Error on non-constant parameters to a type. (#4215)

At present, this is a crash bug. I don't know whether this is the best
fix, but I figure it'll work until zygoloid has a chance to look.

toolchain/check/generic.cpp

@@ -415,4 +415,29 @@ auto ResolveSpecificDefinition(Context& context, SemIR::SpecificId specific_id)
   return true;
 }
 
+auto RequireGenericParams(Context& context, SemIR::InstBlockId block_id)
+    -> void {
+  if (!block_id.is_valid() || block_id == SemIR::InstBlockId::Empty) {
+    return;
+  }
+
+  for (auto& inst_id : context.inst_blocks().Get(block_id)) {
+    if (!context.constant_values().Get(inst_id).is_constant()) {
+      CARBON_DIAGNOSTIC(GenericParamMustBeConstant, Error,
+                        "Parameters of generic types must be constant.");
+      context.emitter().Emit(inst_id, GenericParamMustBeConstant);
+
+      // Replace the parameter with an invalid instruction so that we don't try
+      // constructing a generic based on it. Note this is updating the param
+      // refs block, not the actual params block, so will not be directly
+      // reflected in SemIR output.
+      inst_id = context.AddInstInNoBlock(
+          SemIR::LocIdAndInst::ReusingLoc<SemIR::Param>(
+              context.insts().GetLocId(inst_id),
+              {.type_id = SemIR::TypeId::Error,
+               .name_id = SemIR::NameId::Base}));
+    }
+  }
+}
+
 }  // namespace Carbon::Check

toolchain/check/generic.h

@@ -68,6 +68,13 @@ auto MakeSelfSpecific(Context& context, SemIR::GenericId generic_id)
 auto ResolveSpecificDefinition(Context& context, SemIR::SpecificId specific_id)
     -> bool;
 
+// Requires that a param block only contains generics. Diagnoses and updates the
+// block otherwise. This will typically be called once for each of implicit and
+// explicit parameters, and must occur before constant evaluation of the
+// parameterized instruction.
+auto RequireGenericParams(Context& context, SemIR::InstBlockId block_id)
+    -> void;
+
 }  // namespace Carbon::Check
 
 #endif  // CARBON_TOOLCHAIN_CHECK_GENERIC_H_

toolchain/check/handle_class.cpp

@@ -224,6 +224,9 @@ static auto BuildClassDecl(Context& context, Parse::AnyClassDeclId node_id,
        .self_type_id = SemIR::TypeId::Invalid,
        .inheritance_kind = inheritance_kind}};
 
+  RequireGenericParams(context, class_info.implicit_param_refs_id);
+  RequireGenericParams(context, class_info.param_refs_id);
+
   MergeOrAddName(context, node_id, name_context, class_decl_id, class_decl,
                  class_info, is_definition, is_extern,
                  introducer.modifier_set.GetAccessKind());

toolchain/check/handle_interface.cpp

@@ -53,6 +53,11 @@ static auto BuildInterfaceDecl(Context& context,
   auto interface_decl_id =
       context.AddPlaceholderInst(SemIR::LocIdAndInst(node_id, interface_decl));
 
+  SemIR::Interface interface_info = {name_context.MakeEntityWithParamsBase(
+      name, interface_decl_id, /*is_extern=*/false)};
+  RequireGenericParams(context, interface_info.implicit_param_refs_id);
+  RequireGenericParams(context, interface_info.param_refs_id);
+
   // Check whether this is a redeclaration.
   auto existing_id = context.decl_name_stack().LookupOrAddName(
       name_context, interface_decl_id, introducer.modifier_set.GetAccessKind());
@@ -103,8 +108,6 @@ static auto BuildInterfaceDecl(Context& context,
     // there was an error in the qualifier, we will have lost track of the
     // interface name here. We should keep track of it even if the name is
     // invalid.
-    SemIR::Interface interface_info = {name_context.MakeEntityWithParamsBase(
-        name, interface_decl_id, /*is_extern=*/false)};
     interface_info.generic_id = FinishGenericDecl(context, interface_decl_id);
     interface_decl.interface_id = context.interfaces().Add(interface_info);
     if (interface_info.has_parameters()) {