[clang] [clang] fix getReplacedTemplateParameter for function template specializations (PR #189559)

[Matheus Izvekov via cfe-commits]

https://github.com/mizvekov created https://github.com/llvm/llvm-project/pull/189559

This fixes the transformation of substituted constant template parameters by providing the instantiated parameter type for the function template specialization case.

This fixes a regression introduced in #161029 which will be backported to llvm-22, so there are no release notes.

Fixes #188759

>From 538ce9e6f7aa4cbdb4a8e8389827810a97b286c0 Mon Sep 17 00:00:00 2001
From: Matheus Izvekov <mizvekov at gmail.com>
Date: Sat, 28 Mar 2026 23:54:18 +0000
Subject: [PATCH] [clang] fix getReplacedTemplateParameter for function
 template specializaions

This fixes the transformation of substituted constant template parameters by
providing the instantiated parameter type for the function template
specialization case.

This fixes a regression introduced in #161029 which will be backported to llvm-22, so there are no release notes.

Fixes #188759
---
 clang/lib/AST/DeclTemplate.cpp       | 10 ++++++----
 clang/test/SemaTemplate/GH188759.cpp | 13 +++++++++++++
 2 files changed, 19 insertions(+), 4 deletions(-)
 create mode 100644 clang/test/SemaTemplate/GH188759.cpp

diff --git a/clang/lib/AST/DeclTemplate.cpp b/clang/lib/AST/DeclTemplate.cpp
index 5a8e1ed445f3a..99d02fdc99e92 100644
--- a/clang/lib/AST/DeclTemplate.cpp
+++ b/clang/lib/AST/DeclTemplate.cpp
@@ -1707,10 +1707,12 @@ clang::getReplacedTemplateParameter(Decl *D, unsigned Index) {
   case Decl::Kind::CXXConstructor:
   case Decl::Kind::CXXDestructor:
   case Decl::Kind::CXXMethod:
-  case Decl::Kind::Function:
-    return getReplacedTemplateParameter(
-        cast<FunctionDecl>(D)->getTemplateSpecializationInfo()->getTemplate(),
-        Index);
+  case Decl::Kind::Function: {
+    const FunctionTemplateSpecializationInfo *Info =
+        cast<FunctionDecl>(D)->getTemplateSpecializationInfo();
+    return {Info->getTemplate()->getTemplateParameters()->getParam(Index),
+            Info->TemplateArguments->asArray()[Index]};
+  }
   default:
     llvm_unreachable("Unhandled templated declaration kind");
   }
diff --git a/clang/test/SemaTemplate/GH188759.cpp b/clang/test/SemaTemplate/GH188759.cpp
new file mode 100644
index 0000000000000..f0d579d1fc3e5
--- /dev/null
+++ b/clang/test/SemaTemplate/GH188759.cpp
@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++26 %s
+
+namespace t1 {
+  template <int> struct integer_sequence {};
+  template <int> struct array {};
+  template <int ARRAY_SIZE, array<ARRAY_SIZE> test_apdus> void runBlobs() {
+    []<int... INDEX>(integer_sequence<INDEX...>) { // expected-note {{requested here}}
+      int x{operator0<test_apdus, INDEX>()...};
+      // expected-error at -1 {{use of undeclared identifier 'operator0'}}
+    }(integer_sequence<1>{});
+  }
+  template void runBlobs<2, {}>(); // expected-note {{requested here}}
+} // namespace t1