diff --git a/clang/docs/LanguageExtensions.rst b/clang/docs/LanguageExtensions.rst
--- a/clang/docs/LanguageExtensions.rst
+++ b/clang/docs/LanguageExtensions.rst
@@ -4661,6 +4661,13 @@
     a = b[i] * c[i] + e;
   }
 
+Note: ``math.h`` defines the typedefs ``float_t`` and ``double_t`` based on the active
+evaluation method at the point where the header is included, not where the
+typedefs are used.  Because of this, it is unwise to combine these typedefs with
+``#pragma clang fp eval_method``.  To catch obvious bugs, Clang will emit an
+error for any references to these typedefs within the scope of this pragma;
+however, this is not a fool-proof protection, and programmers must take care.
+
 The ``#pragma float_control`` pragma allows precise floating-point
 semantics and floating-point exception behavior to be specified
 for a section of the source code. This pragma can only appear at file or
diff --git a/clang/include/clang/Basic/Attr.td b/clang/include/clang/Basic/Attr.td
--- a/clang/include/clang/Basic/Attr.td
+++ b/clang/include/clang/Basic/Attr.td
@@ -4193,3 +4193,10 @@
   let Subjects = SubjectList<[Record]>;
   let Documentation = [ReadOnlyPlacementDocs];
 }
+
+def AvailableOnlyInDefaultEvalMethod : InheritableAttr {
+  let Spellings = [Clang<"available_only_in_default_eval_method">];
+  let Subjects = SubjectList<[TypedefName], ErrorDiag>;
+  let Documentation = [Undocumented];
+}
+
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
@@ -11558,6 +11558,10 @@
   "too %select{many|few}0 type arguments for class %1 (have %2, expected %3)">;
 }
 
+def err_type_available_only_in_default_eval_method : Error<
+  "cannot use type '%0' within '#pragma clang fp eval_method'; type is set "
+  "according to the default eval method for the translation unit">;
+
 def err_objc_type_arg_not_id_compatible : Error<
   "type argument %0 is neither an Objective-C object nor a block type">;
 
diff --git a/clang/include/clang/Basic/TokenKinds.def b/clang/include/clang/Basic/TokenKinds.def
--- a/clang/include/clang/Basic/TokenKinds.def
+++ b/clang/include/clang/Basic/TokenKinds.def
@@ -798,13 +798,15 @@
 OBJC_AT_KEYWORD(available)
 
 //===----------------------------------------------------------------------===//
-// Interesting idenitifiers.
+// Interesting identifiers.
 //===----------------------------------------------------------------------===//
 INTERESTING_IDENTIFIER(not_interesting)
 INTERESTING_IDENTIFIER(FILE)
 INTERESTING_IDENTIFIER(jmp_buf)
 INTERESTING_IDENTIFIER(sigjmp_buf)
 INTERESTING_IDENTIFIER(ucontext_t)
+INTERESTING_IDENTIFIER(float_t)
+INTERESTING_IDENTIFIER(double_t)
 
 // TODO: What to do about context-sensitive keywords like:
 //       bycopy/byref/in/inout/oneway/out?
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
@@ -6777,6 +6777,10 @@
       case tok::InterestingIdentifierKind::ucontext_t:
         Context.setucontext_tDecl(NewTD);
         break;
+      case tok::InterestingIdentifierKind::float_t:
+      case tok::InterestingIdentifierKind::double_t:
+        NewTD->addAttr(AvailableOnlyInDefaultEvalMethodAttr::Create(Context));
+        break;
       default:
         break;
       }
diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -8369,6 +8369,12 @@
   D->addAttr(FunctionReturnThunksAttr::Create(S.Context, Kind, AL));
 }
 
+static void handleAvailableOnlyInDefaultEvalMethod(Sema &S, Decl *D,
+                                                   const ParsedAttr &AL) {
+  assert(isa<TypedefNameDecl>(D) && "This attribute only applies to a typedef");
+  handleSimpleAttribute<AvailableOnlyInDefaultEvalMethodAttr>(S, D, AL);
+}
+
 static void handleSYCLKernelAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   // The 'sycl_kernel' attribute applies only to function templates.
   const auto *FD = cast<FunctionDecl>(D);
@@ -9250,6 +9256,10 @@
     handleFunctionReturnThunksAttr(S, D, AL);
     break;
 
+  case ParsedAttr::AT_AvailableOnlyInDefaultEvalMethod:
+    handleAvailableOnlyInDefaultEvalMethod(S, D, AL);
+    break;
+
   // Microsoft attributes:
   case ParsedAttr::AT_LayoutVersion:
     handleLayoutVersion(S, D, AL);
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
@@ -374,6 +374,16 @@
 
   diagnoseUseOfInternalDeclInInlineFunction(*this, D, Loc);
 
+  if (D->hasAttr<AvailableOnlyInDefaultEvalMethodAttr>()) {
+    if (getLangOpts().getFPEvalMethod() !=
+            LangOptions::FPEvalMethodKind::FEM_UnsetOnCommandLine &&
+        PP.getLastFPEvalPragmaLocation().isValid() &&
+        PP.getCurrentFPEvalMethod() != getLangOpts().getFPEvalMethod())
+      Diag(D->getLocation(),
+           diag::err_type_available_only_in_default_eval_method)
+          << D->getName();
+  }
+
   if (auto *VD = dyn_cast<ValueDecl>(D))
     checkTypeSupport(VD->getType(), Loc, VD);
 
diff --git a/clang/test/Misc/pragma-attribute-supported-attributes-list.test b/clang/test/Misc/pragma-attribute-supported-attributes-list.test
--- a/clang/test/Misc/pragma-attribute-supported-attributes-list.test
+++ b/clang/test/Misc/pragma-attribute-supported-attributes-list.test
@@ -21,6 +21,7 @@
 // CHECK-NEXT: AssumeAligned (SubjectMatchRule_objc_method, SubjectMatchRule_function)
 // CHECK-NEXT: Assumption (SubjectMatchRule_function, SubjectMatchRule_objc_method)
 // CHECK-NEXT: Availability ((SubjectMatchRule_record, SubjectMatchRule_enum, SubjectMatchRule_enum_constant, SubjectMatchRule_field, SubjectMatchRule_function, SubjectMatchRule_namespace, SubjectMatchRule_objc_category, SubjectMatchRule_objc_implementation, SubjectMatchRule_objc_interface, SubjectMatchRule_objc_method, SubjectMatchRule_objc_property, SubjectMatchRule_objc_protocol, SubjectMatchRule_record, SubjectMatchRule_type_alias, SubjectMatchRule_variable))
+// CHECK-NEXT: AvailableOnlyInDefaultEvalMethod (SubjectMatchRule_type_alias)
 // CHECK-NEXT: BPFPreserveAccessIndex (SubjectMatchRule_record)
 // CHECK-NEXT: BTFDeclTag (SubjectMatchRule_variable, SubjectMatchRule_function, SubjectMatchRule_record, SubjectMatchRule_field, SubjectMatchRule_type_alias)
 // CHECK-NEXT: BuiltinAlias (SubjectMatchRule_function)
diff --git a/clang/test/Sema/attr-only-in-default-eval.cpp b/clang/test/Sema/attr-only-in-default-eval.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/Sema/attr-only-in-default-eval.cpp
@@ -0,0 +1,26 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+typedef float float_t [[clang::available_only_in_default_eval_method]];
+using double_t __attribute__((available_only_in_default_eval_method)) = double;
+
+// expected-error@+1{{'available_only_in_default_eval_method' attribute only applies to typedefs}}
+class  __attribute__((available_only_in_default_eval_method)) C1 {
+};
+// expected-error@+1{{'available_only_in_default_eval_method' attribute only applies to typedefs}}
+class  [[clang::available_only_in_default_eval_method]] C2 {
+};
+
+// expected-error@+1{{'available_only_in_default_eval_method' attribute only applies to typedefs}}
+struct [[clang::available_only_in_default_eval_method]] S1;
+// expected-error@+1{{'available_only_in_default_eval_method' attribute only applies to typedefs}}
+struct __attribute__((available_only_in_default_eval_method)) S2;
+
+// expected-error@+1{{'available_only_in_default_eval_method' attribute only applies to typedefs}}
+void __attribute__((available_only_in_default_eval_method)) foo();
+// expected-error@+1{{'available_only_in_default_eval_method' attribute cannot be applied to types}}
+void [[clang::available_only_in_default_eval_method]] goo();
+// expected-error@+1{{'available_only_in_default_eval_method' attribute cannot be applied to types}}
+void bar() [[clang::available_only_in_default_eval_method]];
+// expected-error@+1{{'available_only_in_default_eval_method' attribute only applies to typedefs}}
+void barz() __attribute__((available_only_in_default_eval_method));
+
diff --git a/clang/test/Sema/fp-eval-pragma-with-float-double_t-1.c b/clang/test/Sema/fp-eval-pragma-with-float-double_t-1.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Sema/fp-eval-pragma-with-float-double_t-1.c
@@ -0,0 +1,102 @@
+// RUN: %clang_cc1 -verify -fsyntax-only -DNOERROR %s
+// RUN: %clang_cc1 -verify -fsyntax-only -x c++ -DCPP -DNOERROR %s
+
+// RUN: %clang_cc1 -verify -fsyntax-only -ffp-eval-method=source -DNOERROR %s
+// RUN: %clang_cc1 -verify -fsyntax-only -x c++ -DCPP -ffp-eval-method=source \
+// RUN: -DNOERROR %s
+  
+// RUN: %clang_cc1 -verify -fsyntax-only -ffp-eval-method=double %s
+// RUN: %clang_cc1 -verify -fsyntax-only -x c++ -DCPP -ffp-eval-method=double %s
+
+// RUN: %clang_cc1 -verify -fsyntax-only -ffp-eval-method=extended %s
+// RUN: %clang_cc1 -verify -fsyntax-only -x c++ -DCPP \
+// RUN: -ffp-eval-method=extended %s  
+
+
+#ifdef NOERROR
+// expected-no-diagnostics
+typedef float float_t;
+typedef double double_t;
+#else
+#ifdef CPP
+typedef float float_t; //expected-error 9 {{cannot use type 'float_t' within '#pragma clang fp eval_method'; type is set according to the default eval method for the translation unit}}
+  
+typedef double double_t; //expected-error 9 {{cannot use type 'double_t' within '#pragma clang fp eval_method'; type is set according to the default eval method for the translation unit}}
+#else
+typedef float float_t; //expected-error 7 {{cannot use type 'float_t' within '#pragma clang fp eval_method'; type is set according to the default eval method for the translation unit}}
+  
+typedef double double_t; //expected-error 7 {{cannot use type 'double_t' within '#pragma clang fp eval_method'; type is set according to the default eval method for the translation unit}}
+#endif
+#endif
+  
+float foo1() {
+#pragma clang fp eval_method(source)
+  float a;
+  double b;
+  return a - b;
+}
+  
+float foo2() {
+#pragma clang fp eval_method(source)
+  float_t a; 
+  double_t b; 
+  return a - b;
+}
+  
+void foo3() {
+#pragma clang fp eval_method(source)
+  char buff[sizeof(float_t)];
+  char bufd[sizeof(double_t)];
+  buff[1] = bufd[2];
+}
+  
+float foo4() {
+#pragma clang fp eval_method(source)
+  typedef float_t FT;
+  typedef double_t DT;
+  FT a;
+  DT b;
+  return a - b;
+}
+  
+int foo5() {
+#pragma clang fp eval_method(source)
+  int t = _Generic( 1.0L, float_t:1, default:0);
+  int v = _Generic( 1.0L, double_t:1, default:0);
+  return t;
+}
+
+void foo6() {
+#pragma clang fp eval_method(source)
+  float f = (float_t)1; 
+  double d = (double_t)2; 
+}
+  
+void foo7() {
+#pragma clang fp eval_method(source)
+  float c1 = (float_t)12;
+  double c2 = (double_t)13;
+}
+  
+float foo8() {
+#pragma clang fp eval_method(source)
+  extern float_t f;
+  extern double_t g;
+  return f-g;
+}
+
+#ifdef CPP
+void foo9() {
+#pragma clang fp eval_method(source)
+  auto resf = [](float_t f) { return f; };
+  auto resd = [](double_t g) { return g; };
+}
+
+void foo10() {
+#pragma clang fp eval_method(source)
+  using Ft = float_t;
+  using Dt = double_t;
+  Ft a;
+  Dt b;
+}
+#endif
diff --git a/clang/test/Sema/fp-eval-pragma-with-float-double_t-2.c b/clang/test/Sema/fp-eval-pragma-with-float-double_t-2.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Sema/fp-eval-pragma-with-float-double_t-2.c
@@ -0,0 +1,100 @@
+// RUN: %clang_cc1 -verify -fsyntax-only -DNOERROR %s
+// RUN: %clang_cc1 -verify -fsyntax-only -x c++ -DCPP -DNOERROR %s
+
+// RUN: %clang_cc1 -verify -fsyntax-only -ffp-eval-method=double -DNOERROR %s
+// RUN: %clang_cc1 -verify -fsyntax-only -x c++ -DCPP -ffp-eval-method=double -DNOERROR %s
+
+// RUN: %clang_cc1 -verify -fsyntax-only -ffp-eval-method=source %s
+// RUN: %clang_cc1 -verify -fsyntax-only -x c++ -DCPP -ffp-eval-method=source %s
+
+// RUN: %clang_cc1 -verify -fsyntax-only -ffp-eval-method=extended %s
+// RUN: %clang_cc1 -verify -fsyntax-only -x c++ -DCPP -ffp-eval-method=extended %s 
+
+#ifdef NOERROR
+// expected-no-diagnostics
+typedef float float_t;
+typedef double double_t;
+#else
+#ifdef CPP
+typedef float float_t; //expected-error 9 {{cannot use type 'float_t' within '#pragma clang fp eval_method'; type is set according to the default eval method for the translation unit}}
+  
+typedef double double_t; //expected-error 9 {{cannot use type 'double_t' within '#pragma clang fp eval_method'; type is set according to the default eval method for the translation unit}}
+#else
+typedef float float_t; //expected-error 7 {{cannot use type 'float_t' within '#pragma clang fp eval_method'; type is set according to the default eval method for the translation unit}}
+  
+typedef double double_t; //expected-error 7 {{cannot use type 'double_t' within '#pragma clang fp eval_method'; type is set according to the default eval method for the translation unit}}
+#endif
+#endif
+
+float foo1() {
+#pragma clang fp eval_method(double)
+  float a;
+  double b;
+  return a - b;
+}
+  
+float foo2() {
+#pragma clang fp eval_method(double)
+  float_t a; 
+  double_t b; 
+  return a - b;
+}
+  
+void foo3() {
+#pragma clang fp eval_method(double)
+  char buff[sizeof(float_t)];
+  char bufd[sizeof(double_t)];
+  buff[1] = bufd[2];
+}
+  
+float foo4() {
+#pragma clang fp eval_method(double)
+  typedef float_t FT;
+  typedef double_t DT;
+  FT a;
+  DT b;
+  return a - b;
+}
+  
+int foo5() {
+#pragma clang fp eval_method(double)
+  int t = _Generic( 1.0L, float_t:1, default:0);
+  int v = _Generic( 1.0L, double_t:1, default:0);
+  return t;
+}
+
+void foo6() {
+#pragma clang fp eval_method(double)
+  float f = (float_t)1; 
+  double d = (double_t)2; 
+}
+  
+void foo7() {
+#pragma clang fp eval_method(double)
+  float c1 = (float_t)12;
+  double c2 = (double_t)13;
+}
+  
+float foo8() {
+#pragma clang fp eval_method(double)
+  extern float_t f;
+  extern double_t g;
+  return f-g;
+}
+
+#ifdef CPP
+void foo9() {
+#pragma clang fp eval_method(double)
+  auto resf = [](float_t f) { return f; };
+  auto resd = [](double_t g) { return g; };
+}
+
+void foo10() {
+#pragma clang fp eval_method(double)
+  using Ft = float_t;
+  using Dt = double_t;
+  Ft a;
+  Dt b;
+}
+#endif
+ 
diff --git a/clang/test/Sema/fp-eval-pragma-with-float-double_t-3.c b/clang/test/Sema/fp-eval-pragma-with-float-double_t-3.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Sema/fp-eval-pragma-with-float-double_t-3.c
@@ -0,0 +1,102 @@
+// RUN: %clang_cc1 -verify -fsyntax-only -verify -DNOERROR %s
+// RUN: %clang_cc1 -verify -fsyntax-only -x c++ -DCPP -DNOERROR %s
+
+// RUN: %clang_cc1 -verify -fsyntax-only -ffp-eval-method=extended -DNOERROR %s
+// RUN: %clang_cc1 -verify -fsyntax-only -x c++ -DCPP \
+// RUN: -ffp-eval-method=extended -DNOERROR %s
+
+// RUN: %clang_cc1 -verify -fsyntax-only -ffp-eval-method=source %s
+// RUN: %clang_cc1 -verify -fsyntax-only -x c++ -DCPP -ffp-eval-method=source %s
+
+// RUN: %clang_cc1 -verify -fsyntax-only -ffp-eval-method=double %s
+// RUN: %clang_cc1 -verify -fsyntax-only -x c++ -DCPP -ffp-eval-method=double %s
+
+
+#ifdef NOERROR
+// expected-no-diagnostics
+typedef float float_t;
+typedef double double_t;
+#else
+#ifdef CPP
+typedef float float_t; //expected-error 9 {{cannot use type 'float_t' within '#pragma clang fp eval_method'; type is set according to the default eval method for the translation unit}}
+
+typedef double double_t; //expected-error 9 {{cannot use type 'double_t' within '#pragma clang fp eval_method'; type is set according to the default eval method for the translation unit}}
+#else
+typedef float float_t; //expected-error 7 {{cannot use type 'float_t' within '#pragma clang fp eval_method'; type is set according to the default eval method for the translation unit}}
+  
+typedef double double_t; //expected-error 7 {{cannot use type 'double_t' within '#pragma clang fp eval_method'; type is set according to the default eval method for the translation unit}}
+#endif
+#endif
+
+float foo1() {
+#pragma clang fp eval_method(extended)
+  float a;
+  double b;
+  return a - b;
+}
+  
+float foo2() {
+#pragma clang fp eval_method(extended)
+  float_t a; 
+  double_t b; 
+  return a - b;
+}
+  
+void foo3() {
+#pragma clang fp eval_method(extended)
+  char buff[sizeof(float_t)];
+  char bufd[sizeof(double_t)];
+  buff[1] = bufd[2];
+}
+  
+float foo4() {
+#pragma clang fp eval_method(extended)
+  typedef float_t FT;
+  typedef double_t DT;
+  FT a;
+  DT b;
+  return a - b;
+}
+  
+int foo5() {
+#pragma clang fp eval_method(extended)
+  int t = _Generic( 1.0L, float_t:1, default:0);
+  int v = _Generic( 1.0L, double_t:1, default:0);
+  return t;
+}
+
+void foo6() {
+#pragma clang fp eval_method(extended)
+  float f = (float_t)1; 
+  double d = (double_t)2; 
+}
+  
+void foo7() {
+#pragma clang fp eval_method(extended)
+  float c1 = (float_t)12;
+  double c2 = (double_t)13;
+}
+  
+float foo8() {
+#pragma clang fp eval_method(extended)
+  extern float_t f;
+  extern double_t g;
+  return f-g;
+}
+
+#ifdef CPP
+void foo9() {
+#pragma clang fp eval_method(extended)
+  auto resf = [](float_t f) { return f; };
+  auto resd = [](double_t g) { return g; };
+}
+
+void foo10() {
+#pragma clang fp eval_method(extended)
+  using Ft = float_t;
+  using Dt = double_t;
+  Ft a;
+  Dt b;
+}
+#endif
+