Index: test/wasm/fatal-warnings.ll
===================================================================
--- /dev/null
+++ test/wasm/fatal-warnings.ll
@@ -0,0 +1,15 @@
+; RUN: llc -filetype=obj %s -o %t.main.o
+; RUN: lld -flavor wasm --check-signatures -o %t.wasm %t.main.o 2>&1 | FileCheck %s
+
+target triple = "wasm32-unknown-unknown-wasm"
+
+define hidden i32 @_start(i32 %arg) local_unnamed_addr {
+entry:
+  ret i32 %arg
+}
+
+; CHECK: warning: function signature mismatch: _start
+
+; RUN: not lld -flavor wasm --check-signatures --fatal-warnings -o %t.wasm %t.main.o 2>&1 | FileCheck %s -check-prefix=CHECK-FATAL
+
+; CHECK-FATAL: error: function signature mismatch: _start
Index: test/wasm/signature-mismatch.ll
===================================================================
--- test/wasm/signature-mismatch.ll
+++ test/wasm/signature-mismatch.ll
@@ -1,6 +1,6 @@
 ; RUN: llc -filetype=obj %p/Inputs/ret32.ll -o %t.ret32.o
 ; RUN: llc -filetype=obj %s -o %t.main.o
-; RUN: not lld -flavor wasm --check-signatures -o %t.wasm %t.main.o %t.ret32.o 2>&1 | FileCheck %s
+; RUN: not lld -flavor wasm --check-signatures --fatal-warnings -o %t.wasm %t.main.o %t.ret32.o 2>&1 | FileCheck %s
 
 target triple = "wasm32-unknown-unknown-wasm"
 
Index: wasm/Driver.cpp
===================================================================
--- wasm/Driver.cpp
+++ wasm/Driver.cpp
@@ -250,6 +250,8 @@
   Config->CheckSignatures =
       Args.hasFlag(OPT_check_signatures, OPT_no_check_signatures, false);
   Config->Entry = getEntry(Args, Args.hasArg(OPT_relocatable) ? "" : "_start");
+  errorHandler().FatalWarnings =
+      Args.hasFlag(OPT_fatal_warnings, OPT_no_fatal_warnings, false);
   Config->ImportMemory = Args.hasArg(OPT_import_memory);
   Config->OutputFile = Args.getLastArgValue(OPT_o);
   Config->Relocatable = Args.hasArg(OPT_relocatable);
Index: wasm/SymbolTable.cpp
===================================================================
--- wasm/SymbolTable.cpp
+++ wasm/SymbolTable.cpp
@@ -113,10 +113,10 @@
   if (*NewSig == OldSig)
     return;
 
-  error("function signature mismatch: " + Existing.getName() +
-        "\n>>> defined as " + toString(OldSig) + " in " +
-        toString(Existing.getFile()) + "\n>>> defined as " + toString(*NewSig) +
-        " in " + F.getName());
+  warn("function signature mismatch: " + Existing.getName() +
+       "\n>>> defined as " + toString(OldSig) + " in " +
+       toString(Existing.getFile()) + "\n>>> defined as " + toString(*NewSig) +
+       " in " + F.getName());
 }
 
 static void checkSymbolTypes(const Symbol &Existing, const InputFile &F,