diff --git a/clang-tools-extra/clang-tidy/readability/FunctionCognitiveComplexityCheck.cpp b/clang-tools-extra/clang-tidy/readability/FunctionCognitiveComplexityCheck.cpp
--- a/clang-tools-extra/clang-tidy/readability/FunctionCognitiveComplexityCheck.cpp
+++ b/clang-tools-extra/clang-tidy/readability/FunctionCognitiveComplexityCheck.cpp
@@ -501,9 +501,9 @@
 
 void FunctionCognitiveComplexityCheck::registerMatchers(MatchFinder *Finder) {
   Finder->addMatcher(
-      functionDecl(
-          allOf(isDefinition(), unless(anyOf(isDefaulted(), isDeleted(),
-                                             isImplicit(), isInstantiated()))))
+      functionDecl(isDefinition(),
+                   unless(anyOf(isDefaulted(), isDeleted(), isImplicit(),
+                                isInstantiated(), isWeak())))
           .bind("func"),
       this);
 }
diff --git a/clang-tools-extra/test/clang-tidy/checkers/readability-function-cognitive-complexity.cpp b/clang-tools-extra/test/clang-tidy/checkers/readability-function-cognitive-complexity.cpp
--- a/clang-tools-extra/test/clang-tidy/checkers/readability-function-cognitive-complexity.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/readability-function-cognitive-complexity.cpp
@@ -1013,3 +1013,5 @@
 
   templatedFunction<void*>();
 }
+
+static void pr47779_dont_crash_on_weak() __attribute__((__weakref__("__pr47779_dont_crash_on_weak")));
diff --git a/clang/docs/LibASTMatchersReference.html b/clang/docs/LibASTMatchersReference.html
--- a/clang/docs/LibASTMatchersReference.html
+++ b/clang/docs/LibASTMatchersReference.html
@@ -3569,6 +3569,17 @@
 </pre></td></tr>
 
 
+<tr><td>Matcher&lt;<a href="https://clang.llvm.org/doxygen/classclang_1_1FunctionDecl.html">FunctionDecl</a>&gt;</td><td class="name" onclick="toggle('isWeak0')"><a name="isWeak0Anchor">isWeak</a></td><td></td></tr>
+<tr><td colspan="4" class="doc" id="isWeak0"><pre>Matches weak function declarations.
+
+Given:
+  void foo() __attribute__((__weakref__("__foo")));
+  void bar();
+functionDecl(isWeak())
+  matches the weak declaration "foo", but not "bar".
+</pre></td></tr>
+
+
 <tr><td>Matcher&lt;<a href="https://clang.llvm.org/doxygen/classclang_1_1FunctionDecl.html">FunctionDecl</a>&gt;</td><td class="name" onclick="toggle('parameterCountIs0')"><a name="parameterCountIs0Anchor">parameterCountIs</a></td><td>unsigned N</td></tr>
 <tr><td colspan="4" class="doc" id="parameterCountIs0"><pre>Matches FunctionDecls and FunctionProtoTypes that have a
 specific parameter count.
diff --git a/clang/include/clang/ASTMatchers/ASTMatchers.h b/clang/include/clang/ASTMatchers/ASTMatchers.h
--- a/clang/include/clang/ASTMatchers/ASTMatchers.h
+++ b/clang/include/clang/ASTMatchers/ASTMatchers.h
@@ -4647,6 +4647,17 @@
   return Node.isDefaulted();
 }
 
+/// Matches weak function declarations.
+///
+/// Given:
+/// \code
+///   void foo() __attribute__((__weakref__("__foo")));
+///   void bar();
+/// \endcode
+/// functionDecl(isWeak())
+///   matches the weak declaration "foo", but not "bar".
+AST_MATCHER(FunctionDecl, isWeak) { return Node.isWeak(); }
+
 /// Matches functions that have a dynamic exception specification.
 ///
 /// Given:
diff --git a/clang/lib/ASTMatchers/Dynamic/Registry.cpp b/clang/lib/ASTMatchers/Dynamic/Registry.cpp
--- a/clang/lib/ASTMatchers/Dynamic/Registry.cpp
+++ b/clang/lib/ASTMatchers/Dynamic/Registry.cpp
@@ -433,6 +433,7 @@
   REGISTER_MATCHER(isVirtual);
   REGISTER_MATCHER(isVirtualAsWritten);
   REGISTER_MATCHER(isVolatileQualified);
+  REGISTER_MATCHER(isWeak);
   REGISTER_MATCHER(isWritten);
   REGISTER_MATCHER(lValueReferenceType);
   REGISTER_MATCHER(labelDecl);