Add support for default values for non-generic parameters of generic functions. (#8011)

A first step in expanding support for default values for parameters in
generic functions.

Adds code to parameter type deduction to include default values for
absent call arguments. In support of that, this PR also generalizes a
function from `export.cpp` that is useful to extract a default
`ConstantId` value from an arbitrary function paramater pattern.

toolchain/check/cpp/export.cpp

@@ -746,43 +746,6 @@ auto ExportFieldToCpp(Context& context, SemIR::InstId field_inst_id,
 }
 
 namespace {
-
-// Given a block of parameter patterns, returns a vector of the same size,
-// where each element of that vector contains the value used as the default
-// value for that parameter, or `None` if no default value was specified.
-static auto ExtractParameterDefaults(Context& context,
-                                     SemIR::InstBlockId param_patterns_id)
-    -> llvm::SmallVector<SemIR::ConstantId> {
-  llvm::SmallVector<SemIR::ConstantId> parameter_defaults;
-  for (auto inst_id : context.inst_blocks().Get(param_patterns_id)) {
-    auto pattern_id = context.constant_values().GetConstantInstId(inst_id);
-    while (pattern_id.has_value()) {
-      CARBON_KIND_SWITCH(context.insts().Get(pattern_id)) {
-        case CARBON_KIND(SemIR::DefaultValuePattern inst): {
-          parameter_defaults.push_back(
-              context.constant_values().Get(inst.value_id));
-          pattern_id = SemIR::InstId::None;
-          break;
-        }
-        case CARBON_KIND_ANY(SemIR::AnyBindingPattern, inst): {
-          pattern_id = inst.subpattern_id;
-          break;
-        }
-        case CARBON_KIND_ANY(SemIR::AnyParamPattern, _): {
-          parameter_defaults.push_back(SemIR::ConstantId::None);
-          pattern_id = SemIR::InstId::None;
-          break;
-        }
-        default: {
-          pattern_id = SemIR::InstId::None;
-          break;
-        }
-      }
-    }
-  }
-  return parameter_defaults;
-}
-
 struct FunctionInfo {
   struct Param {
     Param(Context& context, SemIR::InstId param_inst_id,
@@ -818,11 +781,7 @@ struct FunctionInfo {
         export_as_constructor(export_as_constructor) {
     auto function_params =
         context.inst_blocks().Get(function.call_param_patterns_id);
-    auto parameter_defaults =
-        ExtractParameterDefaults(context, function.param_patterns_id);
     const auto& ranges = function.call_param_ranges;
-    CARBON_CHECK(parameter_defaults.size() ==
-                 static_cast<size_t>(ranges.explicit_size()));
     auto explicit_begin = ranges.explicit_begin().index;
     auto param_start = explicit_begin;
 
@@ -837,12 +796,15 @@ struct FunctionInfo {
       ++param_start;
     }
 
+    auto param_patterns =
+        context.inst_blocks().GetOrEmpty(function.param_patterns_id);
     // The remaining explicit parameters are the caller-provided arguments.
     for (auto i = param_start; i != ranges.explicit_end().index; ++i) {
       // We only calculated default values for the explicit parameters, so
       // we need to adjust the index within `parameter_defaults` by any elements
       // in `function_params` that may have preceded them.
-      auto default_value_const_id = parameter_defaults[i - explicit_begin];
+      auto default_value_const_id =
+          ExtractParameterDefault(context, param_patterns[i - explicit_begin]);
       explicit_params.push_back(
           Param(context, function_params[i], default_value_const_id));
     }