Index: clang/docs/ReleaseNotes.rst
===================================================================
--- clang/docs/ReleaseNotes.rst
+++ clang/docs/ReleaseNotes.rst
@@ -173,6 +173,9 @@
 
 - ``-Wmisexpect`` warns when the branch weights collected during profiling
   conflict with those added by ``llvm.expect``.
+- Clang now checks for stack resource exhaustion when recursively parsing
+  declarators in order to give a diagnostic before we run out of stack space.
+  This fixes `Issue 51642 <https://github.com/llvm/llvm-project/issues/51642>`_.
 
 Non-comprehensive list of changes in this release
 -------------------------------------------------
Index: clang/include/clang/Parse/Parser.h
===================================================================
--- clang/include/clang/Parse/Parser.h
+++ clang/include/clang/Parse/Parser.h
@@ -791,6 +791,16 @@
     return PP.LookAhead(N-1);
   }
 
+  /// Warn that the stack is nearly exhausted.
+  void warnStackExhausted(SourceLocation Loc);
+
+  /// Run some code with "sufficient" stack space. (Currently, at least 256K is
+  /// guaranteed). Produces a warning if we're low on stack space and allocates
+  /// more in that case. Use this in code that may recurse deeply (for example,
+  /// in template instantiation) to avoid stack overflow.
+  void runWithSufficientStackSpace(SourceLocation Loc,
+                                   llvm::function_ref<void()> Fn);
+
 public:
   /// NextToken - This peeks ahead one token and returns it without
   /// consuming it.
Index: clang/lib/Parse/ParseDecl.cpp
===================================================================
--- clang/lib/Parse/ParseDecl.cpp
+++ clang/lib/Parse/ParseDecl.cpp
@@ -5751,11 +5751,12 @@
 }
 
 /// ParseDeclarator - Parse and verify a newly-initialized declarator.
-///
 void Parser::ParseDeclarator(Declarator &D) {
   /// This implements the 'declarator' production in the C grammar, then checks
   /// for well-formedness and issues diagnostics.
-  ParseDeclaratorInternal(D, &Parser::ParseDirectDeclarator);
+  runWithSufficientStackSpace(D.getBeginLoc(), [&] {
+    ParseDeclaratorInternal(D, &Parser::ParseDirectDeclarator);
+  });
 }
 
 static bool isPtrOperatorToken(tok::TokenKind Kind, const LangOptions &Lang,
@@ -5866,7 +5867,9 @@
       D.ExtendWithDeclSpec(DS);
 
       // Recurse to parse whatever is left.
-      ParseDeclaratorInternal(D, DirectDeclParser);
+      runWithSufficientStackSpace(D.getBeginLoc(), [&] {
+        ParseDeclaratorInternal(D, DirectDeclParser);
+      });
 
       // Sema will have to catch (syntactically invalid) pointers into global
       // scope. It has to catch pointers into namespace scope anyway.
@@ -5915,7 +5918,8 @@
     D.ExtendWithDeclSpec(DS);
 
     // Recursively parse the declarator.
-    ParseDeclaratorInternal(D, DirectDeclParser);
+    runWithSufficientStackSpace(
+        D.getBeginLoc(), [&] { ParseDeclaratorInternal(D, DirectDeclParser); });
     if (Kind == tok::star)
       // Remember that we parsed a pointer type, and remember the type-quals.
       D.AddTypeInfo(DeclaratorChunk::getPointer(
@@ -5960,7 +5964,8 @@
     }
 
     // Recursively parse the declarator.
-    ParseDeclaratorInternal(D, DirectDeclParser);
+    runWithSufficientStackSpace(
+        D.getBeginLoc(), [&] { ParseDeclaratorInternal(D, DirectDeclParser); });
 
     if (D.getNumTypeObjects() > 0) {
       // C++ [dcl.ref]p4: There shall be no references to references.
Index: clang/lib/Parse/Parser.cpp
===================================================================
--- clang/lib/Parse/Parser.cpp
+++ clang/lib/Parse/Parser.cpp
@@ -15,6 +15,7 @@
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/DeclTemplate.h"
 #include "clang/Basic/FileManager.h"
+#include "clang/Basic/Stack.h"
 #include "clang/Parse/ParseDiagnostic.h"
 #include "clang/Parse/RAIIObjectsForParser.h"
 #include "clang/Sema/DeclSpec.h"
@@ -2610,6 +2611,19 @@
   return false;
 }
 
+void Parser::warnStackExhausted(SourceLocation Loc) {
+  // Only warn about this once.
+  if (!Actions.WarnedStackExhausted) {
+    Diag(Loc, diag::warn_stack_exhausted);
+    Actions.WarnedStackExhausted = true;
+  }
+}
+
+void Parser::runWithSufficientStackSpace(SourceLocation Loc,
+                                         llvm::function_ref<void()> Fn) {
+  clang::runWithSufficientStackSpace([&] { warnStackExhausted(Loc); }, Fn);
+}
+
 bool BalancedDelimiterTracker::diagnoseOverflow() {
   P.Diag(P.Tok, diag::err_bracket_depth_exceeded)
     << P.getLangOpts().BracketDepth;
Index: clang/test/Parser/declarators-recursion-crash.c
===================================================================
--- /dev/null
+++ clang/test/Parser/declarators-recursion-crash.c
@@ -0,0 +1,10 @@
+// RUN: not %clang_cc1 %s -fsyntax-only -verify
+
+#define PTR1 * * * * * * * * * *
+#define PTR2 PTR1 PTR1 PTR1 PTR1 PTR1 PTR1 PTR1 PTR1 PTR1 PTR1
+#define PTR3 PTR2 PTR2 PTR2 PTR2 PTR2 PTR2 PTR2 PTR2 PTR2 PTR2
+#define PTR4 PTR3 PTR3 PTR3 PTR3 PTR3 PTR3 PTR3 PTR3 PTR3 PTR3
+#define PTR5 PTR4 PTR4 PTR4 PTR4 PTR4 PTR4 PTR4 PTR4 PTR4 PTR4
+#define PTR6 PTR5 PTR5 PTR5 PTR5 PTR5 PTR5 PTR5 PTR5 PTR5 PTR5
+
+int PTR6 q3_var = 0; // expected-warning {{stack nearly exhausted; compilation time may suffer, and crashes due to stack overflow are likely}}