Repository: carbon-language/carbon-lang (2026-03-15T00:15:19Z)

Issue:
Call to local function inside generic function fails
### Description of the bug:


A call to a local function defined inside a generic function fails with a compiler error.

### What did you do, or what's a simple way to reproduce the bug?


```carbon
fn F(unused T:! type) {
  fn Local() {}
  Local();
}
```

### What did you expect to happen?


This should compile successfully. 

### What actually happened?


The compiler cannot deduce the generic parameters of the enclosing function while deducing parameters of the local function.
```
  // CHECK:STDERR: demo.carbon:[[@LINE+7]]:3: error: cannot deduce value for generic parameter `T` [DeductionIncomplete]
  // CHECK:STDERR:   Local();
  // CHECK:STDERR:   ^~~~~~~
  // CHECK:STDERR: demo.carbon:[[@LINE-4]]:3: note: while deducing parameters of generic declared here [DeductionGenericHere]
  // CHECK:STDERR:   fn Local() {}
  // CHECK:STDERR:   ^~~~~~~~~~~~
  // CHECK:STDERR:
```

### Any other information, logs, or outputs that you want to share?


_No response_


Discussion:
It looks like the problem here is that `ScopeStack::PushForFunctionBody` pushes a scope with a specific of `SpecificId::None`, which is wrong -- it should be pushing the self specific of the function, so that we treat `Local` as referring to the function within `F(T)`, not as a generic function with an unbound and deduced `T:! type` parameter.
@ozguronsoy can i look on this issue ??

Yes, of course.



Patch:
diff --git a/toolchain/check/scope_stack.cpp b/toolchain/check/scope_stack.cpp
index 8b757d7d22c3c..f6d4399950cad 100644
--- a/toolchain/check/scope_stack.cpp
+++ b/toolchain/check/scope_stack.cpp
@@ -72,9 +72,15 @@ auto ScopeStack::Push(SemIR::InstId scope_inst_id, SemIR::NameScopeId scope_id,
   if (scope_stack_.back().is_lexical_scope()) {
     // For lexical lookups, unqualified lookup doesn't know how to find the
     // associated specific, so if we start adding lexical scopes associated with
-    // specifics, we'll need to somehow track them in lookup.
-    CARBON_CHECK(!specific_id.has_value(),
-                 "Lexical scope should not have an associated specific.");
+    // specifics, we'll need to somehow track them in lookup. The self specific
+    // is an exception to this, as its generic will always lexically enclose the
+    // point of use of any looked up name.
+    CARBON_CHECK(!specific_id.has_value() ||
+                     sem_ir().generics().GetSelfSpecific(
+                         sem_ir().specifics().Get(specific_id).generic_id) ==
+                         specific_id,
+                 "Lexical scopes can only have an associated specific if it is "
+                 "a self specific.");
   } else {
     non_lexical_scope_stack_.push_back({.scope_index = next_scope_index_,
                                         .name_scope_id = scope_id,
@@ -111,8 +117,11 @@ auto ScopeStack::PushForSameRegion() -> void {
 }
 
 auto ScopeStack::PushForFunctionBody(SemIR::InstId scope_inst_id) -> void {
-  CARBON_DCHECK(sem_ir().insts().Is<SemIR::FunctionDecl>(scope_inst_id));
-  Push(scope_inst_id, SemIR::NameScopeId::None, SemIR::SpecificId::None,
+  const auto& function_decl =
+      sem_ir().insts().GetAs<SemIR::FunctionDecl>(scope_inst_id);
+  const auto& function = sem_ir().functions().Get(function_decl.function_id);
+  auto self_specific = sem_ir().generics().GetSelfSpecific(function.generic_id);
+  Push(scope_inst_id, SemIR::NameScopeId::None, self_specific,
        /*lexical_lookup_has_load_error=*/false);
 
   return_scope_stack_.push_back({.decl_id = scope_inst_id});


Test patch:
diff --git a/toolchain/check/testdata/generic/local_function.carbon b/toolchain/check/testdata/generic/local_function.carbon
new file mode 100644
index 0000000000000..0560496b08d2b
--- /dev/null
+++ b/toolchain/check/testdata/generic/local_function.carbon
@@ -0,0 +1,94 @@
+// 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-FILE: toolchain/testing/testdata/min_prelude/int.carbon
+// TODO: Add ranges and switch to "--dump-sem-ir-ranges=only".
+// EXTRA-ARGS: --dump-sem-ir-ranges=if-present
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/generic/local_function.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/generic/local_function.carbon
+
+fn Test[unused T:! type]() {
+  fn Local() {
+  }
+  Local();
+}
+
+// CHECK:STDOUT: --- local_function.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %type: type = facet_type <type> [concrete]
+// CHECK:STDOUT:   %.Self: %type = symbolic_binding .Self [symbolic_self]
+// CHECK:STDOUT:   %pattern_type: type = pattern_type type [concrete]
+// CHECK:STDOUT:   %T: type = symbolic_binding T, 0 [symbolic]
+// CHECK:STDOUT:   %Test.type: type = fn_type @Test [concrete]
+// CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
+// CHECK:STDOUT:   %Test: %Test.type = struct_value () [concrete]
+// CHECK:STDOUT:   %Local.type: type = fn_type @Local, @Test(%T) [symbolic]
+// CHECK:STDOUT:   %Local: %Local.type = struct_value () [symbolic]
+// CHECK:STDOUT:   %Local.specific_fn: <specific function> = specific_function %Local, @Local(%T) [symbolic]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT:   %Core: <namespace> = namespace file.%Core.import, [concrete] {
+// CHECK:STDOUT:     import Core//prelude
+// CHECK:STDOUT:     import Core//prelude/...
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [concrete] {
+// CHECK:STDOUT:     .Core = imports.%Core
+// CHECK:STDOUT:     .Test = %Test.decl
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Core.import = import Core
+// CHECK:STDOUT:   %Test.decl: %Test.type = fn_decl @Test [concrete = constants.%Test] {
+// CHECK:STDOUT:     %T.patt: %pattern_type = symbolic_binding_pattern T, 0 [concrete]
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     %.loc15_20.1: type = splice_block %.loc15_20.2 [concrete = type] {
+// CHECK:STDOUT:       %.Self: %type = symbolic_binding .Self [symbolic_self = constants.%.Self]
+// CHECK:STDOUT:       %.loc15_20.2: type = type_literal type [concrete = type]
+// CHECK:STDOUT:     }
+// CHECK:STDOUT:     %T.loc15_17.2: type = symbolic_binding T, 0 [symbolic = %T.loc15_17.1 (constants.%T)]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic fn @Test(%T.loc15_17.2: type) {
+// CHECK:STDOUT:   %T.loc15_17.1: type = symbolic_binding T, 0 [symbolic = %T.loc15_17.1 (constants.%T)]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %Local.type: type = fn_type @Local, @Test(%T.loc15_17.1) [symbolic = %Local.type (constants.%Local.type)]
+// CHECK:STDOUT:   %Local: @Test.%Local.type (%Local.type) = struct_value () [symbolic = %Local (constants.%Local)]
+// CHECK:STDOUT:   %Local.specific_fn.loc18_3.2: <specific function> = specific_function %Local, @Local(%T.loc15_17.1) [symbolic = %Local.specific_fn.loc18_3.2 (constants.%Local.specific_fn)]
+// CHECK:STDOUT:
+// CHECK:STDOUT:   fn() {
+// CHECK:STDOUT:   !entry:
+// CHECK:STDOUT:     %Local.decl: @Test.%Local.type (%Local.type) = fn_decl @Local [symbolic = @Test.%Local (constants.%Local)] {} {}
+// CHECK:STDOUT:     %Local.ref: @Test.%Local.type (%Local.type) = name_ref Local, %Local.decl [symbolic = %Local (constants.%Local)]
+// CHECK:STDOUT:     %Local.specific_fn.loc18_3.1: <specific function> = specific_function %Local.ref, @Local(constants.%T) [symbolic = %Local.specific_fn.loc18_3.2 (constants.%Local.specific_fn)]
+// CHECK:STDOUT:     %Local.call: init %empty_tuple.type = call %Local.specific_fn.loc18_3.1()
+// CHECK:STDOUT:     return
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic fn @Local(@Test.%T.loc15_17.2: type) {
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:
+// CHECK:STDOUT:   fn() {
+// CHECK:STDOUT:   !entry:
+// CHECK:STDOUT:     return
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @Test(constants.%T) {
+// CHECK:STDOUT:   %T.loc15_17.1 => constants.%T
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @Local(constants.%T) {
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT: }
+// CHECK:STDOUT: