From 1a8712d31af6cafa90e5924e65c188b5967aa3e3 Mon Sep 17 00:00:00 2001
From: Ben Boeckel <ben.boeckel@kitware.com>
Date: Mon, 24 Nov 2025 21:15:50 -0500
Subject: [PATCH] cmGeneratorTarget: always provide a compile PDB filename

As of commit f78f592b78 (pchreuse: defer target existence enforcement to
generation time, 2025-06-16) via !10887, the compile PDB directory is
always set. Some codepaths used the total computation as a signal to
build the path themselves. But the filename was not specified, so the
resulting filename ended up changing. Always create the filename as
expected.

Fixes: #27401
---
 Source/cmGeneratorTarget.cxx                      | 13 ++++---------
 Tests/MSVCDebugInformationFormat/CMakeLists.txt   |  8 ++++++++
 .../PdbCompileFileName.c                          | 15 +++++++++++++++
 3 files changed, 27 insertions(+), 9 deletions(-)
 create mode 100644 Tests/MSVCDebugInformationFormat/PdbCompileFileName.c

diff --git a/Source/cmGeneratorTarget.cxx b/Source/cmGeneratorTarget.cxx
index 3ff37b8df1..2fdf06098d 100644
--- a/Source/cmGeneratorTarget.cxx
+++ b/Source/cmGeneratorTarget.cxx
@@ -1312,15 +1312,10 @@ std::string cmGeneratorTarget::GetCompilePDBName(
     return components.prefix + pdbName + ".pdb";
   }
 
-  // If the target is PCH-reused, we need a stable name for the PDB file so
-  // that reusing targets can construct a stable name for it.
-  if (this->PchReused) {
-    NameComponents const& components = GetFullNameInternalComponents(
-      config, cmStateEnums::RuntimeBinaryArtifact);
-    return cmStrCat(components.prefix, this->GetName(), ".pdb");
-  }
-
-  return "";
+  // Always use a name for the compile-time database.
+  NameComponents const& components =
+    GetFullNameInternalComponents(config, cmStateEnums::RuntimeBinaryArtifact);
+  return cmStrCat(components.prefix, this->GetName(), ".pdb");
 }
 
 std::string cmGeneratorTarget::GetCompilePDBPath(
diff --git a/Tests/MSVCDebugInformationFormat/CMakeLists.txt b/Tests/MSVCDebugInformationFormat/CMakeLists.txt
index b09bc6c416..4b3f28092d 100644
--- a/Tests/MSVCDebugInformationFormat/CMakeLists.txt
+++ b/Tests/MSVCDebugInformationFormat/CMakeLists.txt
@@ -79,3 +79,11 @@ endif()
 if(CMake_TEST_Fortran)
   verify(Fortran verify.F90)
 endif()
+
+# Issue 27401; 4.2.0 regression
+add_library(PdbCompileFileName PdbCompileFileName.c)
+target_compile_definitions(PdbCompileFileName
+  PRIVATE
+    "TARGET_DIRECTORY=\"$<TARGET_INTERMEDIATE_DIR:PdbCompileFileName>\"")
+set_property(TARGET PdbCompileFileName PROPERTY
+  MSVC_DEBUG_INFORMATION_FORMAT "ProgramDatabase")
diff --git a/Tests/MSVCDebugInformationFormat/PdbCompileFileName.c b/Tests/MSVCDebugInformationFormat/PdbCompileFileName.c
new file mode 100644
index 0000000000..6c7678b751
--- /dev/null
+++ b/Tests/MSVCDebugInformationFormat/PdbCompileFileName.c
@@ -0,0 +1,15 @@
+#include <stdio.h>
+
+int main(int argc, char* argv[])
+{
+  int ret = 0;
+  char const* fname = TARGET_DIRECTORY "/PdbCompileFileName.pdb";
+  FILE* f = fopen(fname, "r");
+  if (f) {
+    fclose(f);
+  } else {
+    printf("Failed to open PDB file '%s'\n", fname);
+    ret = 1;
+  }
+  return ret;
+}