Index: include/clang/Sema/Sema.h
===================================================================
--- include/clang/Sema/Sema.h
+++ include/clang/Sema/Sema.h
@@ -2113,12 +2113,12 @@
   void checkPartialSpecializationVisibility(SourceLocation Loc,
                                             NamedDecl *Spec);
 
-  /// Retrieve a suitable printing policy.
+  /// Retrieve a suitable printing policy for diagnostics.
   PrintingPolicy getPrintingPolicy() const {
     return getPrintingPolicy(Context, PP);
   }
 
-  /// Retrieve a suitable printing policy.
+  /// Retrieve a suitable printing policy for diagnostics.
   static PrintingPolicy getPrintingPolicy(const ASTContext &Ctx,
                                           const Preprocessor &PP);
 
Index: lib/Frontend/ASTConsumers.cpp
===================================================================
--- lib/Frontend/ASTConsumers.cpp
+++ lib/Frontend/ASTConsumers.cpp
@@ -87,9 +87,10 @@
                 << DC->getPrimaryContext() << "\n";
         } else
           Out << "Not a DeclContext\n";
-      } else if (OutputKind == Print)
-        D->print(Out, /*Indentation=*/0, /*PrintInstantiation=*/true);
-      else if (OutputKind != None)
+      } else if (OutputKind == Print) {
+        PrintingPolicy Policy(D->getASTContext().getLangOpts());
+        D->print(Out, Policy, /*Indentation=*/0, /*PrintInstantiation=*/true);
+      } else if (OutputKind != None)
         D->dump(Out, OutputKind == DumpFull);
     }
 
Index: lib/Sema/Sema.cpp
===================================================================
--- lib/Sema/Sema.cpp
+++ lib/Sema/Sema.cpp
@@ -52,8 +52,8 @@
 PrintingPolicy Sema::getPrintingPolicy(const ASTContext &Context,
                                        const Preprocessor &PP) {
   PrintingPolicy Policy = Context.getPrintingPolicy();
-  // Our printing policy is copied over the ASTContext printing policy whenever
-  // a diagnostic is emitted, so recompute it.
+  // In diagnostics, we print _Bool as bool if the latter is defined as the
+  // former.
   Policy.Bool = Context.getLangOpts().Bool;
   if (!Policy.Bool) {
     if (const MacroInfo *BoolMacro = PP.getMacroInfo(Context.getBoolName())) {
@@ -1287,7 +1287,8 @@
     }
   }
 
-  // Set up the context's printing policy based on our current state.
+  // Copy the diagnostic printing policy over the ASTContext printing policy.
+  // TODO: Stop doing that.  See: https://reviews.llvm.org/D45093#1090292
   Context.setPrintingPolicy(getPrintingPolicy());
 
   // Emit the diagnostic.
Index: test/Misc/ast-print-bool.c
===================================================================
--- test/Misc/ast-print-bool.c
+++ test/Misc/ast-print-bool.c
@@ -0,0 +1,44 @@
+// RUN: %clang_cc1 -verify -ast-print %s -xc -DDEF_BOOL_CBOOL \
+// RUN: | FileCheck %s --check-prefixes=BOOL-AS-CBOOL,CBOOL
+//
+// RUN: %clang_cc1 -verify -ast-print %s -xc -DDEF_BOOL_CBOOL -DDIAG \
+// RUN: | FileCheck %s --check-prefixes=BOOL-AS-CBOOL,CBOOL
+//
+// RUN: %clang_cc1 -verify -ast-print %s -xc -DDEF_BOOL_INT \
+// RUN: | FileCheck %s --check-prefixes=BOOL-AS-INT,CBOOL
+//
+// RUN: %clang_cc1 -verify -ast-print %s -xc -DDEF_BOOL_INT -DDIAG \
+// RUN: | FileCheck %s --check-prefixes=BOOL-AS-INT,CBOOL
+//
+// RUN: %clang_cc1 -verify -ast-print %s -xc++ \
+// RUN: | FileCheck %s --check-prefixes=BOOL-AS-BOOL
+//
+// RUN: %clang_cc1 -verify -ast-print %s -xc++ -DDIAG \
+// RUN: | FileCheck %s --check-prefixes=BOOL-AS-BOOL
+
+#if DEF_BOOL_CBOOL
+# define bool _Bool
+#elif DEF_BOOL_INT
+# define bool int
+#endif
+
+// BOOL-AS-CBOOL: _Bool i;
+// BOOL-AS-INT:   int i;
+// BOOL-AS-BOOL:  bool i;
+bool i;
+
+#ifndef __cplusplus
+// CBOOL: _Bool j;
+_Bool j;
+#endif
+
+// Induce a diagnostic (and verify we actually managed to do so), which used to
+// permanently alter the -ast-print printing policy for _Bool.  How bool is
+// defined by the preprocessor is examined only once per compilation, when the
+// diagnostic is emitted, and it used to affect the entirety of -ast-print, so
+// test only one definition of bool per compilation.
+#if DIAG
+void fn() { 1; } // expected-warning {{expression result unused}}
+#else
+// expected-no-diagnostics
+#endif