Index: ELF/Driver.cpp
===================================================================
--- ELF/Driver.cpp
+++ ELF/Driver.cpp
@@ -756,8 +756,9 @@
       Args.hasFlag(OPT_allow_multiple_definition,
                    OPT_no_allow_multiple_definition, false) ||
       hasZOption(Args, "muldefs");
-  Config->AllowShlibUndefined = Args.hasFlag(
-      OPT_allow_shlib_undefined, OPT_no_allow_shlib_undefined, true);
+  Config->AllowShlibUndefined =
+      Args.hasFlag(OPT_allow_shlib_undefined, OPT_no_allow_shlib_undefined,
+                   Args.hasArg(OPT_shared));
   Config->AuxiliaryList = args::getStrings(Args, OPT_auxiliary);
   Config->Bsymbolic = Args.hasArg(OPT_Bsymbolic);
   Config->BsymbolicFunctions = Args.hasArg(OPT_Bsymbolic_functions);
Index: docs/ld.lld.1
===================================================================
--- docs/ld.lld.1
+++ docs/ld.lld.1
@@ -46,6 +46,9 @@
 .It Fl -allow-multiple-definition
 Do not error if a symbol is defined multiple times.
 The first definition will be used.
+.It Fl -allow-shlib-undefined
+Allow unresolved references in shared libraries.
+This option is enabled by default when linking an shared library.
 .It Fl -apply-dynamic-relocs
 Apply link-time values for dynamic relocations.
 .It Fl -as-needed
@@ -242,6 +245,9 @@
 .It Fl -Map Ns = Ns Ar file , Fl M Ar file
 Print a link map to
 .Ar file .
+.It Fl -no-allow-shlib-undefined
+Do not allow unresolved references in shared libraries.
+This option is enabled by default when linking an executable.
 .It Fl -no-as-needed
 Always set
 .Dv DT_NEEDED
Index: test/ELF/allow-shlib-undefined.s
===================================================================
--- test/ELF/allow-shlib-undefined.s
+++ test/ELF/allow-shlib-undefined.s
@@ -6,9 +6,12 @@
 # RUN:   %p/Inputs/allow-shlib-undefined.s -o %t1.o
 # RUN: ld.lld -shared %t1.o -o %t.so
 
-# RUN: ld.lld %t.o %t.so -o /dev/null
 # RUN: ld.lld --allow-shlib-undefined %t.o %t.so -o /dev/null
 # RUN: not ld.lld --no-allow-shlib-undefined %t.o %t.so -o /dev/null 2>&1 | FileCheck %s
+# Executable defaults to --no-allow-shlib-undefined
+# RUN: not ld.lld %t.o %t.so -o /dev/null 2>&1 | FileCheck %s
+# -shared defaults to --allow-shlib-undefined
+# RUN: ld.lld -shared %t.o %t.so -o /dev/null
 
 # RUN: echo | llvm-mc -filetype=obj -triple=x86_64-unknown-linux -o %tempty.o
 # RUN: ld.lld -shared %tempty.o -o %tempty.so
Index: test/ELF/unresolved-symbols.s
===================================================================
--- test/ELF/unresolved-symbols.s
+++ test/ELF/unresolved-symbols.s
@@ -27,21 +27,21 @@
 # ERRUND: >>> referenced by {{.*}}:(.text+0x1)
 
 ## Also ignore all should not produce error for symbols from DSOs.
-# RUN: ld.lld %t1.o %t.so -o %t1_3 --unresolved-symbols=ignore-all
+# RUN: ld.lld %t1.o %t.so -o %t1_3 --allow-shlib-undefined --unresolved-symbols=ignore-all
 # RUN: llvm-readobj %t1_3 > /dev/null 2>&1
 
 ## Ignoring undefines in objects should not produce error for symbol from object.
 # RUN: ld.lld %t1.o %t2.o -o %t2 --unresolved-symbols=ignore-in-object-files
 # RUN: llvm-readobj %t2 > /dev/null 2>&1
 ## And still should not should produce for undefines from DSOs.
-# RUN: ld.lld %t1.o %t.so -o /dev/null --unresolved-symbols=ignore-in-object-files
+# RUN: ld.lld %t1.o %t.so -o /dev/null --allow-shlib-undefined --unresolved-symbols=ignore-in-object-files
 # RUN: llvm-readobj %t2 > /dev/null 2>&1
 
 ## Ignoring undefines in shared should produce error for symbol from object.
 # RUN: not ld.lld %t2.o -o /dev/null --unresolved-symbols=ignore-in-shared-libs 2>&1 | \
 # RUN:   FileCheck -check-prefix=ERRUND %s
 ## And should not produce errors for symbols from DSO.
-# RUN: ld.lld %t1.o %t.so -o %t3_1 --unresolved-symbols=ignore-in-shared-libs
+# RUN: ld.lld %t1.o %t.so -o %t3_1 --allow-shlib-undefined --unresolved-symbols=ignore-in-shared-libs
 # RUN: llvm-readobj %t3_1 > /dev/null 2>&1
 
 ## Ignoring undefines in shared libs should not produce error for symbol from object