diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -163,6 +163,9 @@
 - Unscoped and scoped enumeration types can no longer be initialized from a
   brace-init-list containing a single element of a different scoped enumeration
   type.
+- Allow use of an elaborated type specifier as a ``_Generic`` selection
+  association in C++ mode. This fixes
+  `Issue 55562 <https://github.com/llvm/llvm-project/issues/55562>`_.
 
 Improvements to Clang's diagnostics
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/include/clang/Parse/Parser.h b/clang/include/clang/Parse/Parser.h
--- a/clang/include/clang/Parse/Parser.h
+++ b/clang/include/clang/Parse/Parser.h
@@ -2195,7 +2195,8 @@
     DSC_template_param, // template parameter context
     DSC_template_type_arg, // template type argument context
     DSC_objc_method_result, // ObjC method result context, enables 'instancetype'
-    DSC_condition // condition declaration context
+    DSC_condition, // condition declaration context
+    DSC_association // A _Generic selection expression's type association
   };
 
   /// Is this a context in which we are parsing just a type-specifier (or
@@ -2214,6 +2215,7 @@
     case DeclSpecContext::DSC_type_specifier:
     case DeclSpecContext::DSC_trailing:
     case DeclSpecContext::DSC_alias_declaration:
+    case DeclSpecContext::DSC_association:
       return true;
     }
     llvm_unreachable("Missing DeclSpecContext case");
@@ -2238,7 +2240,7 @@
   /// so permit class and enum definitions in addition to non-defining class and
   /// enum elaborated-type-specifiers)?
   static AllowDefiningTypeSpec
-  isDefiningTypeSpecifierContext(DeclSpecContext DSC) {
+  isDefiningTypeSpecifierContext(DeclSpecContext DSC, bool IsCPlusPlus) {
     switch (DSC) {
     case DeclSpecContext::DSC_normal:
     case DeclSpecContext::DSC_class:
@@ -2255,6 +2257,10 @@
     case DeclSpecContext::DSC_type_specifier:
       return AllowDefiningTypeSpec::NoButErrorRecovery;
 
+    case DeclSpecContext::DSC_association:
+      return IsCPlusPlus ? AllowDefiningTypeSpec::NoButErrorRecovery
+                         : AllowDefiningTypeSpec::Yes;
+
     case DeclSpecContext::DSC_trailing:
       return AllowDefiningTypeSpec::No;
     }
@@ -2276,6 +2282,7 @@
     case DeclSpecContext::DSC_template_type_arg:
     case DeclSpecContext::DSC_type_specifier:
     case DeclSpecContext::DSC_trailing:
+    case DeclSpecContext::DSC_association:
       return false;
     }
     llvm_unreachable("Missing DeclSpecContext case");
@@ -2291,6 +2298,7 @@
     case DeclSpecContext::DSC_top_level:
     case DeclSpecContext::DSC_condition:
     case DeclSpecContext::DSC_type_specifier:
+    case DeclSpecContext::DSC_association:
       return true;
 
     case DeclSpecContext::DSC_objc_method_result:
diff --git a/clang/include/clang/Sema/DeclSpec.h b/clang/include/clang/Sema/DeclSpec.h
--- a/clang/include/clang/Sema/DeclSpec.h
+++ b/clang/include/clang/Sema/DeclSpec.h
@@ -1786,7 +1786,8 @@
   TemplateTypeArg,     // Template type argument (in default argument).
   AliasDecl,           // C++11 alias-declaration.
   AliasTemplate,       // C++11 alias-declaration template.
-  RequiresExpr         // C++2a requires-expression.
+  RequiresExpr,        // C++2a requires-expression.
+  Association          // C11 _Generic selection expression association.
 };
 
 /// Information about one declarator, including the parsed type
@@ -2024,6 +2025,7 @@
     case DeclaratorContext::TrailingReturn:
     case DeclaratorContext::TrailingReturnVar:
     case DeclaratorContext::RequiresExpr:
+    case DeclaratorContext::Association:
       return true;
     }
     llvm_unreachable("unknown context kind!");
@@ -2063,6 +2065,7 @@
     case DeclaratorContext::TemplateTypeArg:
     case DeclaratorContext::TrailingReturn:
     case DeclaratorContext::TrailingReturnVar:
+    case DeclaratorContext::Association:
       return false;
     }
     llvm_unreachable("unknown context kind!");
@@ -2106,6 +2109,7 @@
     case DeclaratorContext::TemplateTypeArg:
     case DeclaratorContext::TrailingReturn:
     case DeclaratorContext::TrailingReturnVar:
+    case DeclaratorContext::Association:
       return false;
     }
     llvm_unreachable("unknown context kind!");
@@ -2162,6 +2166,7 @@
     case DeclaratorContext::TemplateTypeArg:
     case DeclaratorContext::TrailingReturn:
     case DeclaratorContext::RequiresExpr:
+    case DeclaratorContext::Association:
       return false;
     }
     llvm_unreachable("unknown context kind!");
@@ -2384,6 +2389,7 @@
     case DeclaratorContext::TrailingReturn:
     case DeclaratorContext::TrailingReturnVar:
     case DeclaratorContext::RequiresExpr:
+    case DeclaratorContext::Association:
       return false;
     }
     llvm_unreachable("unknown context kind!");
@@ -2418,6 +2424,7 @@
     case DeclaratorContext::TrailingReturnVar:
     case DeclaratorContext::TemplateTypeArg:
     case DeclaratorContext::RequiresExpr:
+    case DeclaratorContext::Association:
       return false;
 
     case DeclaratorContext::Block:
diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -2870,6 +2870,8 @@
   if (Context == DeclaratorContext::AliasDecl ||
       Context == DeclaratorContext::AliasTemplate)
     return DeclSpecContext::DSC_alias_declaration;
+  if (Context == DeclaratorContext::Association)
+    return DeclSpecContext::DSC_association;
   return DeclSpecContext::DSC_normal;
 }
 
@@ -4573,7 +4575,7 @@
 
   // Determine whether this declaration is permitted to have an enum-base.
   AllowDefiningTypeSpec AllowEnumSpecifier =
-      isDefiningTypeSpecifierContext(DSC);
+      isDefiningTypeSpecifierContext(DSC, getLangOpts().CPlusPlus);
   bool CanBeOpaqueEnumDeclaration =
       DS.isEmpty() && isOpaqueEnumDeclarationContext(DSC);
   bool CanHaveEnumBase = (getLangOpts().CPlusPlus11 || getLangOpts().ObjC ||
diff --git a/clang/lib/Parse/ParseDeclCXX.cpp b/clang/lib/Parse/ParseDeclCXX.cpp
--- a/clang/lib/Parse/ParseDeclCXX.cpp
+++ b/clang/lib/Parse/ParseDeclCXX.cpp
@@ -1752,13 +1752,15 @@
 
   const PrintingPolicy &Policy = Actions.getASTContext().getPrintingPolicy();
   Sema::TagUseKind TUK;
-  if (isDefiningTypeSpecifierContext(DSC) == AllowDefiningTypeSpec::No ||
+  if (isDefiningTypeSpecifierContext(DSC, getLangOpts().CPlusPlus) ==
+          AllowDefiningTypeSpec::No ||
       (getLangOpts().OpenMP && OpenMPDirectiveParsing))
     TUK = Sema::TUK_Reference;
   else if (Tok.is(tok::l_brace) ||
-           (getLangOpts().CPlusPlus && Tok.is(tok::colon)) ||
-           (isClassCompatibleKeyword() &&
-            (NextToken().is(tok::l_brace) || NextToken().is(tok::colon)))) {
+           (DSC != DeclSpecContext::DSC_association &&
+                (getLangOpts().CPlusPlus && Tok.is(tok::colon)) ||
+            (isClassCompatibleKeyword() &&
+             (NextToken().is(tok::l_brace) || NextToken().is(tok::colon))))) {
     if (DS.isFriendSpecified()) {
       // C++ [class.friend]p2:
       //   A class shall not be defined in a friend declaration.
diff --git a/clang/lib/Parse/ParseExpr.cpp b/clang/lib/Parse/ParseExpr.cpp
--- a/clang/lib/Parse/ParseExpr.cpp
+++ b/clang/lib/Parse/ParseExpr.cpp
@@ -3276,7 +3276,7 @@
       Ty = nullptr;
     } else {
       ColonProtectionRAIIObject X(*this);
-      TypeResult TR = ParseTypeName();
+      TypeResult TR = ParseTypeName(nullptr, DeclaratorContext::Association);
       if (TR.isInvalid()) {
         SkipUntil(tok::r_paren, StopAtSemi);
         return ExprError();
diff --git a/clang/lib/Sema/SemaType.cpp b/clang/lib/Sema/SemaType.cpp
--- a/clang/lib/Sema/SemaType.cpp
+++ b/clang/lib/Sema/SemaType.cpp
@@ -3538,6 +3538,7 @@
         break; // auto(x)
       LLVM_FALLTHROUGH;
     case DeclaratorContext::TypeName:
+    case DeclaratorContext::Association:
       Error = 15; // Generic
       break;
     case DeclaratorContext::File:
@@ -3648,6 +3649,7 @@
     case DeclaratorContext::ObjCCatch:
     case DeclaratorContext::TemplateArg:
     case DeclaratorContext::TemplateTypeArg:
+    case DeclaratorContext::Association:
       DiagID = diag::err_type_defined_in_type_specifier;
       break;
     case DeclaratorContext::Prototype:
@@ -4735,6 +4737,7 @@
     case DeclaratorContext::TypeName:
     case DeclaratorContext::FunctionalCast:
     case DeclaratorContext::RequiresExpr:
+    case DeclaratorContext::Association:
       // Don't infer in these contexts.
       break;
     }
@@ -5777,6 +5780,7 @@
     case DeclaratorContext::TrailingReturnVar:
     case DeclaratorContext::TemplateArg:
     case DeclaratorContext::TemplateTypeArg:
+    case DeclaratorContext::Association:
       // FIXME: We may want to allow parameter packs in block-literal contexts
       // in the future.
       S.Diag(D.getEllipsisLoc(),
diff --git a/clang/test/Sema/generic-selection.c b/clang/test/Sema/generic-selection.c
--- a/clang/test/Sema/generic-selection.c
+++ b/clang/test/Sema/generic-selection.c
@@ -78,3 +78,11 @@
       default : 3
     ) == 1, "we had better pick struct Test, not const struct Test!"); // C-specific result
 }
+
+void GH55562(void) {
+  // Ensure that you can still define a type within a generic selection
+  // association (despite it not being particularly useful).
+  (void)_Generic(1, struct S { int a; } : 0, default : 0); // ext-warning {{'_Generic' is a C11 extension}}
+  struct S s = { 0 };
+  int i = s.a;
+}
diff --git a/clang/test/SemaCXX/generic-selection.cpp b/clang/test/SemaCXX/generic-selection.cpp
--- a/clang/test/SemaCXX/generic-selection.cpp
+++ b/clang/test/SemaCXX/generic-selection.cpp
@@ -69,3 +69,24 @@
       default : 3
     ) == 2, "we had better pick const Test, not Test!"); // C++-specific result
 }
+
+namespace GH55562 {
+struct S { // expected-note {{declared here}}
+  int i;
+};
+
+void func(struct S s) {
+  // We would previously reject this because the parser thought 'struct S :'
+  // was the start of a definition (with a base class specifier); it's not, it
+  // is an elaborated type specifier followed by the association's value and
+  // it should work the same as in C.
+  (void)_Generic(s, struct S : 1);
+
+  // The rest of these cases test that we still produce a reasonable diagnostic
+  // when referencing an unknown type or trying to define a type in other ways.
+  (void)_Generic(s, struct T : 1);            // expected-error {{type 'struct T' in generic association incomplete}}
+  (void)_Generic(s, struct U { int a; } : 1); // expected-error {{'U' cannot be defined in a type specifier}}
+  (void)_Generic(s, struct V : S);            // expected-error {{'S' does not refer to a value}}
+  (void)_Generic(s, struct W : S { int b; } : 1); // expected-error {{expected '(' for function-style cast or type construction}}
+}
+} // namespace GH55562