diff --git a/toolchain/check/BUILD b/toolchain/check/BUILD
index bac9db6934f1..6643a9f2e152 100644
--- a/toolchain/check/BUILD
+++ b/toolchain/check/BUILD
@@ -54,10 +54,13 @@ cc_library(
         "convert.cpp",
         "decl_name_stack.cpp",
         "eval.cpp",
+        "function.cpp",
         "import_ref.cpp",
         "inst_block_stack.cpp",
+        "merge.cpp",
         "modifiers.cpp",
         "return.cpp",
+        "subst.cpp",
     ],
     hdrs = [
         "context.h",
@@ -66,12 +69,15 @@ cc_library(
         "decl_state.h",
         "diagnostic_helpers.h",
         "eval.h",
+        "function.h",
         "import_ref.h",
         "inst_block_stack.h",
+        "merge.h",
         "modifiers.h",
         "param_and_arg_refs_stack.h",
         "pending_block.h",
         "return.h",
+        "subst.h",
     ],
     deps = [
         ":node_stack",
@@ -104,7 +110,6 @@ cc_library(
     deps = [
         ":call",
         ":context",
-        ":function",
         ":impl",
         ":import",
         ":interface",
@@ -155,26 +160,12 @@ cc_library(
     ],
 )
 
-cc_library(
-    name = "function",
-    srcs = ["function.cpp"],
-    hdrs = ["function.h"],
-    deps = [
-        ":context",
-        ":subst",
-        "//common:check",
-        "//toolchain/sem_ir:file",
-    ],
-)
-
 cc_library(
     name = "impl",
     srcs = ["impl.cpp"],
     hdrs = ["impl.h"],
     deps = [
         ":context",
-        ":function",
-        ":subst",
         "//common:check",
         "//toolchain/diagnostics:diagnostic_emitter",
         "//toolchain/sem_ir:file",
@@ -219,7 +210,6 @@ cc_library(
     hdrs = ["member_access.h"],
     deps = [
         ":context",
-        ":subst",
         "//common:check",
         "//toolchain/diagnostics:diagnostic_emitter",
         "//toolchain/sem_ir:file",
@@ -262,22 +252,6 @@ cc_library(
     ],
 )
 
-cc_library(
-    name = "subst",
-    srcs = ["subst.cpp"],
-    hdrs = ["subst.h"],
-    deps = [
-        ":context",
-        "//common:check",
-        "//toolchain/diagnostics:diagnostic_emitter",
-        "//toolchain/sem_ir:file",
-        "//toolchain/sem_ir:ids",
-        "//toolchain/sem_ir:inst",
-        "//toolchain/sem_ir:inst_kind",
-        "@llvm-project//llvm:Support",
-    ],
-)
-
 glob_sh_run(
     args = [
         "$(location //toolchain/driver:carbon)",
diff --git a/toolchain/check/function.cpp b/toolchain/check/function.cpp
index a6cdb14506a1..f37233623de0 100644
--- a/toolchain/check/function.cpp
+++ b/toolchain/check/function.cpp
@@ -193,7 +193,7 @@ auto CheckFunctionTypeMatches(Context& context,
 
 // Checks to see if a structurally valid redeclaration is allowed in context.
 // These all still merge.
-static auto CheckIsAllowedRedecl(Context& context, Parse::NodeId node_id,
+static auto CheckIsAllowedRedecl(Context& context, SemIR::LocationId loc_id,
                                  SemIR::Function& new_function,
                                  bool new_is_definition,
                                  SemIR::Function& prev_function,
@@ -207,7 +207,7 @@ static auto CheckIsAllowedRedecl(Context& context, Parse::NodeId node_id,
           "Only one library can declare function {0} without `extern`.",
           SemIR::NameId);
       context.emitter()
-          .Build(node_id, FunctionNonExternRedecl, prev_function.name_id)
+          .Build(loc_id, FunctionNonExternRedecl, prev_function.name_id)
           .Note(prev_function.decl_id, FunctionPreviousDecl)
           .Emit();
       return;
@@ -218,7 +218,7 @@ static auto CheckIsAllowedRedecl(Context& context, Parse::NodeId node_id,
                         "Redundant redeclaration of function {0}.",
                         SemIR::NameId);
       context.emitter()
-          .Build(node_id, FunctionRedecl, prev_function.name_id)
+          .Build(loc_id, FunctionRedecl, prev_function.name_id)
           .Note(prev_function.decl_id, FunctionPreviousDecl)
           .Emit();
       return;
@@ -229,7 +229,7 @@ static auto CheckIsAllowedRedecl(Context& context, Parse::NodeId node_id,
       CARBON_DIAGNOSTIC(FunctionPreviousDefinition, Note,
                         "Previously defined here.");
       context.emitter()
-          .Build(node_id, FunctionRedefinition, prev_function.name_id)
+          .Build(loc_id, FunctionRedefinition, prev_function.name_id)
           .Note(prev_function.definition_id, FunctionPreviousDefinition)
           .Emit();
       return;
@@ -243,7 +243,7 @@ static auto CheckIsAllowedRedecl(Context& context, Parse::NodeId node_id,
       CARBON_DIAGNOSTIC(FunctionPreviousExternDecl, Note,
                         "Previously declared `extern` here.");
       context.emitter()
-          .Build(node_id, FunctionDefiningExtern, prev_function.name_id)
+          .Build(loc_id, FunctionDefiningExtern, prev_function.name_id)
           .Note(prev_function.decl_id, FunctionPreviousExternDecl)
           .Emit();
       return;
@@ -253,7 +253,7 @@ static auto CheckIsAllowedRedecl(Context& context, Parse::NodeId node_id,
 
 // TODO: Detect conflicting cross-file declarations, as well as uses of imported
 // declarations followed by a redeclaration.
-auto MergeFunctionRedecl(Context& context, Parse::NodeId node_id,
+auto MergeFunctionRedecl(Context& context, SemIR::LocationId loc_id,
                          SemIR::Function& new_function, bool new_is_definition,
                          SemIR::FunctionId prev_function_id,
                          bool prev_is_import) -> bool {
@@ -263,7 +263,7 @@ auto MergeFunctionRedecl(Context& context, Parse::NodeId node_id,
     return false;
   }
 
-  CheckIsAllowedRedecl(context, node_id, new_function, new_is_definition,
+  CheckIsAllowedRedecl(context, loc_id, new_function, new_is_definition,
                        prev_function, prev_is_import);
 
   if (new_is_definition) {
diff --git a/toolchain/check/function.h b/toolchain/check/function.h
index 69f2cb0ea9ea..ceb6d351b1ff 100644
--- a/toolchain/check/function.h
+++ b/toolchain/check/function.h
@@ -8,6 +8,7 @@
 #include "toolchain/check/context.h"
 #include "toolchain/check/subst.h"
 #include "toolchain/sem_ir/function.h"
+#include "toolchain/sem_ir/ids.h"
 
 namespace Carbon::Check {
 
@@ -27,7 +28,7 @@ auto CheckFunctionTypeMatches(Context& context,
 //
 // If merging is successful, updates the FunctionId on new_function and returns
 // true. Otherwise, returns false. Prints a diagnostic when appropriate.
-auto MergeFunctionRedecl(Context& context, Parse::NodeId node_id,
+auto MergeFunctionRedecl(Context& context, SemIR::LocationId loc_id,
                          SemIR::Function& new_function, bool new_is_definition,
                          SemIR::FunctionId prev_function_id,
                          bool prev_is_imported) -> bool;
diff --git a/toolchain/check/import.cpp b/toolchain/check/import.cpp
index 8890a2959bf7..5b52eddb5366 100644
--- a/toolchain/check/import.cpp
+++ b/toolchain/check/import.cpp
@@ -6,6 +6,7 @@
 
 #include "common/check.h"
 #include "toolchain/check/context.h"
+#include "toolchain/check/merge.h"
 #include "toolchain/parse/node_ids.h"
 #include "toolchain/sem_ir/file.h"
 #include "toolchain/sem_ir/ids.h"
@@ -242,15 +243,11 @@ auto ImportLibraryFromCurrentPackage(Context& context,
     } else {
       // Leave a placeholder that the inst comes from the other IR.
       auto target_id = context.AddImportRef(ir_id, import_inst_id);
-      // TODO: When importing from other packages, the scope's names should
-      // be changed to allow for ambiguous names. When importing from the
-      // current package, as is currently being done, we should issue a
-      // diagnostic on conflicts.
       auto [it, success] = context.name_scopes()
                                .Get(enclosing_scope_id)
                                .names.insert({name_id, target_id});
       if (!success) {
-        context.DiagnoseDuplicateName(target_id, it->second);
+        MergeImportRef(context, target_id, it->second);
       }
     }
   }
diff --git a/toolchain/check/merge.cpp b/toolchain/check/merge.cpp
new file mode 100644
index 000000000000..9b7720bebc6d
--- /dev/null
+++ b/toolchain/check/merge.cpp
@@ -0,0 +1,81 @@
+// 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
+
+#include "toolchain/check/merge.h"
+
+#include "toolchain/check/function.h"
+#include "toolchain/check/import_ref.h"
+
+namespace Carbon::Check {
+
+// Returns the instruction to consider when merging the given inst_id. Returns
+// nullopt if merging is infeasible and no diagnostic should be printed.
+static auto ResolveMergeableInst(Context& context, SemIR::InstId inst_id)
+    -> std::optional<SemIR::Inst> {
+  auto inst = context.insts().Get(inst_id);
+  switch (inst.kind()) {
+    case SemIR::ImportRefUnused::Kind:
+      // Resolve before merging.
+      TryResolveImportRefUnused(context, inst_id);
+      break;
+
+    case SemIR::ImportRefUsed::Kind:
+      // Already resolved.
+      break;
+
+    case SemIR::Namespace::Kind:
+      // Return back the namespace directly.
+      return inst;
+
+    default:
+      CARBON_FATAL() << "Unexpected inst kind passed to ResolveMergeableInst: "
+                     << inst;
+  }
+
+  auto const_id = context.constant_values().Get(inst_id);
+  // TODO: Function and type declarations are constant, but `var` declarations
+  // are non-constant and should still merge.
+  if (!const_id.is_constant()) {
+    return std::nullopt;
+  }
+  return context.insts().Get(const_id.inst_id());
+}
+
+auto MergeImportRef(Context& context, SemIR::InstId new_inst_id,
+                    SemIR::InstId prev_inst_id) -> void {
+  auto new_inst = ResolveMergeableInst(context, new_inst_id);
+  auto prev_inst = ResolveMergeableInst(context, prev_inst_id);
+  if (!new_inst || !prev_inst) {
+    // TODO: Once `var` declarations get an associated instruction for handling,
+    // it might be more appropriate to return without diagnosing here, to handle
+    // invalid declarations.
+    context.DiagnoseDuplicateName(new_inst_id, prev_inst_id);
+    return;
+  }
+
+  if (new_inst->kind() != prev_inst->kind()) {
+    context.DiagnoseDuplicateName(new_inst_id, prev_inst_id);
+    return;
+  }
+
+  switch (new_inst->kind()) {
+    case SemIR::FunctionDecl::Kind: {
+      auto new_fn = context.functions().Get(
+          new_inst->As<SemIR::FunctionDecl>().function_id);
+      auto prev_fn_id = prev_inst->As<SemIR::FunctionDecl>().function_id;
+      // TODO: May need to "spoil" the new function to prevent it from being
+      // emitted, since it will already be added.
+      MergeFunctionRedecl(context, context.insts().GetLocationId(new_inst_id),
+                          new_fn,
+                          /*new_is_definition=*/false, prev_fn_id,
+                          /*prev_is_imported=*/true);
+      return;
+    }
+    default:
+      context.TODO(new_inst_id, "Merging not yet supported.");
+      return;
+  }
+}
+
+}  // namespace Carbon::Check
diff --git a/toolchain/check/merge.h b/toolchain/check/merge.h
new file mode 100644
index 000000000000..ab5252577b5e
--- /dev/null
+++ b/toolchain/check/merge.h
@@ -0,0 +1,20 @@
+// 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
+
+#ifndef CARBON_TOOLCHAIN_CHECK_MERGE_H_
+#define CARBON_TOOLCHAIN_CHECK_MERGE_H_
+
+#include "toolchain/check/context.h"
+#include "toolchain/sem_ir/ids.h"
+
+namespace Carbon::Check {
+
+// Merges an import ref at new_inst_id another at prev_inst_id. May print a
+// diagnostic if merging is invalid.
+auto MergeImportRef(Context& context, SemIR::InstId new_inst_id,
+                    SemIR::InstId prev_inst_id) -> void;
+
+}  // namespace Carbon::Check
+
+#endif  // CARBON_TOOLCHAIN_CHECK_MERGE_H_
diff --git a/toolchain/check/testdata/function/declaration/extern.carbon b/toolchain/check/testdata/function/declaration/extern.carbon
index 55db48ddc1b9..bdb1d304d193 100644
--- a/toolchain/check/testdata/function/declaration/extern.carbon
+++ b/toolchain/check/testdata/function/declaration/extern.carbon
@@ -61,7 +61,7 @@ class C {
 // CHECK:STDOUT:   %F: <function> = fn_decl @F [template] {}
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @F();
+// CHECK:STDOUT: extern fn @F();
 // CHECK:STDOUT:
 // CHECK:STDOUT: --- fail_redecl.carbon
 // CHECK:STDOUT:
@@ -73,7 +73,7 @@ class C {
 // CHECK:STDOUT:   %F.loc12: <function> = fn_decl @F [template] {}
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @F();
+// CHECK:STDOUT: extern fn @F();
 // CHECK:STDOUT:
 // CHECK:STDOUT: --- fail_redecl_extern.carbon
 // CHECK:STDOUT:
diff --git a/toolchain/check/testdata/function/declaration/import.carbon b/toolchain/check/testdata/function/declaration/import.carbon
index fbf60fed5bf1..6a6f413040fd 100644
--- a/toolchain/check/testdata/function/declaration/import.carbon
+++ b/toolchain/check/testdata/function/declaration/import.carbon
@@ -73,63 +73,11 @@ var b: i32 = B(1);
 var c: {.c: i32} = C((1,));
 var d: () = D();
 
-// --- fail_todo_merge.carbon
+// --- merge.carbon
 
 library "merge" api;
 
 import library "api";
-// CHECK:STDERR: fail_todo_merge.carbon:[[@LINE+52]]:1: In import.
-// CHECK:STDERR: import library "extern_api";
-// CHECK:STDERR: ^~~~~~
-// CHECK:STDERR: extern_api.carbon:4:1: ERROR: Duplicate name being declared in the same scope.
-// CHECK:STDERR: extern fn A();
-// CHECK:STDERR: ^~~~~~~~~~~~~~
-// CHECK:STDERR: fail_todo_merge.carbon:[[@LINE-7]]:1: In import.
-// CHECK:STDERR: import library "api";
-// CHECK:STDERR: ^~~~~~
-// CHECK:STDERR: api.carbon:4:1: Name is previously declared here.
-// CHECK:STDERR: fn A();
-// CHECK:STDERR: ^~~~~~~
-// CHECK:STDERR:
-// CHECK:STDERR: fail_todo_merge.carbon:[[@LINE+39]]:1: In import.
-// CHECK:STDERR: import library "extern_api";
-// CHECK:STDERR: ^~~~~~
-// CHECK:STDERR: extern_api.carbon:5:1: ERROR: Duplicate name being declared in the same scope.
-// CHECK:STDERR: extern fn B(b: i32) -> i32;
-// CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~~~~~~~
-// CHECK:STDERR: fail_todo_merge.carbon:[[@LINE-20]]:1: In import.
-// CHECK:STDERR: import library "api";
-// CHECK:STDERR: ^~~~~~
-// CHECK:STDERR: api.carbon:5:1: Name is previously declared here.
-// CHECK:STDERR: fn B(b: i32) -> i32;
-// CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~
-// CHECK:STDERR:
-// CHECK:STDERR: fail_todo_merge.carbon:[[@LINE+26]]:1: In import.
-// CHECK:STDERR: import library "extern_api";
-// CHECK:STDERR: ^~~~~~
-// CHECK:STDERR: extern_api.carbon:6:1: ERROR: Duplicate name being declared in the same scope.
-// CHECK:STDERR: extern fn C(c: (i32,)) -> {.c: i32};
-// CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-// CHECK:STDERR: fail_todo_merge.carbon:[[@LINE-33]]:1: In import.
-// CHECK:STDERR: import library "api";
-// CHECK:STDERR: ^~~~~~
-// CHECK:STDERR: api.carbon:6:1: Name is previously declared here.
-// CHECK:STDERR: fn C(c: (i32,)) -> {.c: i32};
-// CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-// CHECK:STDERR:
-// CHECK:STDERR: fail_todo_merge.carbon:[[@LINE+13]]:1: In import.
-// CHECK:STDERR: import library "extern_api";
-// CHECK:STDERR: ^~~~~~
-// CHECK:STDERR: extern_api.carbon:7:1: ERROR: Duplicate name being declared in the same scope.
-// CHECK:STDERR: extern fn D();
-// CHECK:STDERR: ^~~~~~~~~~~~~~
-// CHECK:STDERR: fail_todo_merge.carbon:[[@LINE-46]]:1: In import.
-// CHECK:STDERR: import library "api";
-// CHECK:STDERR: ^~~~~~
-// CHECK:STDERR: api.carbon:7:1: Name is previously declared here.
-// CHECK:STDERR: extern fn D();
-// CHECK:STDERR: ^~~~~~~~~~~~~~
-// CHECK:STDERR:
 import library "extern_api";
 
 var a: () = A();
@@ -137,62 +85,11 @@ var b: i32 = B(1);
 var c: {.c: i32} = C((1,));
 var d: () = D();
 
-// --- fail_todo_merge_reverse.carbon
+// --- merge_reverse.carbon
 
 library "merge_reverse" api;
 
 import library "extern_api";
-// CHECK:STDERR: fail_todo_merge_reverse.carbon:[[@LINE+51]]:1: In import.
-// CHECK:STDERR: import library "api";
-// CHECK:STDERR: ^~~~~~
-// CHECK:STDERR: api.carbon:4:1: ERROR: Duplicate name being declared in the same scope.
-// CHECK:STDERR: fn A();
-// CHECK:STDERR: ^~~~~~~
-// CHECK:STDERR: fail_todo_merge_reverse.carbon:[[@LINE-7]]:1: In import.
-// CHECK:STDERR: import library "extern_api";
-// CHECK:STDERR: ^~~~~~
-// CHECK:STDERR: extern_api.carbon:4:1: Name is previously declared here.
-// CHECK:STDERR: extern fn A();
-// CHECK:STDERR: ^~~~~~~~~~~~~~
-// CHECK:STDERR:
-// CHECK:STDERR: fail_todo_merge_reverse.carbon:[[@LINE+38]]:1: In import.
-// CHECK:STDERR: import library "api";
-// CHECK:STDERR: ^~~~~~
-// CHECK:STDERR: api.carbon:5:1: ERROR: Duplicate name being declared in the same scope.
-// CHECK:STDERR: fn B(b: i32) -> i32;
-// CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~
-// CHECK:STDERR: fail_todo_merge_reverse.carbon:[[@LINE-20]]:1: In import.
-// CHECK:STDERR: import library "extern_api";
-// CHECK:STDERR: ^~~~~~
-// CHECK:STDERR: extern_api.carbon:5:1: Name is previously declared here.
-// CHECK:STDERR: extern fn B(b: i32) -> i32;
-// CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~~~~~~~
-// CHECK:STDERR:
-// CHECK:STDERR: fail_todo_merge_reverse.carbon:[[@LINE+25]]:1: In import.
-// CHECK:STDERR: import library "api";
-// CHECK:STDERR: ^~~~~~
-// CHECK:STDERR: api.carbon:6:1: ERROR: Duplicate name being declared in the same scope.
-// CHECK:STDERR: fn C(c: (i32,)) -> {.c: i32};
-// CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-// CHECK:STDERR: fail_todo_merge_reverse.carbon:[[@LINE-33]]:1: In import.
-// CHECK:STDERR: import library "extern_api";
-// CHECK:STDERR: ^~~~~~
-// CHECK:STDERR: extern_api.carbon:6:1: Name is previously declared here.
-// CHECK:STDERR: extern fn C(c: (i32,)) -> {.c: i32};
-// CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-// CHECK:STDERR:
-// CHECK:STDERR: fail_todo_merge_reverse.carbon:[[@LINE+12]]:1: In import.
-// CHECK:STDERR: import library "api";
-// CHECK:STDERR: ^~~~~~
-// CHECK:STDERR: api.carbon:7:1: ERROR: Duplicate name being declared in the same scope.
-// CHECK:STDERR: extern fn D();
-// CHECK:STDERR: ^~~~~~~~~~~~~~
-// CHECK:STDERR: fail_todo_merge_reverse.carbon:[[@LINE-46]]:1: In import.
-// CHECK:STDERR: import library "extern_api";
-// CHECK:STDERR: ^~~~~~
-// CHECK:STDERR: extern_api.carbon:7:1: Name is previously declared here.
-// CHECK:STDERR: extern fn D();
-// CHECK:STDERR: ^~~~~~~~~~~~~~
 import library "api";
 
 var a: () = A();
@@ -238,7 +135,7 @@ var d: () = D();
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @C(%c: (i32,)) -> {.c: i32};
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @D();
+// CHECK:STDOUT: extern fn @D();
 // CHECK:STDOUT:
 // CHECK:STDOUT: --- extern_api.carbon
 // CHECK:STDOUT:
@@ -272,13 +169,13 @@ var d: () = D();
 // CHECK:STDOUT:   %D: <function> = fn_decl @D [template] {}
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @A();
+// CHECK:STDOUT: extern fn @A();
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @B(%b: i32) -> i32;
+// CHECK:STDOUT: extern fn @B(%b: i32) -> i32;
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @C(%c: (i32,)) -> {.c: i32};
+// CHECK:STDOUT: extern fn @C(%c: (i32,)) -> {.c: i32};
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @D();
+// CHECK:STDOUT: extern fn @D();
 // CHECK:STDOUT:
 // CHECK:STDOUT: --- basics.carbon
 // CHECK:STDOUT:
@@ -326,7 +223,7 @@ var d: () = D();
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @C(%c: (i32,)) -> {.c: i32};
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @D();
+// CHECK:STDOUT: extern fn @D();
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @__global_init() {
 // CHECK:STDOUT: !entry:
@@ -412,7 +309,7 @@ var d: () = D();
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @C(%c: (i32,)) -> {.c: i32};
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @D();
+// CHECK:STDOUT: extern fn @D();
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @__global_init() {
 // CHECK:STDOUT: !entry:
@@ -492,13 +389,13 @@ var d: () = D();
 // CHECK:STDOUT:   %d: ref () = bind_name d, %d.var
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @A();
+// CHECK:STDOUT: extern fn @A();
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @B(%b: i32) -> i32;
+// CHECK:STDOUT: extern fn @B(%b: i32) -> i32;
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @C(%c: (i32,)) -> {.c: i32};
+// CHECK:STDOUT: extern fn @C(%c: (i32,)) -> {.c: i32};
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @D();
+// CHECK:STDOUT: extern fn @D();
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @__global_init() {
 // CHECK:STDOUT: !entry:
@@ -522,14 +419,14 @@ var d: () = D();
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: --- fail_todo_merge.carbon
+// CHECK:STDOUT: --- merge.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
-// CHECK:STDOUT:   %.1: type = tuple_type () [template]
-// CHECK:STDOUT:   %.2: i32 = int_literal 1 [template]
-// CHECK:STDOUT:   %.3: type = struct_type {.c: i32} [template]
-// CHECK:STDOUT:   %.4: type = tuple_type (i32) [template]
-// CHECK:STDOUT:   %.5: (i32,) = tuple_value (%.2) [template]
+// CHECK:STDOUT:   %.1: type = tuple_type (i32) [template]
+// CHECK:STDOUT:   %.2: type = struct_type {.c: i32} [template]
+// CHECK:STDOUT:   %.3: type = tuple_type () [template]
+// CHECK:STDOUT:   %.4: i32 = int_literal 1 [template]
+// CHECK:STDOUT:   %.5: (i32,) = tuple_value (%.4) [template]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: file {
@@ -543,67 +440,75 @@ var d: () = D();
 // CHECK:STDOUT:     .c = %c
 // CHECK:STDOUT:     .d = %d
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %import_ref.1: <function> = import_ref ir1, inst+1, used [template = imports.%A]
-// CHECK:STDOUT:   %import_ref.2: <function> = import_ref ir1, inst+5, used [template = imports.%B]
-// CHECK:STDOUT:   %import_ref.3: <function> = import_ref ir1, inst+17, used [template = imports.%C]
-// CHECK:STDOUT:   %import_ref.4: <function> = import_ref ir1, inst+18, used [template = imports.%D]
-// CHECK:STDOUT:   %import_ref.5 = import_ref ir2, inst+1, unused
-// CHECK:STDOUT:   %import_ref.6 = import_ref ir2, inst+5, unused
-// CHECK:STDOUT:   %import_ref.7 = import_ref ir2, inst+17, unused
-// CHECK:STDOUT:   %import_ref.8 = import_ref ir2, inst+18, unused
-// CHECK:STDOUT:   %.loc59_9.1: () = tuple_literal ()
-// CHECK:STDOUT:   %.loc59_9.2: type = converted %.loc59_9.1, constants.%.1 [template = constants.%.1]
+// CHECK:STDOUT:   %import_ref.1: <function> = import_ref ir1, inst+1, used [template = imports.%A.1]
+// CHECK:STDOUT:   %import_ref.2: <function> = import_ref ir1, inst+5, used [template = imports.%B.1]
+// CHECK:STDOUT:   %import_ref.3: <function> = import_ref ir1, inst+17, used [template = imports.%C.1]
+// CHECK:STDOUT:   %import_ref.4: <function> = import_ref ir1, inst+18, used [template = imports.%D.1]
+// CHECK:STDOUT:   %import_ref.5: <function> = import_ref ir2, inst+1, used [template = imports.%A.2]
+// CHECK:STDOUT:   %import_ref.6: <function> = import_ref ir2, inst+5, used [template = imports.%B.2]
+// CHECK:STDOUT:   %import_ref.7: <function> = import_ref ir2, inst+17, used [template = imports.%C.2]
+// CHECK:STDOUT:   %import_ref.8: <function> = import_ref ir2, inst+18, used [template = imports.%D.2]
+// CHECK:STDOUT:   %.loc7_9.1: () = tuple_literal ()
+// CHECK:STDOUT:   %.loc7_9.2: type = converted %.loc7_9.1, constants.%.3 [template = constants.%.3]
 // CHECK:STDOUT:   %a.var: ref () = var a
 // CHECK:STDOUT:   %a: ref () = bind_name a, %a.var
 // CHECK:STDOUT:   %b.var: ref i32 = var b
 // CHECK:STDOUT:   %b: ref i32 = bind_name b, %b.var
-// CHECK:STDOUT:   %.loc61: type = struct_type {.c: i32} [template = constants.%.3]
+// CHECK:STDOUT:   %.loc9: type = struct_type {.c: i32} [template = constants.%.2]
 // CHECK:STDOUT:   %c.var: ref {.c: i32} = var c
 // CHECK:STDOUT:   %c: ref {.c: i32} = bind_name c, %c.var
-// CHECK:STDOUT:   %.loc62_9.1: () = tuple_literal ()
-// CHECK:STDOUT:   %.loc62_9.2: type = converted %.loc62_9.1, constants.%.1 [template = constants.%.1]
+// CHECK:STDOUT:   %.loc10_9.1: () = tuple_literal ()
+// CHECK:STDOUT:   %.loc10_9.2: type = converted %.loc10_9.1, constants.%.3 [template = constants.%.3]
 // CHECK:STDOUT:   %d.var: ref () = var d
 // CHECK:STDOUT:   %d: ref () = bind_name d, %d.var
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @A();
+// CHECK:STDOUT: extern fn @A.1();
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @B(%b: i32) -> i32;
+// CHECK:STDOUT: fn @A.2();
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @C(%c: (i32,)) -> {.c: i32};
+// CHECK:STDOUT: extern fn @B.1(%b: i32) -> i32;
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @D();
+// CHECK:STDOUT: fn @B.2(%b: i32) -> i32;
+// CHECK:STDOUT:
+// CHECK:STDOUT: extern fn @C.1(%c: (i32,)) -> {.c: i32};
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @C.2(%c: (i32,)) -> {.c: i32};
+// CHECK:STDOUT:
+// CHECK:STDOUT: extern fn @D.1();
+// CHECK:STDOUT:
+// CHECK:STDOUT: extern fn @D.2();
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @__global_init() {
 // CHECK:STDOUT: !entry:
-// CHECK:STDOUT:   %A.ref: <function> = name_ref A, file.%import_ref.1 [template = imports.%A]
-// CHECK:STDOUT:   %.loc59: init () = call %A.ref()
-// CHECK:STDOUT:   assign file.%a.var, %.loc59
-// CHECK:STDOUT:   %B.ref: <function> = name_ref B, file.%import_ref.2 [template = imports.%B]
-// CHECK:STDOUT:   %.loc60_16: i32 = int_literal 1 [template = constants.%.2]
-// CHECK:STDOUT:   %.loc60_15: init i32 = call %B.ref(%.loc60_16)
-// CHECK:STDOUT:   assign file.%b.var, %.loc60_15
-// CHECK:STDOUT:   %C.ref: <function> = name_ref C, file.%import_ref.3 [template = imports.%C]
-// CHECK:STDOUT:   %.loc61_23: i32 = int_literal 1 [template = constants.%.2]
-// CHECK:STDOUT:   %.loc61_25.1: (i32,) = tuple_literal (%.loc61_23)
-// CHECK:STDOUT:   %.loc61_25.2: (i32,) = tuple_value (%.loc61_23) [template = constants.%.5]
-// CHECK:STDOUT:   %.loc61_25.3: (i32,) = converted %.loc61_25.1, %.loc61_25.2 [template = constants.%.5]
-// CHECK:STDOUT:   %.loc61_21: init {.c: i32} = call %C.ref(%.loc61_25.3)
-// CHECK:STDOUT:   assign file.%c.var, %.loc61_21
-// CHECK:STDOUT:   %D.ref: <function> = name_ref D, file.%import_ref.4 [template = imports.%D]
-// CHECK:STDOUT:   %.loc62: init () = call %D.ref()
-// CHECK:STDOUT:   assign file.%d.var, %.loc62
+// CHECK:STDOUT:   %A.ref: <function> = name_ref A, file.%import_ref.1 [template = imports.%A.1]
+// CHECK:STDOUT:   %.loc7: init () = call %A.ref()
+// CHECK:STDOUT:   assign file.%a.var, %.loc7
+// CHECK:STDOUT:   %B.ref: <function> = name_ref B, file.%import_ref.2 [template = imports.%B.1]
+// CHECK:STDOUT:   %.loc8_16: i32 = int_literal 1 [template = constants.%.4]
+// CHECK:STDOUT:   %.loc8_15: init i32 = call %B.ref(%.loc8_16)
+// CHECK:STDOUT:   assign file.%b.var, %.loc8_15
+// CHECK:STDOUT:   %C.ref: <function> = name_ref C, file.%import_ref.3 [template = imports.%C.1]
+// CHECK:STDOUT:   %.loc9_23: i32 = int_literal 1 [template = constants.%.4]
+// CHECK:STDOUT:   %.loc9_25.1: (i32,) = tuple_literal (%.loc9_23)
+// CHECK:STDOUT:   %.loc9_25.2: (i32,) = tuple_value (%.loc9_23) [template = constants.%.5]
+// CHECK:STDOUT:   %.loc9_25.3: (i32,) = converted %.loc9_25.1, %.loc9_25.2 [template = constants.%.5]
+// CHECK:STDOUT:   %.loc9_21: init {.c: i32} = call %C.ref(%.loc9_25.3)
+// CHECK:STDOUT:   assign file.%c.var, %.loc9_21
+// CHECK:STDOUT:   %D.ref: <function> = name_ref D, file.%import_ref.4 [template = imports.%D.1]
+// CHECK:STDOUT:   %.loc10: init () = call %D.ref()
+// CHECK:STDOUT:   assign file.%d.var, %.loc10
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: --- fail_todo_merge_reverse.carbon
+// CHECK:STDOUT: --- merge_reverse.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
-// CHECK:STDOUT:   %.1: type = tuple_type () [template]
-// CHECK:STDOUT:   %.2: i32 = int_literal 1 [template]
-// CHECK:STDOUT:   %.3: type = struct_type {.c: i32} [template]
-// CHECK:STDOUT:   %.4: type = tuple_type (i32) [template]
-// CHECK:STDOUT:   %.5: (i32,) = tuple_value (%.2) [template]
+// CHECK:STDOUT:   %.1: type = tuple_type (i32) [template]
+// CHECK:STDOUT:   %.2: type = struct_type {.c: i32} [template]
+// CHECK:STDOUT:   %.3: type = tuple_type () [template]
+// CHECK:STDOUT:   %.4: i32 = int_literal 1 [template]
+// CHECK:STDOUT:   %.5: (i32,) = tuple_value (%.4) [template]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: file {
@@ -617,56 +522,64 @@ var d: () = D();
 // CHECK:STDOUT:     .c = %c
 // CHECK:STDOUT:     .d = %d
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %import_ref.1: <function> = import_ref ir1, inst+1, used [template = imports.%A]
-// CHECK:STDOUT:   %import_ref.2: <function> = import_ref ir1, inst+5, used [template = imports.%B]
-// CHECK:STDOUT:   %import_ref.3: <function> = import_ref ir1, inst+17, used [template = imports.%C]
-// CHECK:STDOUT:   %import_ref.4: <function> = import_ref ir1, inst+18, used [template = imports.%D]
-// CHECK:STDOUT:   %import_ref.5 = import_ref ir2, inst+1, unused
-// CHECK:STDOUT:   %import_ref.6 = import_ref ir2, inst+5, unused
-// CHECK:STDOUT:   %import_ref.7 = import_ref ir2, inst+17, unused
-// CHECK:STDOUT:   %import_ref.8 = import_ref ir2, inst+18, unused
-// CHECK:STDOUT:   %.loc58_9.1: () = tuple_literal ()
-// CHECK:STDOUT:   %.loc58_9.2: type = converted %.loc58_9.1, constants.%.1 [template = constants.%.1]
+// CHECK:STDOUT:   %import_ref.1: <function> = import_ref ir1, inst+1, used [template = imports.%A.1]
+// CHECK:STDOUT:   %import_ref.2: <function> = import_ref ir1, inst+5, used [template = imports.%B.1]
+// CHECK:STDOUT:   %import_ref.3: <function> = import_ref ir1, inst+17, used [template = imports.%C.1]
+// CHECK:STDOUT:   %import_ref.4: <function> = import_ref ir1, inst+18, used [template = imports.%D.1]
+// CHECK:STDOUT:   %import_ref.5: <function> = import_ref ir2, inst+1, used [template = imports.%A.2]
+// CHECK:STDOUT:   %import_ref.6: <function> = import_ref ir2, inst+5, used [template = imports.%B.2]
+// CHECK:STDOUT:   %import_ref.7: <function> = import_ref ir2, inst+17, used [template = imports.%C.2]
+// CHECK:STDOUT:   %import_ref.8: <function> = import_ref ir2, inst+18, used [template = imports.%D.2]
+// CHECK:STDOUT:   %.loc7_9.1: () = tuple_literal ()
+// CHECK:STDOUT:   %.loc7_9.2: type = converted %.loc7_9.1, constants.%.3 [template = constants.%.3]
 // CHECK:STDOUT:   %a.var: ref () = var a
 // CHECK:STDOUT:   %a: ref () = bind_name a, %a.var
 // CHECK:STDOUT:   %b.var: ref i32 = var b
 // CHECK:STDOUT:   %b: ref i32 = bind_name b, %b.var
-// CHECK:STDOUT:   %.loc60: type = struct_type {.c: i32} [template = constants.%.3]
+// CHECK:STDOUT:   %.loc9: type = struct_type {.c: i32} [template = constants.%.2]
 // CHECK:STDOUT:   %c.var: ref {.c: i32} = var c
 // CHECK:STDOUT:   %c: ref {.c: i32} = bind_name c, %c.var
-// CHECK:STDOUT:   %.loc61_9.1: () = tuple_literal ()
-// CHECK:STDOUT:   %.loc61_9.2: type = converted %.loc61_9.1, constants.%.1 [template = constants.%.1]
+// CHECK:STDOUT:   %.loc10_9.1: () = tuple_literal ()
+// CHECK:STDOUT:   %.loc10_9.2: type = converted %.loc10_9.1, constants.%.3 [template = constants.%.3]
 // CHECK:STDOUT:   %d.var: ref () = var d
 // CHECK:STDOUT:   %d: ref () = bind_name d, %d.var
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @A();
+// CHECK:STDOUT: fn @A.1();
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @B(%b: i32) -> i32;
+// CHECK:STDOUT: fn @A.2();
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @C(%c: (i32,)) -> {.c: i32};
+// CHECK:STDOUT: fn @B.1(%b: i32) -> i32;
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @D();
+// CHECK:STDOUT: fn @B.2(%b: i32) -> i32;
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @C.1(%c: (i32,)) -> {.c: i32};
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @C.2(%c: (i32,)) -> {.c: i32};
+// CHECK:STDOUT:
+// CHECK:STDOUT: extern fn @D.1();
+// CHECK:STDOUT:
+// CHECK:STDOUT: extern fn @D.2();
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @__global_init() {
 // CHECK:STDOUT: !entry:
-// CHECK:STDOUT:   %A.ref: <function> = name_ref A, file.%import_ref.1 [template = imports.%A]
-// CHECK:STDOUT:   %.loc58: init () = call %A.ref()
-// CHECK:STDOUT:   assign file.%a.var, %.loc58
-// CHECK:STDOUT:   %B.ref: <function> = name_ref B, file.%import_ref.2 [template = imports.%B]
-// CHECK:STDOUT:   %.loc59_16: i32 = int_literal 1 [template = constants.%.2]
-// CHECK:STDOUT:   %.loc59_15: init i32 = call %B.ref(%.loc59_16)
-// CHECK:STDOUT:   assign file.%b.var, %.loc59_15
-// CHECK:STDOUT:   %C.ref: <function> = name_ref C, file.%import_ref.3 [template = imports.%C]
-// CHECK:STDOUT:   %.loc60_23: i32 = int_literal 1 [template = constants.%.2]
-// CHECK:STDOUT:   %.loc60_25.1: (i32,) = tuple_literal (%.loc60_23)
-// CHECK:STDOUT:   %.loc60_25.2: (i32,) = tuple_value (%.loc60_23) [template = constants.%.5]
-// CHECK:STDOUT:   %.loc60_25.3: (i32,) = converted %.loc60_25.1, %.loc60_25.2 [template = constants.%.5]
-// CHECK:STDOUT:   %.loc60_21: init {.c: i32} = call %C.ref(%.loc60_25.3)
-// CHECK:STDOUT:   assign file.%c.var, %.loc60_21
-// CHECK:STDOUT:   %D.ref: <function> = name_ref D, file.%import_ref.4 [template = imports.%D]
-// CHECK:STDOUT:   %.loc61: init () = call %D.ref()
-// CHECK:STDOUT:   assign file.%d.var, %.loc61
+// CHECK:STDOUT:   %A.ref: <function> = name_ref A, file.%import_ref.1 [template = imports.%A.1]
+// CHECK:STDOUT:   %.loc7: init () = call %A.ref()
+// CHECK:STDOUT:   assign file.%a.var, %.loc7
+// CHECK:STDOUT:   %B.ref: <function> = name_ref B, file.%import_ref.2 [template = imports.%B.1]
+// CHECK:STDOUT:   %.loc8_16: i32 = int_literal 1 [template = constants.%.4]
+// CHECK:STDOUT:   %.loc8_15: init i32 = call %B.ref(%.loc8_16)
+// CHECK:STDOUT:   assign file.%b.var, %.loc8_15
+// CHECK:STDOUT:   %C.ref: <function> = name_ref C, file.%import_ref.3 [template = imports.%C.1]
+// CHECK:STDOUT:   %.loc9_23: i32 = int_literal 1 [template = constants.%.4]
+// CHECK:STDOUT:   %.loc9_25.1: (i32,) = tuple_literal (%.loc9_23)
+// CHECK:STDOUT:   %.loc9_25.2: (i32,) = tuple_value (%.loc9_23) [template = constants.%.5]
+// CHECK:STDOUT:   %.loc9_25.3: (i32,) = converted %.loc9_25.1, %.loc9_25.2 [template = constants.%.5]
+// CHECK:STDOUT:   %.loc9_21: init {.c: i32} = call %C.ref(%.loc9_25.3)
+// CHECK:STDOUT:   assign file.%c.var, %.loc9_21
+// CHECK:STDOUT:   %D.ref: <function> = name_ref D, file.%import_ref.4 [template = imports.%D.1]
+// CHECK:STDOUT:   %.loc10: init () = call %D.ref()
+// CHECK:STDOUT:   assign file.%d.var, %.loc10
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/check/testdata/namespace/fail_conflict_in_imports_namespace_first.carbon b/toolchain/check/testdata/namespace/fail_conflict_in_imports_namespace_first.carbon
index 9913595042dc..20f2bc84f263 100644
--- a/toolchain/check/testdata/namespace/fail_conflict_in_imports_namespace_first.carbon
+++ b/toolchain/check/testdata/namespace/fail_conflict_in_imports_namespace_first.carbon
@@ -81,10 +81,12 @@ fn NS.Bar() {}
 // CHECK:STDOUT:     .Bar = %Bar
 // CHECK:STDOUT:   }
 // CHECK:STDOUT:   %import_ref.2 = import_ref ir1, inst+2, unused
-// CHECK:STDOUT:   %import_ref.3 = import_ref ir2, inst+1, unused
+// CHECK:STDOUT:   %import_ref.3: <function> = import_ref ir2, inst+1, used [template = imports.%NS]
 // CHECK:STDOUT:   %Bar: <function> = fn_decl @Bar [template] {}
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
+// CHECK:STDOUT: fn @NS();
+// CHECK:STDOUT:
 // CHECK:STDOUT: fn @Bar() {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   return
diff --git a/toolchain/check/testdata/packages/cross_package_import.carbon b/toolchain/check/testdata/packages/cross_package_import.carbon
index 89f806f532f9..5ee5e12e2836 100644
--- a/toolchain/check/testdata/packages/cross_package_import.carbon
+++ b/toolchain/check/testdata/packages/cross_package_import.carbon
@@ -199,7 +199,7 @@ fn Other.G() {}
 // CHECK:STDOUT:   %F: <function> = fn_decl @F [template] {}
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @F();
+// CHECK:STDOUT: extern fn @F();
 // CHECK:STDOUT:
 // CHECK:STDOUT: --- other_fn_conflict.carbon
 // CHECK:STDOUT:
@@ -300,7 +300,7 @@ fn Other.G() {}
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @F.1();
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @F.2();
+// CHECK:STDOUT: extern fn @F.2();
 // CHECK:STDOUT:
 // CHECK:STDOUT: --- main_unused_other_ambiguous.carbon
 // CHECK:STDOUT:
diff --git a/toolchain/check/testdata/packages/fail_conflict_no_namespaces.carbon b/toolchain/check/testdata/packages/fail_conflict_no_namespaces.carbon
index ad3927d9f082..8c8a7ae4f32a 100644
--- a/toolchain/check/testdata/packages/fail_conflict_no_namespaces.carbon
+++ b/toolchain/check/testdata/packages/fail_conflict_no_namespaces.carbon
@@ -62,7 +62,9 @@ import library "var";
 // CHECK:STDOUT:   package: <namespace> = namespace [template] {
 // CHECK:STDOUT:     .Foo = %import_ref.1
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %import_ref.1 = import_ref ir1, inst+1, unused
-// CHECK:STDOUT:   %import_ref.2 = import_ref ir2, inst+2, unused
+// CHECK:STDOUT:   %import_ref.1: <function> = import_ref ir1, inst+1, used [template = imports.%Foo]
+// CHECK:STDOUT:   %import_ref.2: ref i32 = import_ref ir2, inst+2, used
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
+// CHECK:STDOUT: fn @Foo();
+// CHECK:STDOUT:
diff --git a/toolchain/sem_ir/formatter.cpp b/toolchain/sem_ir/formatter.cpp
index a5f0cba34ff0..1836e5538508 100644
--- a/toolchain/sem_ir/formatter.cpp
+++ b/toolchain/sem_ir/formatter.cpp
@@ -771,7 +771,13 @@ class Formatter {
   auto FormatFunction(FunctionId id) -> void {
     const Function& fn = sem_ir_.functions().Get(id);
 
-    out_ << "\nfn ";
+    out_ << "\n";
+
+    if (fn.is_extern) {
+      out_ << "extern ";
+    }
+
+    out_ << "fn ";
     FormatFunctionName(id);
 
     llvm::SaveAndRestore function_scope(scope_, inst_namer_.GetScopeFor(id));