[clang-tidy] only diagnose definitions in readability-enum-initial-value

With the `isDefinition` matcher, the analysis and diagnostics will be
constrained to definitions only. Previously forward declarations were
diagnosed as well.

Fixes #107590

Author: 5chmidti

clang-tools-extra/clang-tidy/readability/EnumInitialValueCheck.cpp

@@ -141,16 +141,18 @@ void EnumInitialValueCheck::storeOptions(ClangTidyOptions::OptionMap &Opts) {
 }
 
 void EnumInitialValueCheck::registerMatchers(MatchFinder *Finder) {
-  Finder->addMatcher(
-      enumDecl(unless(isMacro()), unless(hasConsistentInitialValues()))
-          .bind("inconsistent"),
-      this);
+  Finder->addMatcher(enumDecl(isDefinition(), unless(isMacro()),
+                              unless(hasConsistentInitialValues()))
+                         .bind("inconsistent"),
+                     this);
   if (!AllowExplicitZeroFirstInitialValue)
     Finder->addMatcher(
-        enumDecl(hasZeroInitialValueForFirstEnumerator()).bind("zero_first"),
+        enumDecl(isDefinition(), hasZeroInitialValueForFirstEnumerator())
+            .bind("zero_first"),
         this);
   if (!AllowExplicitSequentialInitialValues)
-    Finder->addMatcher(enumDecl(unless(isMacro()), hasSequentialInitialValues())
+    Finder->addMatcher(enumDecl(isDefinition(), unless(isMacro()),
+                                hasSequentialInitialValues())
                            .bind("sequential"),
                        this);
 }

clang-tools-extra/docs/ReleaseNotes.rst

@@ -132,6 +132,10 @@ Changes in existing checks
   <clang-tidy/checks/modernize/use-std-print>` check to support replacing
   member function calls too.
 
+- Improved :doc:`readability-enum-initial-value
+  <clang-tidy/checks/readability-enum-initial-value>` check to only issue
+  diagnostics for the definition of an ``enum``.
+
 - Improved :doc:`performance-avoid-endl
   <clang-tidy/checks/performance/avoid-endl>` check to use ``std::endl`` as
   placeholder when lexer cannot get source text.

clang-tools-extra/test/clang-tidy/checkers/readability/enum-initial-value.c

@@ -78,3 +78,17 @@ enum EnumSequentialInitialValue {
   EnumSequentialInitialValue_2 = 4,
   // CHECK-FIXES-ENABLE: EnumSequentialInitialValue_2 ,
 };
+
+// gh107590
+enum WithFwdDecl : int;
+
+enum WithFwdDecl : int {
+  // CHECK-MESSAGES: :[[@LINE-1]]:1: warning: inital values in enum 'WithFwdDecl' are not consistent
+  // CHECK-MESSAGES-ENABLE: :[[@LINE-2]]:1: warning: inital values in enum 'WithFwdDecl' are not consistent
+  E0,
+  // CHECK-FIXES: E0 = 0,
+  E1 = 1,
+  E2,
+  // CHECK-FIXES: E2 = 2,
+};
+