Index: include/clang/Basic/LangOptions.def
===================================================================
--- include/clang/Basic/LangOptions.def
+++ include/clang/Basic/LangOptions.def
@@ -120,7 +120,8 @@
 LANGOPT(WritableStrings   , 1, 0, "writable string support")
 LANGOPT(ConstStrings      , 1, 0, "const-qualified string support")
 ENUM_LANGOPT(LaxVectorConversions, LaxVectorConversionKind, 2,
-             LaxVectorConversionKind::All, "lax vector conversions")
+             LaxVectorConversionKind::Integer, "lax vector conversions")
+BENIGN_LANGOPT(LaxVectorConversionsSpecified, 1, 0, "whether lax vector conversions was specified")
 LANGOPT(AltiVec           , 1, 0, "AltiVec-style vector initializers")
 LANGOPT(ZVector           , 1, 0, "System z vector extensions")
 LANGOPT(Exceptions        , 1, 0, "exception handling")
Index: lib/Basic/Targets/AArch64.h
===================================================================
--- lib/Basic/Targets/AArch64.h
+++ lib/Basic/Targets/AArch64.h
@@ -57,6 +57,8 @@
     return false;
   }
 
+  void adjust(LangOptions &Opts) override;
+
   void getTargetDefinesARMV81A(const LangOptions &Opts,
                                MacroBuilder &Builder) const;
   void getTargetDefinesARMV82A(const LangOptions &Opts,
Index: lib/Basic/Targets/AArch64.cpp
===================================================================
--- lib/Basic/Targets/AArch64.cpp
+++ lib/Basic/Targets/AArch64.cpp
@@ -117,6 +117,17 @@
   llvm::AArch64::fillValidCPUArchList(Values);
 }
 
+void AArch64TargetInfo::adjust(LangOptions &Opts) {
+  TargetInfo::adjust(Opts);
+
+  // FIXME: arm_neon.h makes use of lax vector conversions (including
+  // conversions between integer and floating-point type), so forcibly enable
+  // support for that language extension / bug whenever arm_neon.h would be
+  // usable.
+  if ((FPU & NeonMode) && !Opts.LaxVectorConversionsSpecified)
+    Opts.setLaxVectorConversions(LangOptions::LaxVectorConversionKind::All);
+}
+
 void AArch64TargetInfo::getTargetDefinesARMV81A(const LangOptions &Opts,
                                                 MacroBuilder &Builder) const {
   Builder.defineMacro("__ARM_FEATURE_QRDMX", "1");
Index: lib/Basic/Targets/ARM.h
===================================================================
--- lib/Basic/Targets/ARM.h
+++ lib/Basic/Targets/ARM.h
@@ -146,6 +146,8 @@
     return false;
   }
 
+  void adjust(LangOptions &Opts) override;
+
   void getTargetDefinesARMV81A(const LangOptions &Opts,
                                MacroBuilder &Builder) const;
 
Index: lib/Basic/Targets/ARM.cpp
===================================================================
--- lib/Basic/Targets/ARM.cpp
+++ lib/Basic/Targets/ARM.cpp
@@ -570,6 +570,18 @@
   return false;
 }
 
+void ARMTargetInfo::adjust(LangOptions &Opts) {
+  TargetInfo::adjust(Opts);
+
+  // FIXME: arm_neon.h makes use of lax vector conversions (including
+  // conversions between integer and floating-point type), so forcibly enable
+  // support for that language extension / bug whenever arm_neon.h would be
+  // usable.
+  if ((FPU & NeonFPU) && !SoftFloat && ArchVersion >= 7 &&
+      !Opts.LaxVectorConversionsSpecified)
+    Opts.setLaxVectorConversions(LangOptions::LaxVectorConversionKind::All);
+}
+
 void ARMTargetInfo::getTargetDefinesARMV81A(const LangOptions &Opts,
                                             MacroBuilder &Builder) const {
   Builder.defineMacro("__ARM_FEATURE_QRDMX", "1");
Index: lib/Frontend/CompilerInvocation.cpp
===================================================================
--- lib/Frontend/CompilerInvocation.cpp
+++ lib/Frontend/CompilerInvocation.cpp
@@ -2669,6 +2669,7 @@
                                    Opts.ConstStrings);
   if (Arg *A = Args.getLastArg(OPT_flax_vector_conversions_EQ)) {
     using LaxKind = LangOptions::LaxVectorConversionKind;
+    Opts.LaxVectorConversionsSpecified = true;
     if (auto Kind = llvm::StringSwitch<Optional<LaxKind>>(A->getValue())
                         .Case("none", LaxKind::None)
                         .Case("integer", LaxKind::Integer)
Index: test/Headers/altivec-header.c
===================================================================
--- test/Headers/altivec-header.c
+++ test/Headers/altivec-header.c
@@ -1,5 +1,5 @@
-// RUN: %clang_cc1 -triple powerpc64-unknown-unknown -target-feature +altivec -ffreestanding -emit-llvm -o - %s | FileCheck %s
 // RUN: %clang_cc1 -triple powerpc64-unknown-unknown -target-feature +altivec -ffreestanding -emit-llvm -flax-vector-conversions=none -o - %s | FileCheck %s
+// RUN: %clang_cc1 -triple powerpc64-unknown-unknown -target-feature +altivec -ffreestanding -emit-llvm -flax-vector-conversions=all -o - %s | FileCheck %s
 // RUN: %clang_cc1 -triple powerpc64-unknown-unknown -target-feature +altivec -ffreestanding -emit-llvm -x c++ -o - %s | FileCheck %s
 
 #include <altivec.h>
Index: test/Headers/arm-neon-header.c
===================================================================
--- test/Headers/arm-neon-header.c
+++ test/Headers/arm-neon-header.c
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -triple thumbv7-apple-darwin10 -target-cpu cortex-a8 -fsyntax-only -Wvector-conversions -ffreestanding %s
+// RUN: %clang_cc1 -triple thumbv7-apple-darwin10 -target-cpu cortex-a8 -fsyntax-only -flax-vector-conversions=all -Wvector-conversions -ffreestanding %s
 // RUN: %clang_cc1 -triple thumbv7-apple-darwin10 -target-cpu cortex-a8 -fsyntax-only -flax-vector-conversions=none -ffreestanding %s
 // RUN: %clang_cc1 -x c++ -triple thumbv7-apple-darwin10 -target-cpu cortex-a8 -fsyntax-only -Wvector-conversions -ffreestanding %s
 
Index: test/Headers/x86-intrinsics-headers.c
===================================================================
--- test/Headers/x86-intrinsics-headers.c
+++ test/Headers/x86-intrinsics-headers.c
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only -ffreestanding %s
+// RUN: %clang_cc1 -fsyntax-only -ffreestanding -flax-vector-conversions=all %s
 // RUN: %clang_cc1 -fsyntax-only -ffreestanding -flax-vector-conversions=none %s
 // RUN: %clang_cc1 -fsyntax-only -ffreestanding -x c++ %s
 
Index: test/Headers/x86intrin-2.c
===================================================================
--- test/Headers/x86intrin-2.c
+++ test/Headers/x86intrin-2.c
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only -ffreestanding %s -verify
+// RUN: %clang_cc1 -fsyntax-only -ffreestanding -flax-vector-conversions=all %s -verify
 // RUN: %clang_cc1 -fsyntax-only -ffreestanding -flax-vector-conversions=none %s -verify
 // RUN: %clang_cc1 -fsyntax-only -ffreestanding -x c++ %s -verify
 // expected-no-diagnostics
Index: test/Headers/x86intrin.c
===================================================================
--- test/Headers/x86intrin.c
+++ test/Headers/x86intrin.c
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only -ffreestanding %s -verify
+// RUN: %clang_cc1 -fsyntax-only -ffreestanding -flax-vector-conversions=all %s -verify
 // RUN: %clang_cc1 -fsyntax-only -ffreestanding -flax-vector-conversions=none %s -verify
 // RUN: %clang_cc1 -fsyntax-only -ffreestanding -x c++ %s -verify
 // expected-no-diagnostics
Index: test/Sema/vector-assign.c
===================================================================
--- test/Sema/vector-assign.c
+++ test/Sema/vector-assign.c
@@ -14,12 +14,12 @@
   
   v1 = v2; // expected-warning {{incompatible vector types assigning to 'v2s' (vector of 2 'int' values) from 'v2u' (vector of 2 'unsigned int' values)}}
   v1 = v3; // expected-error {{assigning to 'v2s' (vector of 2 'int' values) from incompatible type 'v1s' (vector of 1 'int' value)}}
-  v1 = v4; // expected-warning {{incompatible vector types assigning to 'v2s' (vector of 2 'int' values) from 'v2f' (vector of 2 'float' values)}}
+  v1 = v4; // expected-error {{assigning to 'v2s' (vector of 2 'int' values) from incompatible type 'v2f' (vector of 2 'float' values)}}
   v1 = v5; // expected-warning {{incompatible vector types assigning to 'v2s' (vector of 2 'int' values) from 'v4ss' (vector of 4 'short' values)}}
   
   v2 = v1; // expected-warning {{incompatible vector types assigning to 'v2u' (vector of 2 'unsigned int' values) from 'v2s' (vector of 2 'int' values)}}
   v2 = v3; // expected-error {{assigning to 'v2u' (vector of 2 'unsigned int' values) from incompatible type 'v1s' (vector of 1 'int' value)}}
-  v2 = v4; // expected-warning {{incompatible vector types assigning to 'v2u' (vector of 2 'unsigned int' values) from 'v2f' (vector of 2 'float' values)}}
+  v2 = v4; // expected-error {{assigning to 'v2u' (vector of 2 'unsigned int' values) from incompatible type 'v2f' (vector of 2 'float' values)}}
   v2 = v5; // expected-warning {{incompatible vector types assigning to 'v2u' (vector of 2 'unsigned int' values) from 'v4ss' (vector of 4 'short' values)}}
   
   v3 = v1; // expected-error {{assigning to 'v1s' (vector of 1 'int' value) from incompatible type 'v2s' (vector of 2 'int' values)}}
@@ -27,15 +27,15 @@
   v3 = v4; // expected-error {{assigning to 'v1s' (vector of 1 'int' value) from incompatible type 'v2f' (vector of 2 'float' values)}}
   v3 = v5; // expected-error {{assigning to 'v1s' (vector of 1 'int' value) from incompatible type 'v4ss'}}
   
-  v4 = v1; // expected-warning {{incompatible vector types assigning to 'v2f' (vector of 2 'float' values) from 'v2s' (vector of 2 'int' values)}}
-  v4 = v2; // expected-warning {{incompatible vector types assigning to 'v2f' (vector of 2 'float' values) from 'v2u' (vector of 2 'unsigned int' values)}}
+  v4 = v1; // expected-error {{assigning to 'v2f' (vector of 2 'float' values) from incompatible type 'v2s' (vector of 2 'int' values)}}
+  v4 = v2; // expected-error {{assigning to 'v2f' (vector of 2 'float' values) from incompatible type 'v2u' (vector of 2 'unsigned int' values)}}
   v4 = v3; // expected-error {{assigning to 'v2f' (vector of 2 'float' values) from incompatible type 'v1s' (vector of 1 'int' value)}}
-  v4 = v5; // expected-warning {{incompatible vector types assigning to 'v2f' (vector of 2 'float' values) from 'v4ss' (vector of 4 'short' values)}}
+  v4 = v5; // expected-error {{assigning to 'v2f' (vector of 2 'float' values) from incompatible type 'v4ss' (vector of 4 'short' values)}}
   
   v5 = v1; // expected-warning {{incompatible vector types assigning to 'v4ss' (vector of 4 'short' values) from 'v2s' (vector of 2 'int' values)}}
   v5 = v2; // expected-warning {{incompatible vector types assigning to 'v4ss' (vector of 4 'short' values) from 'v2u' (vector of 2 'unsigned int' values)}}
   v5 = v3; // expected-error {{assigning to 'v4ss' (vector of 4 'short' values) from incompatible type 'v1s' (vector of 1 'int' value)}}
-  v5 = v4; // expected-warning {{incompatible vector types assigning to 'v4ss' (vector of 4 'short' values) from 'v2f'}}
+  v5 = v4; // expected-error {{assigning to 'v4ss' (vector of 4 'short' values) from incompatible type 'v2f'}}
 }
 
 // PR2263
Index: test/Sema/vector-cast.c
===================================================================
--- test/Sema/vector-cast.c
+++ test/Sema/vector-cast.c
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only %s -verify -Wvector-conversion
+// RUN: %clang_cc1 -fsyntax-only %s -verify -Wvector-conversion -flax-vector-conversions=all
 
 typedef long long t1 __attribute__ ((vector_size (8)));
 typedef char t2 __attribute__ ((vector_size (16)));
Index: test/Sema/vector-ops.c
===================================================================
--- test/Sema/vector-ops.c
+++ test/Sema/vector-ops.c
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 %s -verify -fsyntax-only -Wvector-conversion -triple x86_64-apple-darwin10
+// RUN: %clang_cc1 %s -verify -fsyntax-only -Wvector-conversion -triple x86_64-apple-darwin10 -flax-vector-conversions=all
 typedef unsigned int v2u __attribute__ ((vector_size (8)));
 typedef int v2s __attribute__ ((vector_size (8)));
 typedef float v2f __attribute__ ((vector_size(8)));