if: Add DIAGNOSTIC <category> condition

Add an existence check to the `if` command for diagnostic categories,
mirroring the check for policies. Aside from being desirable for
consistency's sake, this will make it easier for users to write logic to
manipulate diagnostics from newer versions of CMake that don't exist
today.

Help/command/if.rst

@@ -43,8 +43,9 @@ Compound conditions are evaluated in the following order of precedence:
 
 2. Unary tests such as:
 
-  * The `Existence Checks`_ :cref:`COMMAND`, :cref:`POLICY`, :cref:`TARGET`,
-    :cref:`TEST`, :cref:`EXISTS`, and :cref:`DEFINED`.
+  * The `Existence Checks`_ :cref:`COMMAND`, :cref:`DEFINED`,
+    :cref:`DIAGNOSTIC`, :cref:`EXISTS`, :cref:`POLICY`, :cref:`TARGET`, and
+    :cref:`TEST`.
   * The `File Operations`_ :cref:`IS_READABLE`, :cref:`IS_WRITABLE`,
     :cref:`IS_EXECUTABLE`, :cref:`IS_DIRECTORY`, :cref:`IS_SYMLINK`, and
     :cref:`IS_ABSOLUTE`.
@@ -131,6 +132,12 @@ Existence Checks
   True if the given name is a command, macro or function that can be
   invoked.
 
+.. signature:: if(DIAGNOSTIC <category>)
+
+  .. versionadded:: 4.4
+
+  True if the given name is an existing diagnostic category.
+
 .. signature:: if(POLICY <policy-id>)
 
   True if the given name is an existing policy (of the form ``CMP<NNNN>``).

Help/release/4.4.rst

@@ -186,6 +186,9 @@ Commands
 * The :command:`file(READ_SYMLINK)` command gained a ``RESULT``
   option to capture errors without failing.
 
+* The :command:`if` command gained a new ``DIAGNOSTIC`` operator that evaluates
+  to true if a given diagnostic category name is known to CMake.
+
 * The :command:`include` command now has a ``NO_DIAGNOSTIC_SCOPE`` option.
 
 * The :command:`list(FILTER)` command gained a new ``PREDICATE`` mode

Source/cmConditionEvaluator.cxx

@@ -11,12 +11,14 @@
 #include <sstream>
 #include <utility>
 
+#include <cm/optional>
 #include <cm/string_view>
 #include <cmext/algorithm>
 
 #include "cmsys/RegularExpression.hxx"
 
 #include "cmCMakePath.h"
+#include "cmDiagnostics.h"
 #include "cmExpandedCommandArgument.h"
 #include "cmList.h"
 #include "cmMakefile.h"
@@ -30,6 +32,7 @@ namespace {
 auto const keyAND = "AND"_s;
 auto const keyCOMMAND = "COMMAND"_s;
 auto const keyDEFINED = "DEFINED"_s;
+auto const keyDIAGNOSTIC = "DIAGNOSTIC"_s;
 auto const keyEQUAL = "EQUAL"_s;
 auto const keyEXISTS = "EXISTS"_s;
 auto const keyIS_READABLE = "IS_READABLE"_s;
@@ -479,6 +482,13 @@ bool cmConditionEvaluator::HandleLevel1(cmArgumentList& newArgs, std::string&,
           this->Makefile.GetState()->GetCommand(args.next->GetValue())),
         args);
     }
+    // does a diagnostic exist
+    else if (this->IsKeyword(keyDIAGNOSTIC, *args.current)) {
+      newArgs.ReduceOneArg(
+        cmDiagnostics::GetDiagnosticCategory(args.next->GetValue())
+          .has_value(),
+        args);
+    }
     // does a policy exist
     else if (this->IsKeyword(keyPOLICY, *args.current)) {
       cmPolicies::PolicyID pid;

Tests/RunCMake/Diagnostics/ExistenceCheck.cmake

@@ -0,0 +1,7 @@
+if(NOT DIAGNOSTIC CMD_AUTHOR)
+  message(SEND_ERROR "existence check for CMD_AUTHOR failed?!")
+endif()
+
+if(DIAGNOSTIC BOGUS)
+  message(SEND_ERROR "existence check for bogus category succeeded?!")
+endif()

Tests/RunCMake/Diagnostics/RunCMakeTest.cmake

@@ -1,6 +1,7 @@
 include(RunCMake)
 
 run_cmake(Actions)
+run_cmake(ExistenceCheck)
 
 run_cmake_with_options(
   CacheInit