Support float template params (#6801)

https://github.com/carbon-language/carbon-lang/issues/6717

toolchain/check/cpp/call.cpp

@@ -154,27 +154,29 @@ static auto ConvertArgToTemplateArg(
       }
     }
 
-    // Handle integer parameters.
-    if (param_type->isIntegerType()) {
-      // Get the Carbon type corresponding to the parameter's Clang type.
-      const auto type_expr =
-          ImportCppType(context, SemIR::LocId(arg_id), param_type);
+    // Get the Carbon type corresponding to the parameter's Clang type.
+    const auto type_expr =
+        ImportCppType(context, SemIR::LocId(arg_id), param_type);
 
-      // Try to convert the argument to the parameter type.
-      const auto converted_inst_id =
-          Convert(context, SemIR::LocId(arg_id), arg_id,
-                  {
-                      .kind = ConversionTarget::Value,
-                      .type_id = type_expr.type_id,
-                  });
+    // Try to convert the argument to the parameter type.
+    const auto converted_inst_id =
+        Convert(context, SemIR::LocId(arg_id), arg_id,
+                {
+                    .kind = ConversionTarget::Value,
+                    .type_id = type_expr.type_id,
+                });
 
-      if (converted_inst_id == SemIR::ErrorInst::InstId) {
-        return std::nullopt;
-      }
+    if (converted_inst_id == SemIR::ErrorInst::InstId) {
+      return std::nullopt;
+    }
+
+    // TODO: provide a better location.
+    auto template_loc = clang::TemplateArgumentLocInfo();
 
-      auto const_inst_id =
-          context.constant_values().GetConstantInstId(converted_inst_id);
-      if (const_inst_id.has_value()) {
+    auto const_inst_id =
+        context.constant_values().GetConstantInstId(converted_inst_id);
+    if (const_inst_id.has_value()) {
+      if (param_type->isIntegerType()) {
         if (auto int_value =
                 context.insts().TryGetAs<SemIR::IntValue>(const_inst_id)) {
           const auto& ap_int = context.ints().Get(int_value->int_id);
@@ -183,11 +185,17 @@ static auto ConvertArgToTemplateArg(
           auto aps_int =
               llvm::APSInt(ap_int, is_unsigned)
                   .extOrTrunc(context.ast_context().getIntWidth(param_type));
-          auto template_arg = clang::TemplateArgument(context.ast_context(),
-                                                      aps_int, param_type);
-          // TODO: provide a better location.
-          auto loc = clang::TemplateArgumentLocInfo();
-          return clang::TemplateArgumentLoc(template_arg, loc);
+          clang::TemplateArgument template_arg(context.ast_context(), aps_int,
+                                               param_type);
+          return clang::TemplateArgumentLoc(template_arg, template_loc);
+        }
+      } else if (param_type->isFloatingType()) {
+        if (auto float_value =
+                context.insts().TryGetAs<SemIR::FloatValue>(const_inst_id)) {
+          const auto& ap_float = context.floats().Get(float_value->float_id);
+          clang::TemplateArgument template_arg(
+              context.ast_context(), param_type, clang::APValue(ap_float));
+          return clang::TemplateArgumentLoc(template_arg, template_loc);
         }
       }
     }

toolchain/check/testdata/interop/cpp/template/non_type_param.carbon

@@ -3,6 +3,7 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 // INCLUDE-FILE: toolchain/testing/testdata/min_prelude/full.carbon
+// EXTRA-ARGS: --clang-arg=-std=c++20
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run:
@@ -75,6 +76,16 @@ impl C as Core.ImplicitAs(i32) {
 }
 var s: Cpp.Simple({} as C) = Cpp.MakeSimple123();
 
+// --- float.carbon
+
+library "[[@TEST_NAME]]";
+
+import Cpp inline '''
+template<float N> struct Struct {};
+Struct<123.5f> MakeStruct() { return Struct<123.5f>(); }
+''';
+let v: Cpp.Struct(123.5) = Cpp.MakeStruct();
+
 // --- valid.carbon
 
 library "[[@TEST_NAME]]";