diff --git a/clang/docs/CommandGuide/clang.rst b/clang/docs/CommandGuide/clang.rst
--- a/clang/docs/CommandGuide/clang.rst
+++ b/clang/docs/CommandGuide/clang.rst
@@ -278,9 +278,18 @@
  Make all string literals default to writable.  This disables uniquing of
  strings and other optimizations.
 
-.. option:: -flax-vector-conversions
+.. option:: -flax-vector-conversions, -flax-vector-conversions=<kind>, -fno-lax-vector-conversions
 
  Allow loose type checking rules for implicit vector conversions.
+ Possible values of <kind>:
+
+ - ``none``: allow no implicit conversions between vectors
+ - ``integer``: allow implicit bitcasts between integer vectors of the same
+   overall bit-width
+ - ``all``: allow implicit bitcasts between any vectors of the same
+   overall bit-width
+
+ <kind> defaults to ``integer`` if unspecified.
 
 .. option:: -fblocks
 
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -62,6 +62,15 @@
 -------------------------------------------------
 
 
+* Lax vector conversions involving floating-point vectors have been disabled
+  by default, and can no longer be enabled with ``-flax-vector-conversions``.
+  This matches the behavior of these flags in GCC, but code relying on implicit
+  vector bitcasts between integer and floating-point types that used to compile
+  with older versions of Clang is no longer accepted by default in Clang 10.
+  The old behavior can be restored with ``-flax-vector-conversions=all``.
+  In a future release of Clang, we intend to change the default to
+  ``-fno-lax-vector-conversions``.
+
 New Compiler Flags
 ------------------
 
@@ -78,6 +87,21 @@
 -----------------------
 
 
+- ``-flax-vector-conversions`` has been split into three different levels of
+  laxness:
+
+  - ``-flax-vector-conversions=all``: This is Clang's historical default, and
+    permits implicit vector conversions (performed as bitcasts) between any
+    two vector types of the same overall bit-width.
+
+  - ``-flax-vector-conversions=integer``: This is Clang's current default,
+    and permits implicit vector conversions (performed as bitcasts) between
+    any two integer vector types of the same overall bit-width.
+    Synonym: ``-flax-vector-conversions``.
+
+  - ``-flax-vector-conversions=none``: Do not perform any implicit bitcasts
+    between vector types. Synonym: ``-fno-lax-vector-conversions``.
+
 New Pragmas in Clang
 --------------------
 
diff --git a/clang/include/clang/Basic/LangOptions.def b/clang/include/clang/Basic/LangOptions.def
--- a/clang/include/clang/Basic/LangOptions.def
+++ b/clang/include/clang/Basic/LangOptions.def
@@ -121,7 +121,7 @@
 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")
 LANGOPT(ConvergentFunctions, 1, 1, "Assume convergent functions")
 LANGOPT(AltiVec           , 1, 0, "AltiVec-style vector initializers")
 LANGOPT(ZVector           , 1, 0, "System z vector extensions")
diff --git a/clang/test/Headers/altivec-header.c b/clang/test/Headers/altivec-header.c
--- a/clang/test/Headers/altivec-header.c
+++ b/clang/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>
diff --git a/clang/test/Headers/arm-neon-header.c b/clang/test/Headers/arm-neon-header.c
--- a/clang/test/Headers/arm-neon-header.c
+++ b/clang/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
 
diff --git a/clang/test/Headers/x86-intrinsics-headers.c b/clang/test/Headers/x86-intrinsics-headers.c
--- a/clang/test/Headers/x86-intrinsics-headers.c
+++ b/clang/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
 
diff --git a/clang/test/Headers/x86intrin-2.c b/clang/test/Headers/x86intrin-2.c
--- a/clang/test/Headers/x86intrin-2.c
+++ b/clang/test/Headers/x86intrin-2.c
@@ -1,5 +1,5 @@
-// RUN: %clang_cc1 -fsyntax-only -ffreestanding -Wcast-qual %s -verify
-// RUN: %clang_cc1 -fsyntax-only -ffreestanding -flax-vector-conversions=none -Wcast-qual %s -verify
+// RUN: %clang_cc1 -fsyntax-only -ffreestanding -Wcast-qual -flax-vector-conversions=all %s -verify
+// RUN: %clang_cc1 -fsyntax-only -ffreestanding -Wcast-qual -flax-vector-conversions=none %s -verify
 // RUN: %clang_cc1 -fsyntax-only -ffreestanding -Wcast-qual -x c++ %s -verify
 // expected-no-diagnostics
 
diff --git a/clang/test/Headers/x86intrin.c b/clang/test/Headers/x86intrin.c
--- a/clang/test/Headers/x86intrin.c
+++ b/clang/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
diff --git a/clang/test/Sema/vector-assign.c b/clang/test/Sema/vector-assign.c
--- a/clang/test/Sema/vector-assign.c
+++ b/clang/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
diff --git a/clang/test/Sema/vector-cast.c b/clang/test/Sema/vector-cast.c
--- a/clang/test/Sema/vector-cast.c
+++ b/clang/test/Sema/vector-cast.c
@@ -1,4 +1,5 @@
-// RUN: %clang_cc1 -fsyntax-only %s -verify -Wvector-conversion
+// RUN: %clang_cc1 -fsyntax-only %s -verify=expected,no-lax -Wvector-conversion -flax-vector-conversions=none
+// RUN: %clang_cc1 -fsyntax-only %s -verify=expected,lax -Wvector-conversion -flax-vector-conversions=all
 
 typedef long long t1 __attribute__ ((vector_size (8)));
 typedef char t2 __attribute__ ((vector_size (16)));
@@ -41,7 +42,9 @@
 void f2(t2 X); // expected-note{{passing argument to parameter 'X' here}}
 
 void f3(t3 Y) {
-  f2(Y);  // expected-warning {{incompatible vector types passing 't3' (vector of 4 'float' values) to parameter of type 't2' (vector of 16 'char' values)}}
+  f2(Y);
+  // lax-warning@-1 {{incompatible vector types passing 't3' (vector of 4 'float' values) to parameter of type 't2' (vector of 16 'char' values)}}
+  // no-lax-error@-2 {{passing 't3' (vector of 4 'float' values) to parameter of incompatible type 't2' (vector of 16 'char' values)}}
 }
 
 typedef float float2 __attribute__ ((vector_size (8)));
@@ -58,13 +61,15 @@
   float64x2_t v = {0.0, 1.0};
   f2 += d; // expected-error {{cannot convert between scalar type 'double' and vector type 'float2' (vector of 2 'float' values) as implicit conversion would cause truncation}}
   d += f2; // expected-error {{assigning to 'double' from incompatible type 'float2' (vector of 2 'float' values)}}
-  a = 3.0 + vget_low_f64(v);
-  b = vget_low_f64(v) + 3.0;
-  c = vget_low_f64(v);
-  c -= vget_low_f64(v);
+  a = 3.0 + vget_low_f64(v); // no-lax-error {{assigning to 'double' from incompatible type 'float64x1_t' (vector of 1 'double' value)}}
+  b = vget_low_f64(v) + 3.0; // no-lax-error {{assigning to 'double' from incompatible type 'float64x1_t' (vector of 1 'double' value)}}
+  c = vget_low_f64(v); // no-lax-error {{assigning to 'double' from incompatible type 'float64x1_t' (vector of 1 'double' value)}}
+  c -= vget_low_f64(v); // no-lax-error {{assigning to 'double' from incompatible type 'float64x1_t' (vector of 1 'double' value)}}
   // LAX conversions between scalar and vector types require same size and one element sized vectors.
   d = f2; // expected-error {{assigning to 'double' from incompatible type 'float2'}}
-  d = d + f2; // expected-error {{assigning to 'double' from incompatible type 'float2'}}
+  d = d + f2;
+  // lax-error@-1 {{assigning to 'double' from incompatible type 'float2' (vector of 2 'float' values)}}
+  // no-lax-error@-2 {{cannot convert between scalar type 'double' and vector type 'float2' (vector of 2 'float' values) as implicit conversion would cause truncation}}
 }
 
 // rdar://15931426
@@ -78,6 +83,8 @@
 }
 
 void f6(vSInt32 a0) {
-  vUInt32 counter = (float16){0.0f, 0.0f, 0.0f, 0.0f}; // expected-warning {{incompatible vector types initializing 'vUInt32' (vector of 4 'unsigned int' values) with an expression of type 'float16' (vector of 4 'float' values)}}
+  vUInt32 counter = (float16){0.0f, 0.0f, 0.0f, 0.0f};
+  // lax-warning@-1 {{incompatible vector types initializing 'vUInt32' (vector of 4 'unsigned int' values) with an expression of type 'float16' (vector of 4 'float' values)}}
+  // no-lax-error@-2 {{initializing 'vUInt32' (vector of 4 'unsigned int' values) with an expression of incompatible type 'float16' (vector of 4 'float' values)}}
   counter -= a0;
 }
diff --git a/clang/test/Sema/vector-ops.c b/clang/test/Sema/vector-ops.c
--- a/clang/test/Sema/vector-ops.c
+++ b/clang/test/Sema/vector-ops.c
@@ -1,4 +1,5 @@
-// 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
+// FIXME: We get worse diagnostics here with -flax-vector-conversions disabled.
 typedef unsigned int v2u __attribute__ ((vector_size (8)));
 typedef int v2s __attribute__ ((vector_size (8)));
 typedef float v2f __attribute__ ((vector_size(8)));