diff --git a/clang-tools-extra/clang-tidy/ClangTidy.cpp b/clang-tools-extra/clang-tidy/ClangTidy.cpp
index 499144830eabe..47253570e91da 100644
--- a/clang-tools-extra/clang-tidy/ClangTidy.cpp
+++ b/clang-tools-extra/clang-tidy/ClangTidy.cpp
@@ -133,7 +133,7 @@ class ErrorReporter {
       for (const FileByteRange &FBR : Error.Message.Ranges)
         Diag << getRange(FBR);
       // FIXME: explore options to support interactive fix selection.
-      const llvm::StringMap<Replacements> *ChosenFix;
+      const llvm::StringMap<Replacements> *ChosenFix = nullptr;
       if (ApplyFixes != FB_NoFix &&
           (ChosenFix = getFixIt(Error, ApplyFixes == FB_FixNotes))) {
         for (const auto &FileAndReplacements : *ChosenFix) {
diff --git a/clang-tools-extra/clang-tidy/ClangTidyCheck.cpp b/clang-tools-extra/clang-tidy/ClangTidyCheck.cpp
index 8b261268449cf..3e926236adb45 100644
--- a/clang-tools-extra/clang-tidy/ClangTidyCheck.cpp
+++ b/clang-tools-extra/clang-tidy/ClangTidyCheck.cpp
@@ -97,7 +97,7 @@ static std::optional<bool> getAsBool(StringRef Value,
     return Parsed;
   // To maintain backwards compatability, we support parsing numbers as
   // booleans, even though its not supported in YAML.
-  long long Number;
+  long long Number = 0;
   if (!Value.getAsInteger(10, Number))
     return Number != 0;
   return std::nullopt;
diff --git a/clang-tools-extra/clang-tidy/abseil/DurationFactoryScaleCheck.cpp b/clang-tools-extra/clang-tidy/abseil/DurationFactoryScaleCheck.cpp
index 5688dc16d1106..121892fd0daa9 100644
--- a/clang-tools-extra/clang-tidy/abseil/DurationFactoryScaleCheck.cpp
+++ b/clang-tools-extra/clang-tidy/abseil/DurationFactoryScaleCheck.cpp
@@ -159,7 +159,7 @@ void DurationFactoryScaleCheck::check(const MatchFinder::MatchResult &Result) {
     return;
 
   DurationScale Scale = *MaybeScale;
-  const Expr *Remainder;
+  const Expr *Remainder = nullptr;
   std::optional<DurationScale> NewScale;
 
   // We next handle the cases of multiplication and division.
diff --git a/clang-tools-extra/clang-tidy/abseil/RedundantStrcatCallsCheck.cpp b/clang-tools-extra/clang-tidy/abseil/RedundantStrcatCallsCheck.cpp
index 2eed10bbf6e6c..fafb029e7de1b 100644
--- a/clang-tools-extra/clang-tidy/abseil/RedundantStrcatCallsCheck.cpp
+++ b/clang-tools-extra/clang-tidy/abseil/RedundantStrcatCallsCheck.cpp
@@ -103,9 +103,9 @@ StrCatCheckResult processCall(const CallExpr *RootCall, bool IsAppend,
 }  // namespace
 
 void RedundantStrcatCallsCheck::check(const MatchFinder::MatchResult& Result) {
-  bool IsAppend;
+  bool IsAppend = false;
 
-  const CallExpr* RootCall;
+  const CallExpr *RootCall = nullptr;
   if ((RootCall = Result.Nodes.getNodeAs<CallExpr>("StrCat"))) 
   	IsAppend = false;
   else if ((RootCall = Result.Nodes.getNodeAs<CallExpr>("StrAppend"))) 
diff --git a/clang-tools-extra/clang-tidy/altera/UnrollLoopsCheck.cpp b/clang-tools-extra/clang-tidy/altera/UnrollLoopsCheck.cpp
index 197fa81250160..37eacf565f199 100644
--- a/clang-tools-extra/clang-tidy/altera/UnrollLoopsCheck.cpp
+++ b/clang-tools-extra/clang-tidy/altera/UnrollLoopsCheck.cpp
@@ -203,7 +203,7 @@ bool UnrollLoopsCheck::hasLargeNumIterations(const Stmt *Statement,
   // If increment is binary and not one of +, -, *, /, we can't know the loop
   // bounds.
   if (const auto *Op = dyn_cast<BinaryOperator>(Increment)) {
-    int ConstantValue;
+    int ConstantValue = 0;
     if (!extractValue(ConstantValue, Op, Context))
       return true;
     switch (Op->getOpcode()) {
diff --git a/clang-tools-extra/clang-tidy/bugprone/ImplicitWideningOfMultiplicationResultCheck.cpp b/clang-tools-extra/clang-tidy/bugprone/ImplicitWideningOfMultiplicationResultCheck.cpp
index e27012a91c3e5..95a7c521eabb0 100644
--- a/clang-tools-extra/clang-tidy/bugprone/ImplicitWideningOfMultiplicationResultCheck.cpp
+++ b/clang-tools-extra/clang-tidy/bugprone/ImplicitWideningOfMultiplicationResultCheck.cpp
@@ -151,7 +151,7 @@ void ImplicitWideningOfMultiplicationResultCheck::handlePointerOffsetting(
 
   // We are looking for a pointer offset operation,
   // with one hand being a pointer, and another one being an offset.
-  const Expr *PointerExpr, *IndexExpr;
+  const Expr *PointerExpr = nullptr, *IndexExpr = nullptr;
   if (const auto *BO = dyn_cast<BinaryOperator>(E)) {
     PointerExpr = BO->getLHS();
     IndexExpr = BO->getRHS();
diff --git a/clang-tools-extra/clang-tidy/bugprone/MisplacedWideningCastCheck.cpp b/clang-tools-extra/clang-tidy/bugprone/MisplacedWideningCastCheck.cpp
index bdf388f04a779..a1f92aae55448 100644
--- a/clang-tools-extra/clang-tidy/bugprone/MisplacedWideningCastCheck.cpp
+++ b/clang-tools-extra/clang-tidy/bugprone/MisplacedWideningCastCheck.cpp
@@ -163,7 +163,7 @@ static int relativeCharSizesW(BuiltinType::Kind Kind) {
 }
 
 static bool isFirstWider(BuiltinType::Kind First, BuiltinType::Kind Second) {
-  int FirstSize, SecondSize;
+  int FirstSize = 0, SecondSize = 0;
   if ((FirstSize = relativeIntSizes(First)) != 0 &&
       (SecondSize = relativeIntSizes(Second)) != 0)
     return FirstSize > SecondSize;
diff --git a/clang-tools-extra/clang-tidy/bugprone/MultipleNewInOneExpressionCheck.cpp b/clang-tools-extra/clang-tidy/bugprone/MultipleNewInOneExpressionCheck.cpp
index bd8a0b359c2e0..41191a3cfed23 100644
--- a/clang-tools-extra/clang-tidy/bugprone/MultipleNewInOneExpressionCheck.cpp
+++ b/clang-tools-extra/clang-tidy/bugprone/MultipleNewInOneExpressionCheck.cpp
@@ -29,7 +29,7 @@ bool isExprValueStored(const Expr *E, ASTContext &C) {
   DynTypedNodeList P = PMap.getParents(*E);
   if (P.size() != 1)
     return false;
-  const Expr *ParentE;
+  const Expr *ParentE = nullptr;
   while ((ParentE = P[0].get<Expr>()) && ParentE->IgnoreParenCasts() == E) {
     P = PMap.getParents(P[0]);
     if (P.size() != 1)
diff --git a/clang-tools-extra/clang-tidy/bugprone/RedundantBranchConditionCheck.cpp b/clang-tools-extra/clang-tidy/bugprone/RedundantBranchConditionCheck.cpp
index 3e04df656a67a..95fbca2ad0083 100644
--- a/clang-tools-extra/clang-tidy/bugprone/RedundantBranchConditionCheck.cpp
+++ b/clang-tools-extra/clang-tidy/bugprone/RedundantBranchConditionCheck.cpp
@@ -76,7 +76,7 @@ void RedundantBranchConditionCheck::check(const MatchFinder::MatchResult &Result
   const auto *CondVar = Result.Nodes.getNodeAs<VarDecl>(CondVarStr);
   const auto *Func = Result.Nodes.getNodeAs<FunctionDecl>(FuncStr);
 
-  const DeclRefExpr *OuterIfVar, *InnerIfVar;
+  const DeclRefExpr *OuterIfVar = nullptr, *InnerIfVar = nullptr;
   if (const auto *Inner = Result.Nodes.getNodeAs<DeclRefExpr>(InnerIfVar1Str))
     InnerIfVar = Inner;
   else
diff --git a/clang-tools-extra/clang-tidy/bugprone/SignalHandlerCheck.cpp b/clang-tools-extra/clang-tidy/bugprone/SignalHandlerCheck.cpp
index efa7dd66c816e..fd3ca76e68a61 100644
--- a/clang-tools-extra/clang-tidy/bugprone/SignalHandlerCheck.cpp
+++ b/clang-tools-extra/clang-tidy/bugprone/SignalHandlerCheck.cpp
@@ -480,7 +480,7 @@ bool SignalHandlerCheck::checkFunctionCPP14(
     return true;
   }
 
-  const FunctionDecl *FBody;
+  const FunctionDecl *FBody = nullptr;
   const Stmt *BodyS = FD->getBody(FBody);
   if (!BodyS)
     return false;
diff --git a/clang-tools-extra/clang-tidy/cert/StrToNumCheck.cpp b/clang-tools-extra/clang-tidy/cert/StrToNumCheck.cpp
index 3c7e599a1acfa..3b59d2357fe29 100644
--- a/clang-tools-extra/clang-tidy/cert/StrToNumCheck.cpp
+++ b/clang-tools-extra/clang-tidy/cert/StrToNumCheck.cpp
@@ -178,7 +178,7 @@ StringRef classifyReplacement(ConversionKind K) {
 void StrToNumCheck::check(const MatchFinder::MatchResult &Result) {
   const auto *Call = Result.Nodes.getNodeAs<CallExpr>("expr");
   const FunctionDecl *FuncDecl = nullptr;
-  ConversionKind Conversion;
+  ConversionKind Conversion = ConversionKind::None;
 
   if (const auto *ConverterFunc =
           Result.Nodes.getNodeAs<FunctionDecl>("converter")) {
diff --git a/clang-tools-extra/clang-tidy/cppcoreguidelines/NarrowingConversionsCheck.cpp b/clang-tools-extra/clang-tidy/cppcoreguidelines/NarrowingConversionsCheck.cpp
index f22c9f599a262..1b858db511f50 100644
--- a/clang-tools-extra/clang-tidy/cppcoreguidelines/NarrowingConversionsCheck.cpp
+++ b/clang-tools-extra/clang-tidy/cppcoreguidelines/NarrowingConversionsCheck.cpp
@@ -487,7 +487,7 @@ void NarrowingConversionsCheck::handleFloatingCast(const ASTContext &Context,
       // not within destination range. We convert the value to the destination
       // type and check if the resulting value is infinity.
       llvm::APFloat Tmp = Constant.getFloat();
-      bool UnusedLosesInfo;
+      bool UnusedLosesInfo = false;
       Tmp.convert(Context.getFloatTypeSemantics(ToType->desugar()),
                   llvm::APFloatBase::rmNearestTiesToEven, &UnusedLosesInfo);
       if (Tmp.isInfinity())
diff --git a/clang-tools-extra/clang-tidy/cppcoreguidelines/PreferMemberInitializerCheck.cpp b/clang-tools-extra/clang-tidy/cppcoreguidelines/PreferMemberInitializerCheck.cpp
index 152db151f78ec..aac824f15fa57 100644
--- a/clang-tools-extra/clang-tidy/cppcoreguidelines/PreferMemberInitializerCheck.cpp
+++ b/clang-tools-extra/clang-tidy/cppcoreguidelines/PreferMemberInitializerCheck.cpp
@@ -175,8 +175,8 @@ void PreferMemberInitializerCheck::check(
         return;
     }
 
-    const FieldDecl *Field;
-    const Expr *InitValue;
+    const FieldDecl *Field = nullptr;
+    const Expr *InitValue = nullptr;
     std::tie(Field, InitValue) = isAssignmentToMemberOf(Class, S, Ctor);
     if (Field) {
       if (IsUseDefaultMemberInitEnabled && getLangOpts().CPlusPlus11 &&
diff --git a/clang-tools-extra/clang-tidy/fuchsia/MultipleInheritanceCheck.cpp b/clang-tools-extra/clang-tidy/fuchsia/MultipleInheritanceCheck.cpp
index f73667f21ae87..fa000e21b281c 100644
--- a/clang-tools-extra/clang-tidy/fuchsia/MultipleInheritanceCheck.cpp
+++ b/clang-tools-extra/clang-tidy/fuchsia/MultipleInheritanceCheck.cpp
@@ -62,7 +62,7 @@ bool MultipleInheritanceCheck::isInterface(const CXXRecordDecl *Node) {
     return false;
 
   // Short circuit the lookup if we have analyzed this record before.
-  bool PreviousIsInterfaceResult;
+  bool PreviousIsInterfaceResult = false;
   if (getInterfaceStatus(Node, PreviousIsInterfaceResult))
     return PreviousIsInterfaceResult;
 
diff --git a/clang-tools-extra/clang-tidy/google/IntegerTypesCheck.cpp b/clang-tools-extra/clang-tidy/google/IntegerTypesCheck.cpp
index 53c6f5aa7940d..bb4e1de8cc4b1 100644
--- a/clang-tools-extra/clang-tidy/google/IntegerTypesCheck.cpp
+++ b/clang-tools-extra/clang-tidy/google/IntegerTypesCheck.cpp
@@ -88,8 +88,8 @@ void IntegerTypesCheck::check(const MatchFinder::MatchResult &Result) {
                    tok::kw_signed))
     return;
 
-  bool IsSigned;
-  unsigned Width;
+  bool IsSigned = false;
+  unsigned Width = 0;
   const TargetInfo &TargetInfo = Result.Context->getTargetInfo();
 
   // Look for uses of short, long, long long and their unsigned versions.
diff --git a/clang-tools-extra/clang-tidy/google/UpgradeGoogletestCaseCheck.cpp b/clang-tools-extra/clang-tidy/google/UpgradeGoogletestCaseCheck.cpp
index b100fae2be299..ce57b9fc3cac1 100644
--- a/clang-tools-extra/clang-tidy/google/UpgradeGoogletestCaseCheck.cpp
+++ b/clang-tools-extra/clang-tidy/google/UpgradeGoogletestCaseCheck.cpp
@@ -267,8 +267,8 @@ void UpgradeGoogletestCaseCheck::check(const MatchFinder::MatchResult &Result) {
   if (const auto *Method = Result.Nodes.getNodeAs<CXXMethodDecl>("method")) {
     ReplacementText = getNewMethodName(Method->getName());
 
-    bool IsInInstantiation;
-    bool IsInTemplate;
+    bool IsInInstantiation = false;
+    bool IsInTemplate = false;
     bool AddFix = true;
     if (const auto *Call = Result.Nodes.getNodeAs<CXXMemberCallExpr>("call")) {
       const auto *Callee = llvm::cast<MemberExpr>(Call->getCallee());
diff --git a/clang-tools-extra/clang-tidy/hicpp/MultiwayPathsCoveredCheck.cpp b/clang-tools-extra/clang-tidy/hicpp/MultiwayPathsCoveredCheck.cpp
index ebd20f29a641c..47dafca2d03ff 100644
--- a/clang-tools-extra/clang-tidy/hicpp/MultiwayPathsCoveredCheck.cpp
+++ b/clang-tools-extra/clang-tidy/hicpp/MultiwayPathsCoveredCheck.cpp
@@ -95,8 +95,8 @@ void MultiwayPathsCoveredCheck::check(const MatchFinder::MatchResult &Result) {
     return;
   }
   const auto *Switch = Result.Nodes.getNodeAs<SwitchStmt>("switch");
-  std::size_t SwitchCaseCount;
-  bool SwitchHasDefault;
+  std::size_t SwitchCaseCount = 0;
+  bool SwitchHasDefault = false;
   std::tie(SwitchCaseCount, SwitchHasDefault) = countCaseLabels(Switch);
 
   // Checks the sanity of 'switch' statements that actually do define
diff --git a/clang-tools-extra/clang-tidy/llvm/IncludeOrderCheck.cpp b/clang-tools-extra/clang-tidy/llvm/IncludeOrderCheck.cpp
index 022ac10baae4c..3fb2e8daaebed 100644
--- a/clang-tools-extra/clang-tidy/llvm/IncludeOrderCheck.cpp
+++ b/clang-tools-extra/clang-tidy/llvm/IncludeOrderCheck.cpp
@@ -143,7 +143,7 @@ void IncludeOrderPPCallbacks::EndOfMainFile() {
     // block.
     for (unsigned BI = 0, BE = Blocks.size() - 1; BI != BE; ++BI) {
       // Find the first include that's not in the right position.
-      unsigned I, E;
+      unsigned I = 0, E = 0;
       for (I = Blocks[BI], E = Blocks[BI + 1]; I != E; ++I)
         if (IncludeIndices[I] != I)
           break;
diff --git a/clang-tools-extra/clang-tidy/misc/ConfusableIdentifierCheck.cpp b/clang-tools-extra/clang-tidy/misc/ConfusableIdentifierCheck.cpp
index 551f26ab9b924..5b1b1cd152fff 100644
--- a/clang-tools-extra/clang-tidy/misc/ConfusableIdentifierCheck.cpp
+++ b/clang-tools-extra/clang-tidy/misc/ConfusableIdentifierCheck.cpp
@@ -56,7 +56,7 @@ static llvm::SmallString<64U> skeleton(StringRef Name) {
   while (Curr < End) {
 
     const char *Prev = Curr;
-    UTF32 CodePoint;
+    UTF32 CodePoint = 0;
     ConversionResult Result = convertUTF8Sequence(
         reinterpret_cast<const UTF8 **>(&Curr),
         reinterpret_cast<const UTF8 *>(End), &CodePoint, strictConversion);
diff --git a/clang-tools-extra/clang-tidy/misc/ConfusableTable/BuildConfusableTable.cpp b/clang-tools-extra/clang-tidy/misc/ConfusableTable/BuildConfusableTable.cpp
index 9fd718c270e59..79ebb2387179e 100644
--- a/clang-tools-extra/clang-tidy/misc/ConfusableTable/BuildConfusableTable.cpp
+++ b/clang-tools-extra/clang-tidy/misc/ConfusableTable/BuildConfusableTable.cpp
@@ -38,14 +38,14 @@ int main(int argc, char *argv[]) {
     }
 
     llvm::StringRef From = Values[0].trim();
-    llvm::UTF32 CodePoint;
+    llvm::UTF32 CodePoint = 0;
     From.getAsInteger(16, CodePoint);
 
     SmallVector<llvm::UTF32> To;
     SmallVector<StringRef> ToN;
     Values[1].split(ToN, ' ', -1, false);
     for (StringRef To_ : ToN) {
-      llvm::UTF32 ToCodePoint;
+      llvm::UTF32 ToCodePoint = 0;
       To_.trim().getAsInteger(16, ToCodePoint);
       To.push_back(ToCodePoint);
     }
diff --git a/clang-tools-extra/clang-tidy/misc/MisleadingBidirectional.cpp b/clang-tools-extra/clang-tidy/misc/MisleadingBidirectional.cpp
index dad6737ab40cd..da7139255bfaa 100644
--- a/clang-tools-extra/clang-tidy/misc/MisleadingBidirectional.cpp
+++ b/clang-tools-extra/clang-tidy/misc/MisleadingBidirectional.cpp
@@ -51,7 +51,7 @@ static bool containsMisleadingBidi(StringRef Buffer,
         BidiContexts.clear();
       continue;
     }
-    llvm::UTF32 CodePoint;
+    llvm::UTF32 CodePoint = 0;
     llvm::ConversionResult Result = llvm::convertUTF8Sequence(
         (const llvm::UTF8 **)&CurPtr, (const llvm::UTF8 *)Buffer.end(),
         &CodePoint, llvm::strictConversion);
diff --git a/clang-tools-extra/clang-tidy/misc/MisleadingIdentifier.cpp b/clang-tools-extra/clang-tidy/misc/MisleadingIdentifier.cpp
index e1b98b86ecbd6..0448a71260b13 100644
--- a/clang-tools-extra/clang-tidy/misc/MisleadingIdentifier.cpp
+++ b/clang-tools-extra/clang-tidy/misc/MisleadingIdentifier.cpp
@@ -124,7 +124,7 @@ static bool hasRTLCharacters(StringRef Buffer) {
   const char *CurPtr = Buffer.begin();
   const char *EndPtr = Buffer.end();
   while (CurPtr < EndPtr) {
-    llvm::UTF32 CodePoint;
+    llvm::UTF32 CodePoint = 0;
     llvm::ConversionResult Result = llvm::convertUTF8Sequence(
         (const llvm::UTF8 **)&CurPtr, (const llvm::UTF8 *)EndPtr, &CodePoint,
         llvm::strictConversion);
diff --git a/clang-tools-extra/clang-tidy/misc/MisplacedConstCheck.cpp b/clang-tools-extra/clang-tidy/misc/MisplacedConstCheck.cpp
index 4aaf64c9c5361..0cdd48c13b2a6 100644
--- a/clang-tools-extra/clang-tidy/misc/MisplacedConstCheck.cpp
+++ b/clang-tools-extra/clang-tidy/misc/MisplacedConstCheck.cpp
@@ -51,7 +51,7 @@ void MisplacedConstCheck::check(const MatchFinder::MatchResult &Result) {
   QualType CanQT = Var->getType().getCanonicalType();
 
   SourceLocation AliasLoc;
-  const char *AliasType;
+  const char *AliasType = nullptr;
   if (const auto *Typedef = Result.Nodes.getNodeAs<TypedefDecl>("typedef")) {
     AliasLoc = Typedef->getLocation();
     AliasType = "typedef";
diff --git a/clang-tools-extra/clang-tidy/misc/NewDeleteOverloadsCheck.cpp b/clang-tools-extra/clang-tidy/misc/NewDeleteOverloadsCheck.cpp
index e772bcc9ed771..32ea0c0cdf48c 100644
--- a/clang-tools-extra/clang-tidy/misc/NewDeleteOverloadsCheck.cpp
+++ b/clang-tools-extra/clang-tidy/misc/NewDeleteOverloadsCheck.cpp
@@ -17,7 +17,7 @@ namespace clang::tidy::misc {
 namespace {
 
 AST_MATCHER(FunctionDecl, isPlacementOverload) {
-  bool New;
+  bool New = false;
   switch (Node.getOverloadedOperator()) {
   default:
     return false;
diff --git a/clang-tools-extra/clang-tidy/misc/RedundantExpressionCheck.cpp b/clang-tools-extra/clang-tidy/misc/RedundantExpressionCheck.cpp
index 92a541c9e59be..fa8343220dd05 100644
--- a/clang-tools-extra/clang-tidy/misc/RedundantExpressionCheck.cpp
+++ b/clang-tools-extra/clang-tidy/misc/RedundantExpressionCheck.cpp
@@ -1027,7 +1027,7 @@ void RedundantExpressionCheck::checkArithmeticExpr(
     const MatchFinder::MatchResult &Result) {
   APSInt LhsValue, RhsValue;
   const Expr *LhsSymbol = nullptr, *RhsSymbol = nullptr;
-  BinaryOperatorKind LhsOpcode, RhsOpcode;
+  BinaryOperatorKind LhsOpcode{}, RhsOpcode{};
 
   if (const auto *ComparisonOperator = Result.Nodes.getNodeAs<BinaryOperator>(
           "binop-const-compare-to-sym")) {
@@ -1104,7 +1104,7 @@ void RedundantExpressionCheck::checkBitwiseExpr(
 
     APSInt LhsValue, RhsValue;
     const Expr *LhsSymbol = nullptr;
-    BinaryOperatorKind LhsOpcode;
+    BinaryOperatorKind LhsOpcode{};
     if (!retrieveBinOpIntegerConstantExpr(Result, "lhs", LhsOpcode, LhsSymbol,
                                           LhsValue) ||
         !retrieveIntegerConstantExpr(Result, "rhs", RhsValue))
@@ -1180,7 +1180,7 @@ void RedundantExpressionCheck::checkRelationalExpr(
     const Expr *LhsExpr = nullptr, *RhsExpr = nullptr;
     const Expr *LhsSymbol = nullptr, *RhsSymbol = nullptr;
     const Expr *LhsConst = nullptr, *RhsConst = nullptr;
-    BinaryOperatorKind LhsOpcode, RhsOpcode;
+    BinaryOperatorKind LhsOpcode{}, RhsOpcode{};
     APSInt LhsValue, RhsValue;
 
     if (!retrieveRelationalIntegerConstantExpr(
@@ -1240,7 +1240,7 @@ void RedundantExpressionCheck::check(const MatchFinder::MatchResult &Result) {
     // intentional.
     if (areSidesBinaryConstExpressions(BinOp, Result.Context)) {
       const Expr *LhsConst = nullptr, *RhsConst = nullptr;
-      BinaryOperatorKind MainOpcode, SideOpcode;
+      BinaryOperatorKind MainOpcode{}, SideOpcode{};
 
       if (!retrieveConstExprFromBothSides(BinOp, MainOpcode, SideOpcode,
                                           LhsConst, RhsConst, Result.Context))
diff --git a/clang-tools-extra/clang-tidy/modernize/LoopConvertCheck.cpp b/clang-tools-extra/clang-tidy/modernize/LoopConvertCheck.cpp
index bf340984bd381..a37551b42a0e0 100644
--- a/clang-tools-extra/clang-tidy/modernize/LoopConvertCheck.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/LoopConvertCheck.cpp
@@ -962,8 +962,8 @@ void LoopConvertCheck::check(const MatchFinder::MatchResult &Result) {
   Confidence ConfidenceLevel(Confidence::CL_Safe);
   ASTContext *Context = Result.Context;
 
-  const ForStmt *Loop;
-  LoopFixerKind FixerKind;
+  const ForStmt *Loop = nullptr;
+  LoopFixerKind FixerKind{};
   RangeDescriptor Descriptor;
 
   if ((Loop = Nodes.getNodeAs<ForStmt>(LoopNameArray))) {
diff --git a/clang-tools-extra/clang-tidy/modernize/UseEqualsDefaultCheck.cpp b/clang-tools-extra/clang-tidy/modernize/UseEqualsDefaultCheck.cpp
index afac9f7497ac8..5134eb51a0322 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseEqualsDefaultCheck.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/UseEqualsDefaultCheck.cpp
@@ -310,7 +310,7 @@ void UseEqualsDefaultCheck::check(const MatchFinder::MatchResult &Result) {
                   bodyEmpty(Result.Context, Body);
 
   std::vector<FixItHint> RemoveInitializers;
-  unsigned MemberType;
+  unsigned MemberType = 0;
   if (const auto *Ctor = dyn_cast<CXXConstructorDecl>(SpecialFunctionDecl)) {
     if (Ctor->getNumParams() == 0) {
       MemberType = 0;
diff --git a/clang-tools-extra/clang-tidy/performance/UnnecessaryCopyInitialization.cpp b/clang-tools-extra/clang-tidy/performance/UnnecessaryCopyInitialization.cpp
index d4e216239ea94..990e20400fbfc 100644
--- a/clang-tools-extra/clang-tidy/performance/UnnecessaryCopyInitialization.cpp
+++ b/clang-tools-extra/clang-tidy/performance/UnnecessaryCopyInitialization.cpp
@@ -42,7 +42,7 @@ void recordFixes(const VarDecl &Var, ASTContext &Context,
 
 std::optional<SourceLocation> firstLocAfterNewLine(SourceLocation Loc,
                                                    SourceManager &SM) {
-  bool Invalid;
+  bool Invalid = false;
   const char *TextAfter = SM.getCharacterData(Loc, &Invalid);
   if (Invalid) {
     return std::nullopt;
diff --git a/clang-tools-extra/clang-tidy/readability/FunctionCognitiveComplexityCheck.cpp b/clang-tools-extra/clang-tidy/readability/FunctionCognitiveComplexityCheck.cpp
index b5dc09c5f13ad..831614148c7c2 100644
--- a/clang-tools-extra/clang-tidy/readability/FunctionCognitiveComplexityCheck.cpp
+++ b/clang-tools-extra/clang-tidy/readability/FunctionCognitiveComplexityCheck.cpp
@@ -100,8 +100,8 @@ struct CognitiveComplexity final {
     std::pair<unsigned, unsigned short> process() const {
       assert(C != Criteria::None && "invalid criteria");
 
-      unsigned MsgId;           // The id of the message to output.
-      unsigned short Increment; // How much of an increment?
+      unsigned MsgId = 0;           // The id of the message to output.
+      unsigned short Increment = 0; // How much of an increment?
 
       if (C == Criteria::All) {
         Increment = 1 + Nesting;
@@ -196,8 +196,8 @@ void CognitiveComplexity::account(SourceLocation Loc, unsigned short Nesting,
   Details.emplace_back(Loc, Nesting, C);
   const Detail &D = Details.back();
 
-  unsigned MsgId;
-  unsigned short Increase;
+  unsigned MsgId = 0;
+  unsigned short Increase = 0;
   std::tie(MsgId, Increase) = D.process();
 
   Total += Increase;
@@ -242,9 +242,8 @@ class FunctionASTVisitor final
       return Base::TraverseIfStmt(Node);
 
     {
-      CognitiveComplexity::Criteria Reasons;
-
-      Reasons = CognitiveComplexity::Criteria::None;
+      CognitiveComplexity::Criteria Reasons =
+          CognitiveComplexity::Criteria::None;
 
       // "If" increases cognitive complexity.
       Reasons |= CognitiveComplexity::Criteria::Increment;
@@ -290,9 +289,8 @@ class FunctionASTVisitor final
       return TraverseIfStmt(E, true);
 
     {
-      CognitiveComplexity::Criteria Reasons;
-
-      Reasons = CognitiveComplexity::Criteria::None;
+      CognitiveComplexity::Criteria Reasons =
+          CognitiveComplexity::Criteria::None;
 
       // "Else" increases cognitive complexity.
       Reasons |= CognitiveComplexity::Criteria::Increment;
@@ -549,8 +547,8 @@ void FunctionCognitiveComplexityCheck::check(
 
   // Output all the basic increments of complexity.
   for (const auto &Detail : Visitor.CC.Details) {
-    unsigned MsgId;          // The id of the message to output.
-    unsigned short Increase; // How much of an increment?
+    unsigned MsgId = 0;          // The id of the message to output.
+    unsigned short Increase = 0; // How much of an increment?
     std::tie(MsgId, Increase) = Detail.process();
     assert(MsgId < Msgs.size() && "MsgId should always be valid");
     // Increase, on the other hand, can be 0.
diff --git a/clang-tools-extra/clang-tidy/readability/MagicNumbersCheck.cpp b/clang-tools-extra/clang-tidy/readability/MagicNumbersCheck.cpp
index cffe858c39ad1..7f3c2cb9a0434 100644
--- a/clang-tools-extra/clang-tidy/readability/MagicNumbersCheck.cpp
+++ b/clang-tools-extra/clang-tidy/readability/MagicNumbersCheck.cpp
@@ -93,7 +93,7 @@ MagicNumbersCheck::MagicNumbersCheck(StringRef Name, ClangTidyContext *Context)
   IgnoredIntegerValues.resize(IgnoredIntegerValuesInput.size());
   llvm::transform(IgnoredIntegerValuesInput, IgnoredIntegerValues.begin(),
                   [](StringRef Value) {
-                    int64_t Res;
+                    int64_t Res = 0;
                     Value.getAsInteger(10, Res);
                     return Res;
                   });
diff --git a/clang-tools-extra/clang-tidy/readability/SimplifyBooleanExprCheck.cpp b/clang-tools-extra/clang-tidy/readability/SimplifyBooleanExprCheck.cpp
index 89e0c89fa933a..26d9287f07049 100644
--- a/clang-tools-extra/clang-tidy/readability/SimplifyBooleanExprCheck.cpp
+++ b/clang-tools-extra/clang-tidy/readability/SimplifyBooleanExprCheck.cpp
@@ -613,8 +613,8 @@ void SimplifyBooleanExprCheck::reportBinOp(const ASTContext &Context,
   const auto *LHS = Op->getLHS()->IgnoreParenImpCasts();
   const auto *RHS = Op->getRHS()->IgnoreParenImpCasts();
 
-  const CXXBoolLiteralExpr *Bool;
-  const Expr *Other;
+  const CXXBoolLiteralExpr *Bool = nullptr;
+  const Expr *Other = nullptr;
   if ((Bool = dyn_cast<CXXBoolLiteralExpr>(LHS)) != nullptr)
     Other = RHS;
   else if ((Bool = dyn_cast<CXXBoolLiteralExpr>(RHS)) != nullptr)
diff --git a/clang-tools-extra/clang-tidy/readability/SuspiciousCallArgumentCheck.cpp b/clang-tools-extra/clang-tidy/readability/SuspiciousCallArgumentCheck.cpp
index bfca8c3b6c3d4..3eb80019ae753 100644
--- a/clang-tools-extra/clang-tidy/readability/SuspiciousCallArgumentCheck.cpp
+++ b/clang-tools-extra/clang-tidy/readability/SuspiciousCallArgumentCheck.cpp
@@ -239,7 +239,7 @@ static bool applyJaroWinklerHeuristic(StringRef Arg, StringRef Param,
   std::ptrdiff_t L = 0;
   for (std::ptrdiff_t I = 0; I < ParamLen; ++I) {
     if (ParamFlags[I] == 1) {
-      std::ptrdiff_t J;
+      std::ptrdiff_t J = 0;
       for (J = L; J < ArgLen; ++J)
         if (ArgFlags[J] == 1) {
           L = J + 1;
diff --git a/clang-tools-extra/clang-tidy/readability/UppercaseLiteralSuffixCheck.cpp b/clang-tools-extra/clang-tidy/readability/UppercaseLiteralSuffixCheck.cpp
index 549331c2d9938..66b5f3922789d 100644
--- a/clang-tools-extra/clang-tidy/readability/UppercaseLiteralSuffixCheck.cpp
+++ b/clang-tools-extra/clang-tidy/readability/UppercaseLiteralSuffixCheck.cpp
@@ -128,7 +128,7 @@ shouldReplaceLiteralSuffix(const Expr &Literal,
   // Else keep the naive literal location!
 
   // Get the whole literal from the source buffer.
-  bool Invalid;
+  bool Invalid = false;
   const StringRef LiteralSourceText = Lexer::getSourceText(
       CharSourceRange::getTokenRange(*Range), SM, LO, &Invalid);
   assert(!Invalid && "Failed to retrieve the source text.");
diff --git a/clang-tools-extra/clang-tidy/utils/ExceptionSpecAnalyzer.cpp b/clang-tools-extra/clang-tidy/utils/ExceptionSpecAnalyzer.cpp
index 40a3916fde478..4dd4a95f880ac 100644
--- a/clang-tools-extra/clang-tidy/utils/ExceptionSpecAnalyzer.cpp
+++ b/clang-tools-extra/clang-tidy/utils/ExceptionSpecAnalyzer.cpp
@@ -14,19 +14,17 @@ namespace clang::tidy::utils {
 
 ExceptionSpecAnalyzer::State
 ExceptionSpecAnalyzer::analyze(const FunctionDecl *FuncDecl) {
-  ExceptionSpecAnalyzer::State State;
-
   // Check if the function has already been analyzed and reuse that result.
   const auto CacheEntry = FunctionCache.find(FuncDecl);
   if (CacheEntry == FunctionCache.end()) {
-    State = analyzeImpl(FuncDecl);
+    ExceptionSpecAnalyzer::State State = analyzeImpl(FuncDecl);
 
     // Cache the result of the analysis.
     FunctionCache.try_emplace(FuncDecl, State);
-  } else
-    State = CacheEntry->getSecond();
+    return State;
+  }
 
-  return State;
+  return CacheEntry->getSecond();
 }
 
 ExceptionSpecAnalyzer::State
@@ -144,7 +142,7 @@ ExceptionSpecAnalyzer::analyzeFunctionEST(const FunctionDecl *FuncDecl,
     return State::NotThrowing;
   case CT_Dependent: {
     const Expr *NoexceptExpr = FuncProto->getNoexceptExpr();
-    bool Result;
+    bool Result = false;
     return (NoexceptExpr && !NoexceptExpr->isValueDependent() &&
             NoexceptExpr->EvaluateAsBooleanCondition(
                 Result, FuncDecl->getASTContext(), true) &&
diff --git a/clang-tools-extra/clang-tidy/utils/HeaderGuard.cpp b/clang-tools-extra/clang-tidy/utils/HeaderGuard.cpp
index 46c36d62bdd56..cd5cbdc25e10e 100644
--- a/clang-tools-extra/clang-tidy/utils/HeaderGuard.cpp
+++ b/clang-tools-extra/clang-tidy/utils/HeaderGuard.cpp
@@ -188,7 +188,7 @@ class HeaderGuardPPCallbacks : public PPCallbacks {
   void checkEndifComment(StringRef FileName, SourceLocation EndIf,
                          StringRef HeaderGuard,
                          std::vector<FixItHint> &FixIts) {
-    size_t EndIfLen;
+    size_t EndIfLen = 0;
     if (wouldFixEndifComment(FileName, EndIf, HeaderGuard, &EndIfLen)) {
       FixIts.push_back(FixItHint::CreateReplacement(
           CharSourceRange::getCharRange(EndIf,