[clang-tidy] readability-redundant-declaration: fix false positive with C "extern inline"

[mgehre]

Summary

readability-redundant-declaration was diagnosing a redundant declaration
on "extern inline void f();", which is needed in C code to force an external definition
of the inline function f. (This is different to how inline behaves in C++).

[aaron.ballman]

LGTM with some testing nits.

clang-tools-extra/test/clang-tidy/readability-redundant-declaration.c

32

Please add a newline.

clang-tools-extra/test/clang-tidy/readability-redundant-declaration.cpp

81

Please add a newline.

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

 #include "clang/Lex/Lexer.h"
 
 using namespace clang::ast_matchers;
 
 namespace clang {
 namespace tidy {
 namespace readability {
 
+AST_MATCHER(FunctionDecl, doesDeclarationForceExternallyVisibleDefinition) {
+  return Node.doesDeclarationForceExternallyVisibleDefinition();
+}
+
 RedundantDeclarationCheck::RedundantDeclarationCheck(StringRef Name,
                                                      ClangTidyContext *Context)
     : ClangTidyCheck(Name, Context),
       IgnoreMacros(Options.getLocalOrGlobal("IgnoreMacros", true)) {}
 
 void RedundantDeclarationCheck::registerMatchers(MatchFinder *Finder) {
   Finder->addMatcher(
       namedDecl(anyOf(varDecl(unless(isDefinition())),
-                      functionDecl(unless(anyOf(isDefinition(), isDefaulted(),
-                                                hasParent(friendDecl()))))))
+                      functionDecl(unless(anyOf(
+                          isDefinition(), isDefaulted(),
+                          doesDeclarationForceExternallyVisibleDefinition(),
           .bind("Decl"),
       this);
 }
 
 void RedundantDeclarationCheck::check(const MatchFinder::MatchResult &Result) {
   const auto *D = Result.Nodes.getNodeAs<NamedDecl>("Decl");
   const auto *Prev = D->getPreviousDecl();
   if (!Prev)

clang-tools-extra/test/clang-tidy/readability-redundant-declaration.c

+// RUN: %check_clang_tidy %s readability-redundant-declaration %t
+
+extern int Xyz;
+extern int Xyz; // Xyz
+// CHECK-MESSAGES: :[[@LINE-1]]:12: warning: redundant 'Xyz' declaration [readability-redundant-declaration]
+// CHECK-FIXES: {{^}}// Xyz{{$}}
+int Xyz = 123;
+
+extern int A;
+extern int A, B;
+// CHECK-MESSAGES: :[[@LINE-1]]:12: warning: redundant 'A' declaration
+// CHECK-FIXES: {{^}}extern int A, B;{{$}}
+
+extern int Buf[10];
+extern int Buf[10]; // Buf[10]
+// CHECK-MESSAGES: :[[@LINE-1]]:12: warning: redundant 'Buf' declaration
+// CHECK-FIXES: {{^}}// Buf[10]{{$}}
+
+static int f();
+static int f(); // f
+// CHECK-MESSAGES: :[[@LINE-1]]:12: warning: redundant 'f' declaration
+// CHECK-FIXES: {{^}}// f{{$}}
+static int f() {}
+
+inline void g() {}
+
+inline void g();
+// CHECK-MESSAGES: :[[@LINE-1]]:13: warning: redundant 'g' declaration
+
+// OK: Needed to emit an external definition.
+extern inline void g();
 No newline at end of file

clang-tools-extra/test/clang-tidy/readability-redundant-declaration.cpp

 DEFINE(test);
 // CHECK-MESSAGES: :[[@LINE-1]]:8: warning: redundant 'test' declaration
 // CHECK-FIXES: {{^}}#define DECLARE(x) extern int x{{$}}
 // CHECK-FIXES: {{^}}#define DEFINE(x) extern int x; int x = 42{{$}}
 // CHECK-FIXES: {{^}}DECLARE(test);{{$}}
 // CHECK-FIXES: {{^}}DEFINE(test);{{$}}
 
 } // namespace macros
+
+inline void g() {}
+
+inline void g(); // g
+// CHECK-MESSAGES: :[[@LINE-1]]:13: warning: redundant 'g' declaration
+// CHECK-FIXES: {{^}}// g{{$}}
+
+extern inline void g(); // extern g
+// CHECK-MESSAGES: :[[@LINE-1]]:20: warning: redundant 'g' declaration
+// CHECK-FIXES: {{^}}// extern g{{$}}
 No newline at end of file