Index: clang/include/clang/Basic/DiagnosticGroups.td
===================================================================
--- clang/include/clang/Basic/DiagnosticGroups.td
+++ clang/include/clang/Basic/DiagnosticGroups.td
@@ -693,6 +693,7 @@
 def GNUZeroLineDirective : DiagGroup<"gnu-zero-line-directive">;
 def GNUZeroVariadicMacroArguments : DiagGroup<"gnu-zero-variadic-macro-arguments">;
 def Fallback : DiagGroup<"fallback">;
+def MisleadingIndentation : DiagGroup<"misleading-indentation">;
 
 // This covers both the deprecated case (in C++98)
 // and the extension case (in C++11 onwards).
@@ -884,7 +885,7 @@
 // Note that putting warnings in -Wall will not disable them by default. If a
 // warning should be active _only_ when -Wall is passed in, mark it as
 // DefaultIgnore in addition to putting it here.
-def All : DiagGroup<"all", [Most, Parentheses, Switch, SwitchBool]>;
+def All : DiagGroup<"all", [Most, Parentheses, Switch, SwitchBool, MisleadingIndentation]>;
 
 // Warnings that should be in clang-cl /w4.
 def : DiagGroup<"CL4", [All, Extra]>;
Index: clang/include/clang/Basic/DiagnosticParseKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticParseKinds.td
+++ clang/include/clang/Basic/DiagnosticParseKinds.td
@@ -61,6 +61,13 @@
   "remove unnecessary ';' to silence this warning">,
   InGroup<ExtraSemiStmt>, DefaultIgnore;
 
+def warn_misleading_indentation : Warning<
+  "misleading indentation; statement is not part of "
+  "the previous '%select{if|else|for|while|else if}0'">,
+  InGroup<MisleadingIndentation>, DefaultIgnore;
+def note_previous_statement : Note<
+  "previous statement is here">;
+
 def ext_thread_before : Extension<"'__thread' before '%0'">;
 def ext_keyword_as_ident : ExtWarn<
   "keyword '%0' will be made available as an identifier "
Index: clang/include/clang/Lex/Preprocessor.h
===================================================================
--- clang/include/clang/Lex/Preprocessor.h
+++ clang/include/clang/Lex/Preprocessor.h
@@ -932,6 +932,12 @@
     return TheModuleLoader.HadFatalFailure;
   }
 
+  /// Retrieve the number of Directives that have been processed by the
+  /// Preprocessor.
+  unsigned getNumDirectives() const {
+    return NumDirectives;
+  }
+
   /// True if we are currently preprocessing a #if or #elif directive
   bool isParsingIfOrElifDirective() const {
     return ParsingIfOrElifDirective;
Index: clang/include/clang/Parse/Parser.h
===================================================================
--- clang/include/clang/Parse/Parser.h
+++ clang/include/clang/Parse/Parser.h
@@ -1961,7 +1961,9 @@
                                  Sema::ConditionResult &CondResult,
                                  SourceLocation Loc,
                                  Sema::ConditionKind CK);
-  StmtResult ParseIfStatement(SourceLocation *TrailingElseLoc);
+  StmtResult
+  ParseIfStatement(SourceLocation *TrailingElseLoc,
+                   SourceLocation ElseLocOnElseIf = SourceLocation());
   StmtResult ParseSwitchStatement(SourceLocation *TrailingElseLoc);
   StmtResult ParseWhileStatement(SourceLocation *TrailingElseLoc);
   StmtResult ParseDoStatement();
Index: clang/lib/Parse/ParseStmt.cpp
===================================================================
--- clang/lib/Parse/ParseStmt.cpp
+++ clang/lib/Parse/ParseStmt.cpp
@@ -1191,6 +1191,56 @@
   return false;
 }
 
+namespace {
+
+enum MisleadingStatementKind { MSK_if, MSK_else, MSK_for, MSK_while, MSK_else_if };
+
+struct MisleadingIndentationChecker {
+  Parser &P;
+  SourceLocation StmtLoc;
+  SourceLocation PrevLoc;
+  unsigned NumDirectives;
+  MisleadingStatementKind Kind;
+  bool NeedsChecking;
+  bool ShouldSkip;
+  MisleadingIndentationChecker(Parser &P, MisleadingStatementKind K,
+                               SourceLocation SL)
+      : P(P), StmtLoc(SL), PrevLoc(P.getCurToken().getLocation()),
+        NumDirectives(P.getPreprocessor().getNumDirectives()), Kind(K),
+        NeedsChecking(true),
+        ShouldSkip(P.getCurToken().is(tok::l_brace) ||
+                   (K == MSK_else && P.getCurToken().is(tok::kw_if))) {}
+  void Check(bool ShouldCheck) {
+    NeedsChecking = false;
+    Token Tok = P.getCurToken();
+    if (!ShouldCheck || ShouldSkip ||
+        NumDirectives != P.getPreprocessor().getNumDirectives() ||
+        Tok.isOneOf(tok::semi, tok::r_brace) || Tok.isAnnotation() ||
+        Tok.getLocation().isMacroID() || PrevLoc.isMacroID() ||
+        StmtLoc.isMacroID())
+      return;
+    SourceManager &SM = P.getPreprocessor().getSourceManager();
+    unsigned PrevColNum = SM.getSpellingColumnNumber(PrevLoc);
+    unsigned CurColNum = SM.getSpellingColumnNumber(Tok.getLocation());
+    unsigned StmtColNum = SM.getSpellingColumnNumber(StmtLoc);
+
+    if (PrevColNum != 0 && CurColNum != 0 && StmtColNum != 0 &&
+        ((PrevColNum > StmtColNum && PrevColNum == CurColNum) ||
+         !Tok.isAtStartOfLine())) {
+      if (SM.getPresumedLineNumber(StmtLoc) ==
+          SM.getPresumedLineNumber(Tok.getLocation()))
+        return;
+      P.Diag(Tok.getLocation(), diag::warn_misleading_indentation)
+          << Kind;
+      P.Diag(StmtLoc, diag::note_previous_statement);
+    }
+  }
+  ~MisleadingIndentationChecker() {
+    assert(!NeedsChecking && "Check Has not been called");
+  }
+};
+
+}
 
 /// ParseIfStatement
 ///       if-statement: [C99 6.8.4.1]
@@ -1199,7 +1249,8 @@
 /// [C++]   'if' '(' condition ')' statement
 /// [C++]   'if' '(' condition ')' statement 'else' statement
 ///
-StmtResult Parser::ParseIfStatement(SourceLocation *TrailingElseLoc) {
+StmtResult Parser::ParseIfStatement(SourceLocation *TrailingElseLoc,
+                                    SourceLocation ElseLocOnElseIf) {
   assert(Tok.is(tok::kw_if) && "Not an if stmt!");
   SourceLocation IfLoc = ConsumeToken();  // eat the 'if'.
 
@@ -1265,6 +1316,10 @@
   //
   ParseScope InnerScope(this, Scope::DeclScope, C99orCXX, Tok.is(tok::l_brace));
 
+  MisleadingIndentationChecker MIChecker(
+      *this, ElseLocOnElseIf.isInvalid() ? MSK_if : MSK_else_if,
+      ElseLocOnElseIf.isInvalid() ? IfLoc : ElseLocOnElseIf);
+
   // Read the 'then' stmt.
   SourceLocation ThenStmtLoc = Tok.getLocation();
 
@@ -1278,6 +1333,8 @@
     ThenStmt = ParseStatement(&InnerStatementTrailingElseLoc);
   }
 
+  MIChecker.Check(Tok.isNot(tok::kw_else));
+
   // Pop the 'if' scope if needed.
   InnerScope.Exit();
 
@@ -1305,11 +1362,18 @@
     ParseScope InnerScope(this, Scope::DeclScope, C99orCXX,
                           Tok.is(tok::l_brace));
 
+    MisleadingIndentationChecker MIChecker(*this, MSK_else, ElseLoc);
+
     EnterExpressionEvaluationContext PotentiallyDiscarded(
         Actions, Sema::ExpressionEvaluationContext::DiscardedStatement, nullptr,
         Sema::ExpressionEvaluationContextRecord::EK_Other,
         /*ShouldEnter=*/ConstexprCondition && *ConstexprCondition);
-    ElseStmt = ParseStatement();
+    if (Tok.is(tok::kw_if))
+      ElseStmt = ParseIfStatement(nullptr, ElseLoc);
+    else
+      ElseStmt = ParseStatement();
+
+    MIChecker.Check(ElseStmt.isUsable());
 
     // Pop the 'else' scope if needed.
     InnerScope.Exit();
@@ -1484,9 +1548,12 @@
   //
   ParseScope InnerScope(this, Scope::DeclScope, C99orCXX, Tok.is(tok::l_brace));
 
+  MisleadingIndentationChecker MIChecker(*this, MSK_while, WhileLoc);
+
   // Read the body statement.
   StmtResult Body(ParseStatement(TrailingElseLoc));
 
+  MIChecker.Check(Body.isUsable());
   // Pop the body scope if needed.
   InnerScope.Exit();
   WhileScope.Exit();
@@ -1918,9 +1985,13 @@
   if (C99orCXXorObjC)
     getCurScope()->decrementMSManglingNumber();
 
+  MisleadingIndentationChecker MIChecker(*this, MSK_for, ForLoc);
+
   // Read the body statement.
   StmtResult Body(ParseStatement(TrailingElseLoc));
 
+  MIChecker.Check(Body.isUsable());
+
   // Pop the body scope if needed.
   InnerScope.Exit();
 
Index: clang/test/Index/pragma-diag-reparse.c
===================================================================
--- clang/test/Index/pragma-diag-reparse.c
+++ clang/test/Index/pragma-diag-reparse.c
@@ -11,6 +11,7 @@
   return x;
 }
 
+#pragma clang diagnostic ignored "-Wmisleading-indentation"
 void foo() { int b=0; while (b==b); }
 
 // RUN: env CINDEXTEST_EDITING=1 CINDEXTEST_FAILONERROR=1 c-index-test -test-load-source-reparse 5 local \
Index: clang/test/Misc/warning-wall.c
===================================================================
--- clang/test/Misc/warning-wall.c
+++ clang/test/Misc/warning-wall.c
@@ -90,6 +90,7 @@
 CHECK-NEXT:    -Wdangling-else
 CHECK-NEXT:  -Wswitch
 CHECK-NEXT:  -Wswitch-bool
+CHECK-NEXT:  -Wmisleading-indentation
 
 
 CHECK-NOT:-W
Index: clang/test/Parser/warn-misleading-indentation.cpp
===================================================================
--- /dev/null
+++ clang/test/Parser/warn-misleading-indentation.cpp
@@ -0,0 +1,208 @@
+// RUN: %clang_cc1 -x c -fsyntax-only -verify %s
+// RUN: %clang_cc1 -x c -fsyntax-only -verify -Wmisleading-indentation -DWITH_WARN %s
+// RUN: %clang_cc1 -std=c++17 -fsyntax-only -verify -Wall -Wno-unused -DWITH_WARN -DCXX17 %s
+// RUN: %clang_cc1 -std=c++17 -fsyntax-only -verify -Wall -Wno-unused -Wno-misleading-indentation -DCXX17 %s
+
+#ifndef WITH_WARN
+// expected-no-diagnostics
+#endif
+
+void f0(int i) {
+  if (i)
+#ifdef WITH_WARN
+// expected-note@-2 {{here}}
+#endif
+    i = i + 1;
+    int x = 0;
+#ifdef WITH_WARN
+// expected-warning@-2 {{misleading indentation; statement is not part of the previous 'if'}}
+#endif
+  return;
+#ifdef CXX17
+  if constexpr (false)
+#ifdef WITH_WARN
+// expected-note@-2 {{here}}
+#endif
+    i = 0;
+    i += 1;
+#ifdef WITH_WARN
+// expected-warning@-2 {{misleading indentation; statement is not part of the previous 'if'}}
+#endif
+#endif
+}
+
+void f1(int i) {
+  for (;i;)
+#ifdef WITH_WARN
+// expected-note@-2 {{here}}
+#endif
+    i = i + 1;
+    i *= 2;
+#ifdef WITH_WARN
+// expected-warning@-2 {{misleading indentation; statement is not part of the previous 'for'}}
+#endif
+  return;
+}
+
+void f2(int i) {
+  while (i)
+#ifdef WITH_WARN
+// expected-note@-2 {{here}}
+#endif
+    i = i + 1; i *= 2;
+#ifdef WITH_WARN
+// expected-warning@-2 {{misleading indentation; statement is not part of the previous 'while'}}
+#endif
+  return;
+}
+
+void f3(int i) {
+  if (i)
+    i = i + 1;
+  else
+#ifdef WITH_WARN
+// expected-note@-2 {{here}}
+#endif
+    i *= 2;
+    const int x = 0;
+#ifdef WITH_WARN
+// expected-warning@-2 {{misleading indentation; statement is not part of the previous 'else'}}
+#endif
+}
+
+#ifdef CXX17
+struct Range {
+  int *begin() {return nullptr;}
+  int *end() {return nullptr;}
+};
+#endif
+
+void f4(int i) {
+  if (i)
+  i *= 2;
+  return;
+  if (i)
+    i *= 2;
+    ;
+  if (i)
+#ifdef WITH_WARN
+// expected-note@-2 {{here}}
+#endif
+    i *= 2;
+    typedef int Int;
+#ifdef WITH_WARN
+// expected-warning@-2 {{misleading indentation; statement is not part of the previous 'if'}}
+#endif
+#ifdef CXX17
+  Range R;
+  for (auto e : R)
+#ifdef WITH_WARN
+// expected-note@-2 {{here}}
+#endif
+    i *= 2;
+    using Int2 = int;
+#ifdef WITH_WARN
+// expected-warning@-2 {{misleading indentation; statement is not part of the previous 'for'}}
+#endif
+#endif
+}
+
+int bar(void);
+
+int foo(int* dst)
+{   
+    if (dst)
+       return
+    bar();
+  if (dst)
+    dst = dst + \
+    bar();
+  return 0;
+}
+
+void g(int i) {
+  if (1)
+    i = 2;
+  else
+         if (i == 3)
+#ifdef WITH_WARN
+// expected-note@-2 {{here}}
+#endif
+    i = 4;
+    i = 5;
+#ifdef WITH_WARN
+// expected-warning@-2 {{misleading indentation; statement is not part of the previous 'else if'}}
+#endif
+}
+
+// Or this
+#define TEST i = 5
+void g0(int i) {
+  if (1)
+    i = 2;
+  else
+    i = 5;
+    TEST;
+}
+
+void g1(int i) {
+  if (1)
+    i = 2;
+  else if (i == 3)
+#ifdef WITH_WARN
+// expected-note@-2 {{here}}
+#endif
+      i = 4;
+      i = 5;
+#ifdef WITH_WARN
+// expected-warning@-2 {{misleading indentation; statement is not part of the previous 'else if'}}
+#endif
+}
+
+void g2(int i) {
+  if (1)
+    i = 2;
+  else
+    if (i == 3)
+    {i = 4;}
+    i = 5;
+}
+
+void g6(int i) {
+        if (1)
+                if (i == 3)
+#ifdef WITH_WARN
+// expected-note@-2 {{here}}
+#endif
+                        i = 4;
+                        i = 5;
+#ifdef WITH_WARN
+// expected-warning@-2 {{misleading indentation; statement is not part of the previous 'if'}}
+#endif
+}
+
+void g7(int i) {
+  if (1)
+    i = 4;
+#ifdef TEST1
+#endif
+    i = 5;
+}
+
+void a1(int i) { if (1) i = 4; return; }
+
+void a2(int i) {
+  {
+    if (1)
+      i = 4;
+      }
+  return;
+}
+
+void a3(int i) {
+  if (1)
+    {
+    i = 4;
+    }
+    return;
+}
Index: clang/test/Preprocessor/pragma_diagnostic_sections.cpp
===================================================================
--- clang/test/Preprocessor/pragma_diagnostic_sections.cpp
+++ clang/test/Preprocessor/pragma_diagnostic_sections.cpp
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only -Wall -Wunused-macros -Wunused-parameter -Wno-uninitialized -verify %s
+// RUN: %clang_cc1 -fsyntax-only -Wall -Wunused-macros -Wunused-parameter -Wno-uninitialized -Wno-misleading-indentation -verify %s
 
 // rdar://8365684
 struct S {