return_slot_pattern [concrete]
+// CHECK:STDOUT:     %return.param_patt: %pattern_type.7ce = out_param_pattern %return.patt, call_param0 [concrete]
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     %int_32: Core.IntLiteral = int_value 32 [concrete = constants.%int_32]
+// CHECK:STDOUT:     %i32: type = class_type @Int, @Int(constants.%int_32) [concrete = constants.%i32]
+// CHECK:STDOUT:     %return.param: ref %i32 = out_param call_param0
+// CHECK:STDOUT:     %return: ref %i32 = return_slot %return.param
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Mul.decl: %Mul.type = fn_decl @Mul [concrete = constants.%Mul] {} {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @Add() -> %i32;
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @Mul;
+// CHECK:STDOUT: