diff --git a/clang/lib/Basic/TargetInfo.cpp b/clang/lib/Basic/TargetInfo.cpp
index 6ef306d27e75..947cbfc3f6d2 100644
--- a/clang/lib/Basic/TargetInfo.cpp
+++ b/clang/lib/Basic/TargetInfo.cpp
@@ -350,6 +350,10 @@ bool TargetInfo::validateInputConstraint(ConstraintInfo *OutputConstraints,
         if (i >= NumOutputs)
           return false;
 
+        // A number must refer to an output only operand.
+        if (OutputConstraints[i].isReadWrite())
+          return false;
+
         // The constraint should have the same info as the respective
         // output constraint.
         Info.setTiedOperand(i, OutputConstraints[i]);
diff --git a/clang/test/Sema/asm.c b/clang/test/Sema/asm.c
index 52611faf3dbc..73d081ddd385 100644
--- a/clang/test/Sema/asm.c
+++ b/clang/test/Sema/asm.c
@@ -79,3 +79,9 @@ int test7(unsigned long long b) {
 
 // <rdar://problem/7574870>
 asm volatile (""); // expected-warning {{meaningless 'volatile' on asm outside function}}
+
+// PR3904
+int test8(int i) {
+  // A number in an input constraint can't point to a read-write constraint.
+  asm ("" : "+r" (i), "=r"(i) :  "0" (i)); // expected-error{{invalid input constraint '0' in asm}}
+}