enable_language(): Fail if called before project()

Fixes: #25550

Help/command/enable_language.rst

@@ -9,15 +9,19 @@ Enable languages (CXX/C/OBJC/OBJCXX/Fortran/etc)
 
 Enables support for the named languages in CMake.  This is the same as
 the :command:`project` command but does not create any of the extra
-variables that are created by the project command.
+variables that are created by the :command:`project` command.
 
 .. include:: SUPPORTED_LANGUAGES.txt
 
-This command must be called in file scope, not in a function call.
+The following restrictions apply to where ``enable_language()`` may be called:
-Furthermore, it must be called in the highest directory common to all
+
-targets using the named language directly for compiling sources or
+* It must be called in file scope, not in a function call.
-indirectly through link dependencies.  It is simplest to enable all
+* It must not be called before the first call to :command:`project`.
-needed languages in the top-level directory of a project.
+  See policy :policy:`CMP0165`.
+* It must be called in the highest directory common to all targets
+  using the named language directly for compiling sources or
+  indirectly through link dependencies.  It is simplest to enable all
+  needed languages in the top-level directory of a project.
 
 The ``OPTIONAL`` keyword is a placeholder for future implementation and
 does not currently work. Instead you can use the :module:`CheckLanguage`

Help/manual/cmake-policies.7.rst

@@ -57,6 +57,7 @@ Policies Introduced by CMake 3.30
 .. toctree::
    :maxdepth: 1
 
+   CMP0165: enable_language() must not be called before project(). </policy/CMP0165>
    CMP0164: add_library() rejects SHARED libraries when not supported by the platform. </policy/CMP0164>
    CMP0163: The GENERATED source file property is now visible in all directories. </policy/CMP0163>
    CMP0162: Visual Studio generators add UseDebugLibraries indicators by default. </policy/CMP0162>

Help/policy/CMP0165.rst

@@ -0,0 +1,26 @@
+CMP0165
+-------
+
+.. versionadded:: 3.30
+
+:command:`enable_language` must not be called before :command:`project`.
+
+In CMake 3.29 and below, if a project called :command:`enable_language`
+before the first call to :command:`project`, the language would be enabled
+but possibly using unset details that were expected to be set.
+In CMake 3.30 and above, :command:`enable_language` prefers to reject this
+case and stop with a fatal error instead if it detects that :command:`project`
+has not yet been called.  This policy provides compatibility for projects that
+happened to work when :command:`enable_language` was called before
+:command:`project` and have not been updated to call these commands in the
+required order.
+
+The ``OLD`` behavior for this policy is to allow :command:`enable_language`
+to be called before :command:`project`.  The ``NEW`` behavior for this policy
+is to fail with a fatal error in such cases.
+
+.. |INTRODUCED_IN_CMAKE_VERSION| replace:: 3.30
+.. |WARNS_OR_DOES_NOT_WARN| replace:: warns
+.. include:: STANDARD_ADVICE.txt
+
+.. include:: DEPRECATED.txt

Help/release/dev/enable_language-before-project.rst

@@ -0,0 +1,6 @@
+enable_language-before-project
+------------------------------
+
+* The :command:`enable_language` command now fails with an error
+  if it is called before the first :command:`project` call.
+  See policy :policy:`CMP0165`.

Source/cmEnableLanguageCommand.cxx

@@ -4,6 +4,9 @@
 
 #include "cmExecutionStatus.h"
 #include "cmMakefile.h"
+#include "cmMessageType.h"
+#include "cmPolicies.h"
+#include "cmSystemTools.h"
 
 bool cmEnableLanguageCommand(std::vector<std::string> const& args,
                              cmExecutionStatus& status)
@@ -13,6 +16,27 @@ bool cmEnableLanguageCommand(std::vector<std::string> const& args,
     return false;
   }
 
+  cmMakefile& mf = status.GetMakefile();
+  if (!mf.IsNormalDefinitionSet("PROJECT_NAME")) {
+    switch (mf.GetPolicyStatus(cmPolicies::CMP0165)) {
+      case cmPolicies::WARN:
+        mf.IssueMessage(
+          MessageType::AUTHOR_WARNING,
+          "project() should be called prior to this enable_language() call.");
+        break;
+      case cmPolicies::OLD:
+        break;
+      case cmPolicies::NEW:
+        mf.IssueMessage(
+          MessageType::FATAL_ERROR,
+          "project() must be called prior to this enable_language() call.");
+        cmSystemTools::SetFatalErrorOccurred();
+        return false;
+      default:
+        break;
+    }
+  }
+
   bool optional = false;
   std::vector<std::string> languages;
   for (std::string const& it : args) {
@@ -23,6 +47,6 @@ bool cmEnableLanguageCommand(std::vector<std::string> const& args,
     }
   }
 
-  status.GetMakefile().EnableLanguage(languages, optional);
+  mf.EnableLanguage(languages, optional);
   return true;
 }

Source/cmPolicies.h

@@ -505,7 +505,10 @@ class cmMakefile;
   SELECT(POLICY, CMP0164,                                                     \
          "add_library() rejects SHARED libraries when not supported by the "  \
          "platform.",                                                         \
-         3, 30, 0, cmPolicies::WARN)
+         3, 30, 0, cmPolicies::WARN)                                          \
+  SELECT(POLICY, CMP0165,                                                     \
+         "enable_language() must not be called before project().", 3, 30, 0,  \
+         cmPolicies::WARN)
 
 #define CM_SELECT_ID(F, A1, A2, A3, A4, A5, A6) F(A1)
 #define CM_FOR_EACH_POLICY_ID(POLICY)                                         \

Tests/RunCMake/CMP0165/CMP0165-NEW-result.txt

@@ -0,0 +1 @@
+1

Tests/RunCMake/CMP0165/CMP0165-NEW-stderr.txt

@@ -0,0 +1,4 @@
+CMake Error at CMP0165-NEW\.cmake:[0-9]+ \(enable_language\):
+  project\(\) must be called prior to this enable_language\(\) call\.
+Call Stack \(most recent call first\):
+  CMakeLists\.txt:[0-9]+ \(include\)

Tests/RunCMake/CMP0165/CMP0165-NEW.cmake

@@ -0,0 +1,2 @@
+cmake_policy(SET CMP0165 NEW)
+enable_language(C)

Tests/RunCMake/CMP0165/CMP0165-OLD.cmake

@@ -0,0 +1,2 @@
+cmake_policy(SET CMP0165 OLD)
+enable_language(C)

Tests/RunCMake/CMP0165/CMP0165-WARN-stderr.txt

@@ -0,0 +1,5 @@
+CMake Warning \(dev\) at CMP0165-WARN\.cmake:[0-9]+ \(enable_language\):
+  project\(\) should be called prior to this enable_language\(\) call\.
+Call Stack \(most recent call first\):
+  CMakeLists\.txt:[0-9]+ \(include\)
+This warning is for project developers\.  Use -Wno-dev to suppress it\.

Tests/RunCMake/CMP0165/CMP0165-WARN.cmake

@@ -0,0 +1,2 @@
+# Don't touch the policy, it should warn by default
+enable_language(C)

Tests/RunCMake/CMP0165/CMakeLists.txt

@@ -0,0 +1,7 @@
+cmake_minimum_required(VERSION 3.29)
+
+# This is different to the usual RunCMake test pattern. We are specifically
+# testing the scenario where enable_language() is called before project().
+include(${RunCMake_TEST}.cmake)
+
+project(${RunCMake_TEST} NONE)

Tests/RunCMake/CMP0165/RunCMakeTest.cmake

@@ -0,0 +1,5 @@
+include(RunCMake)
+
+run_cmake(CMP0165-WARN)
+run_cmake(CMP0165-OLD)
+run_cmake(CMP0165-NEW)

Tests/RunCMake/CMakeLists.txt

@@ -174,6 +174,7 @@ add_RunCMake_test(CMP0156 -DCMAKE_C_COMPILER_ID=${CMAKE_C_COMPILER_ID}
                           -DCMAKE_C_COMPILER_VERSION=${CMAKE_C_COMPILER_VERSION})
 add_RunCMake_test(CMP0160)
 add_RunCMake_test(CMP0163)
+add_RunCMake_test(CMP0165)
 
 # The test for Policy 65 requires the use of the
 # CMAKE_SHARED_LIBRARY_LINK_CXX_FLAGS variable, which both the VS and Xcode