diff --git a/clang/include/clang/Basic/DiagnosticGroups.td b/clang/include/clang/Basic/DiagnosticGroups.td
--- a/clang/include/clang/Basic/DiagnosticGroups.td
+++ b/clang/include/clang/Basic/DiagnosticGroups.td
@@ -720,6 +720,8 @@
 def UnusedLabel : DiagGroup<"unused-label">;
 def UnusedLambdaCapture : DiagGroup<"unused-lambda-capture">;
 def UnusedParameter : DiagGroup<"unused-parameter">;
+def UnusedButSetParameter : DiagGroup<"unused-but-set-parameter">;
+def UnusedButSetVariable : DiagGroup<"unused-but-set-variable">;
 def UnusedResult : DiagGroup<"unused-result">;
 def PotentiallyEvaluatedExpression : DiagGroup<"potentially-evaluated-expression">;
 def UnevaluatedExpression : DiagGroup<"unevaluated-expression",
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -310,8 +310,12 @@
   "repeated RISC-V 'interrupt' attribute is here">;
 def warn_unused_parameter : Warning<"unused parameter %0">,
   InGroup<UnusedParameter>, DefaultIgnore;
+def warn_unused_but_set_parameter : Warning<"parameter %0 set but not used">,
+  InGroup<UnusedButSetParameter>, DefaultIgnore;
 def warn_unused_variable : Warning<"unused variable %0">,
   InGroup<UnusedVariable>, DefaultIgnore;
+def warn_unused_but_set_variable : Warning<"variable %0 set but not used">,
+  InGroup<UnusedButSetVariable>, DefaultIgnore;
 def warn_unused_local_typedef : Warning<
   "unused %select{typedef|type alias}0 %1">,
   InGroup<UnusedLocalTypedef>, DefaultIgnore;
diff --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
--- a/clang/include/clang/Sema/Sema.h
+++ b/clang/include/clang/Sema/Sema.h
@@ -2827,6 +2827,13 @@
   /// ParmVarDecl pointers.
   void DiagnoseUnusedParameters(ArrayRef<ParmVarDecl *> Parameters);
 
+  /// Diagnose any unused but set parameters in the given sequence of
+  /// ParmVarDecl pointers.
+  void DiagnoseUnusedButSetParameters(ArrayRef<ParmVarDecl *> Parameters);
+
+  /// Diagnose any unused but set variables declared in this CompoundStmt
+  void DiagnoseUnusedButSetVariables(CompoundStmt *CS);
+
   /// Diagnose whether the size of parameters or return value of a
   /// function or obj-c method definition is pass-by-value and larger than a
   /// specified threshold.
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -24,6 +24,7 @@
 #include "clang/AST/Expr.h"
 #include "clang/AST/ExprCXX.h"
 #include "clang/AST/NonTrivialTypeVisitor.h"
+#include "clang/AST/RecursiveASTVisitor.h"
 #include "clang/AST/StmtCXX.h"
 #include "clang/Basic/Builtins.h"
 #include "clang/Basic/PartialDiagnostic.h"
@@ -43,6 +44,7 @@
 #include "clang/Sema/ScopeInfo.h"
 #include "clang/Sema/SemaInternal.h"
 #include "clang/Sema/Template.h"
+#include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/Triple.h"
 #include <algorithm>
@@ -13733,6 +13735,140 @@
   }
 }
 
+// values in Map should be true. traverses Body; if any key is used in any way
+// other than assigning to it, sets the corresponding value to false.
+static void AreAllUsesSets(Stmt *Body,
+                           llvm::SmallDenseMap<NamedDecl *, bool> *Map) {
+
+  struct AllUsesAreSetsVisitor : RecursiveASTVisitor<AllUsesAreSetsVisitor> {
+    llvm::SmallDenseMap<NamedDecl *, bool> *M;
+    unsigned FalseCount = 0;
+
+    AllUsesAreSetsVisitor(llvm::SmallDenseMap<NamedDecl *, bool> *Map)
+        : M(Map) {}
+
+    bool TraverseBinaryOperator(BinaryOperator *BO) {
+      auto *LHS = BO->getLHS();
+      auto *DRE = dyn_cast<DeclRefExpr>(LHS);
+      if (!DRE || M->find(DRE->getFoundDecl()) == M->end()) {
+        // the LHS is not a reference to one of our NamedDecls
+        TraverseStmt(LHS);
+      }
+      TraverseStmt(BO->getRHS());
+      return true;
+    }
+
+    bool VisitDeclRefExpr(const DeclRefExpr *DRE) {
+      auto iter = M->find(DRE->getFoundDecl());
+      if (iter != M->end()) {
+        iter->second = false;
+        ++FalseCount;
+        if (FalseCount == M->size()) {
+          return false;
+        }
+      }
+      return true;
+    }
+  };
+
+  if (!Map->size())
+    return;
+
+  AllUsesAreSetsVisitor Visitor(Map);
+  Visitor.TraverseStmt(Body);
+}
+
+template <typename R>
+static void DiagnoseUnusedDecls(Sema *Se, Stmt *Body, R Decls,
+                                unsigned DiagID) {
+  // put the Decls in a map so we only have to traverse the body once for all of
+  // them
+  llvm::SmallDenseMap<NamedDecl *, bool> AllUsesAreSets;
+
+  for (NamedDecl *ND : Decls) {
+    AllUsesAreSets.insert(std::make_pair(ND, true));
+  }
+
+  AreAllUsesSets(Body, &AllUsesAreSets);
+
+  for (const auto Pair : AllUsesAreSets) {
+    if (Pair.second) {
+      Se->Diag(Pair.first->getLocation(), DiagID) << Pair.first->getDeclName();
+    }
+  }
+}
+
+void Sema::DiagnoseUnusedButSetParameters(ArrayRef<ParmVarDecl *> Parameters) {
+  // Don't diagnose unused-but-set-parameter errors in template instantiations;
+  // we will already have done so in the template itself.
+  if (inTemplateInstantiation())
+    return;
+
+  bool CPlusPlus = getLangOpts().CPlusPlus;
+
+  auto IsCandidate = [CPlusPlus, &Diags = Diags](ParmVarDecl *P) {
+    // check for Ignored here, because if we have no candidates we can avoid
+    // walking the AST
+    if (Diags.getDiagnosticLevel(diag::warn_unused_but_set_parameter,
+                                 P->getLocation()) ==
+        DiagnosticsEngine::Ignored)
+      return false;
+    if (!P->isReferenced() || !P->getDeclName() || P->hasAttr<UnusedAttr>())
+      return false;
+    // mimic gcc's behavior regarding nonscalar types
+    if (CPlusPlus && !P->getType()->isScalarType())
+      return false;
+    return true;
+  };
+
+  auto Candidates = llvm::make_filter_range(Parameters, IsCandidate);
+
+  if (Parameters.begin() != Parameters.end()) {
+    Decl *D =
+        Decl::castFromDeclContext((*Parameters.begin())->getDeclContext());
+    Stmt *Body = D->getBody();
+    if (Body)
+      DiagnoseUnusedDecls(this, Body, Candidates,
+                          diag::warn_unused_but_set_parameter);
+  }
+}
+
+void Sema::DiagnoseUnusedButSetVariables(CompoundStmt *CS) {
+  bool CPlusPlus = getLangOpts().CPlusPlus;
+
+  auto IsCandidate = [CPlusPlus, &Diags = Diags](Stmt *S) {
+    DeclStmt *SD = dyn_cast<DeclStmt>(S);
+    if (!SD || !SD->isSingleDecl())
+      return false;
+    VarDecl *VD = dyn_cast<VarDecl>(SD->getSingleDecl());
+    if (!VD || !VD->isReferenced() || !VD->getDeclName() ||
+        VD->hasAttr<UnusedAttr>())
+      return false;
+    // check for Ignored here, because if we have no candidates we can avoid
+    // walking the AST
+    if (Diags.getDiagnosticLevel(diag::warn_unused_but_set_variable,
+                                 VD->getLocation()) ==
+        DiagnosticsEngine::Ignored)
+      return false;
+    // mimic gcc's behavior regarding nonscalar types
+    if (CPlusPlus && !VD->getType()->isScalarType())
+      return false;
+    return true;
+  };
+
+  auto Candidates = llvm::make_filter_range(CS->body(), IsCandidate);
+
+  auto ToNamedDecl = [](Stmt *S) {
+    DeclStmt *SD = dyn_cast<DeclStmt>(S);
+    return dyn_cast<NamedDecl>(SD->getSingleDecl());
+  };
+
+  auto CandidateDecls = llvm::map_range(Candidates, ToNamedDecl);
+
+  DiagnoseUnusedDecls(this, CS, CandidateDecls,
+                      diag::warn_unused_but_set_variable);
+}
+
 void Sema::DiagnoseSizeOfParametersAndReturnValue(
     ArrayRef<ParmVarDecl *> Parameters, QualType ReturnTy, NamedDecl *D) {
   if (LangOpts.NumLargeByValueCopy == 0) // No check.
@@ -14435,8 +14571,10 @@
 
     if (!FD->isInvalidDecl()) {
       // Don't diagnose unused parameters of defaulted or deleted functions.
-      if (!FD->isDeleted() && !FD->isDefaulted() && !FD->hasSkippedBody())
+      if (!FD->isDeleted() && !FD->isDefaulted() && !FD->hasSkippedBody()) {
         DiagnoseUnusedParameters(FD->parameters());
+        DiagnoseUnusedButSetParameters(FD->parameters());
+      }
       DiagnoseSizeOfParametersAndReturnValue(FD->parameters(),
                                              FD->getReturnType(), FD);
 
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -15498,6 +15498,9 @@
 
   BD->setBody(cast<CompoundStmt>(Body));
 
+  // wait to diagnose unused but set parameters until after setBody
+  DiagnoseUnusedButSetParameters(BD->parameters());
+
   if (Body && getCurFunction()->HasPotentialAvailabilityViolations)
     DiagnoseUnguardedAvailabilityViolations(BD);
 
diff --git a/clang/lib/Sema/SemaStmt.cpp b/clang/lib/Sema/SemaStmt.cpp
--- a/clang/lib/Sema/SemaStmt.cpp
+++ b/clang/lib/Sema/SemaStmt.cpp
@@ -433,7 +433,9 @@
       DiagnoseEmptyLoopBody(Elts[i], Elts[i + 1]);
   }
 
-  return CompoundStmt::Create(Context, Elts, L, R);
+  CompoundStmt *CS = CompoundStmt::Create(Context, Elts, L, R);
+  DiagnoseUnusedButSetVariables(CS);
+  return CS;
 }
 
 ExprResult
diff --git a/clang/test/Sema/vector-gcc-compat.c b/clang/test/Sema/vector-gcc-compat.c
--- a/clang/test/Sema/vector-gcc-compat.c
+++ b/clang/test/Sema/vector-gcc-compat.c
@@ -35,7 +35,7 @@
 
 void arithmeticTest(void) {
   v2i64 v2i64_a = (v2i64){0, 1};
-  v2i64 v2i64_r;
+  v2i64 v2i64_r; // expected-warning{{variable 'v2i64_r' set but not used}}
 
   v2i64_r = v2i64_a + 1;
   v2i64_r = v2i64_a - 1;
@@ -58,7 +58,7 @@
 
 void comparisonTest(void) {
   v2i64 v2i64_a = (v2i64){0, 1};
-  v2i64 v2i64_r;
+  v2i64 v2i64_r; // expected-warning{{variable 'v2i64_r' set but not used}}
 
   v2i64_r = v2i64_a == 1;
   v2i64_r = v2i64_a != 1;
@@ -78,8 +78,8 @@
 void logicTest(void) {
   v2i64 v2i64_a = (v2i64){0, 1};
   v2i64 v2i64_b = (v2i64){2, 1};
-  v2i64 v2i64_c = (v2i64){3, 1};
-  v2i64 v2i64_r;
+  v2i64 v2i64_c = (v2i64){3, 1}; // expected-warning{{variable 'v2i64_c' set but not used}}
+  v2i64 v2i64_r;                 // expected-warning{{variable 'v2i64_r' set but not used}}
 
   v2i64_r = !v2i64_a; // expected-error {{invalid argument type 'v2i64' (vector of 2 'long long' values) to unary expression}}
   v2i64_r = ~v2i64_a;
@@ -132,7 +132,7 @@
 
 void floatTestConstantComparison(void) {
   v4f32 v4f32_a = {0.4f, 0.4f, 0.4f, 0.4f};
-  v4i32 v4i32_r;
+  v4i32 v4i32_r; // expected-warning{{variable 'v4i32_r' set but not used}}
   v4i32_r = v4f32_a > 0.4f;
   v4i32_r = v4f32_a >= 0.4f;
   v4i32_r = v4f32_a < 0.4f;
@@ -143,7 +143,7 @@
 
 void doubleTestConstantComparison(void) {
   v2f64 v2f64_a = {0.4, 0.4};
-  v2i64 v2i64_r;
+  v2i64 v2i64_r; // expected-warning{{variable 'v2i64_r' set but not used}}
   v2i64_r = v2f64_a > 0.4;
   v2i64_r = v2f64_a >= 0.4;
   v2i64_r = v2f64_a < 0.4;
diff --git a/clang/test/Sema/warn-unused-but-set-parameters-cpp.cpp b/clang/test/Sema/warn-unused-but-set-parameters-cpp.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/Sema/warn-unused-but-set-parameters-cpp.cpp
@@ -0,0 +1,27 @@
+// RUN: %clang_cc1 -fblocks -fsyntax-only -Wunused-but-set-parameter -verify %s
+
+int f0(int x,
+       int y, // expected-warning{{parameter 'y' set but not used}}
+       int z __attribute__((unused))) {
+  y = 0;
+  return x;
+}
+
+void f1(void) {
+  (void)^(int x,
+          int y, // expected-warning{{parameter 'y' set but not used}}
+          int z __attribute__((unused))) {
+    y = 0;
+    return x;
+  };
+}
+
+struct S {
+  int i;
+};
+
+// in C++, don't warn for a struct
+void f3(struct S s) {
+  struct S t;
+  s = t;
+}
diff --git a/clang/test/Sema/warn-unused-but-set-parameters.c b/clang/test/Sema/warn-unused-but-set-parameters.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Sema/warn-unused-but-set-parameters.c
@@ -0,0 +1,26 @@
+// RUN: %clang_cc1 -fblocks -fsyntax-only -Wunused-but-set-parameter -verify %s
+
+int f0(int x,
+       int y, // expected-warning{{parameter 'y' set but not used}}
+       int z __attribute__((unused))) {
+  y = 0;
+  return x;
+}
+
+void f1(void) {
+  (void)^(int x,
+          int y, // expected-warning{{parameter 'y' set but not used}}
+          int z __attribute__((unused))) {
+    y = 0;
+    return x;
+  };
+}
+
+struct S {
+  int i;
+};
+
+void f3(struct S s) { // expected-warning{{parameter 's' set but not used}}
+  struct S t;
+  s = t;
+}
diff --git a/clang/test/Sema/warn-unused-but-set-variables-cpp.cpp b/clang/test/Sema/warn-unused-but-set-variables-cpp.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/Sema/warn-unused-but-set-variables-cpp.cpp
@@ -0,0 +1,33 @@
+// RUN: %clang_cc1 -fblocks -fsyntax-only -Wunused-but-set-variable -verify %s
+
+struct S {
+  int i;
+};
+
+int f0() {
+  int y; // expected-warning{{variable 'y' set but not used}}
+  y = 0;
+
+  int z __attribute__((unused));
+  z = 0;
+
+  // no warning for structs in C++
+  struct S s;
+  struct S t;
+  s = t;
+
+  int x;
+  x = 0;
+  return x;
+}
+
+void f1(void) {
+  (void)^() {
+    int y; // expected-warning{{variable 'y' set but not used}}
+    y = 0;
+
+    int x;
+    x = 0;
+    return x;
+  };
+}
diff --git a/clang/test/Sema/warn-unused-but-set-variables.c b/clang/test/Sema/warn-unused-but-set-variables.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Sema/warn-unused-but-set-variables.c
@@ -0,0 +1,32 @@
+// RUN: %clang_cc1 -fblocks -fsyntax-only -Wunused-but-set-variable -verify %s
+
+struct S {
+  int i;
+};
+
+int f0() {
+  int y; // expected-warning{{variable 'y' set but not used}}
+  y = 0;
+
+  int z __attribute__((unused));
+  z = 0;
+
+  struct S s; // expected-warning{{variable 's' set but not used}}
+  struct S t;
+  s = t;
+
+  int x;
+  x = 0;
+  return x;
+}
+
+void f1(void) {
+  (void)^() {
+    int y; // expected-warning{{variable 'y' set but not used}}
+    y = 0;
+
+    int x;
+    x = 0;
+    return x;
+  };
+}