Index: test/wasm/undefined-weak-call.test
===================================================================
--- /dev/null
+++ test/wasm/undefined-weak-call.test
@@ -0,0 +1,47 @@
+# RUN: yaml2obj %s > %t.o
+# RUN: not lld -flavor wasm --no-entry %t.o -o %t.wasm 2>&1 | FileCheck %s
+
+# Check linking fails when the input file contains weak symbols, and relocations
+# which reference those symbols and cannot be resolved.
+
+# CHECK: Cannot generate relocation against undefined function: weakFunc
+
+--- !WASM
+FileHeader:
+  Version:         0x00000001
+Sections:
+  - Type:            TYPE
+    Signatures:
+      - Index:           0
+        ReturnType:      NORESULT
+        ParamTypes:
+  - Type:            IMPORT
+    Imports:
+      - Module:          env
+        Field:           weakFunc
+        Kind:            FUNCTION
+        SigIndex:        0
+  - Type:            FUNCTION
+    FunctionTypes:   [ 0 ]
+  - Type:            CODE
+    Relocations:
+      - Type:            R_WEBASSEMBLY_FUNCTION_INDEX_LEB
+        Index:           0
+        Offset:          0x00000004
+    Functions:
+      - Index:           1
+        Locals:
+        Body:            1080808080000F0B
+  - Type:            CUSTOM
+    Name:            linking
+    SymbolTable:
+      - Index:           0
+        Kind:            FUNCTION
+        Name:            weakFunc
+        Flags:           [ UNDEFINED, BINDING_WEAK ]
+        Function:        0
+      - Index:           1
+        Kind:            FUNCTION
+        Name:            callWeakFunc
+        Flags:           [  ]
+        Function:        1
Index: test/wasm/undefined-weak-getglobal.test
===================================================================
--- /dev/null
+++ test/wasm/undefined-weak-getglobal.test
@@ -0,0 +1,48 @@
+# RUN: yaml2obj %s > %t.o
+# RUN: not lld -flavor wasm --no-entry %t.o -o %t.wasm 2>&1 | FileCheck %s
+
+# Check linking fails when the input file contains weak symbols, and relocations
+# which reference those symbols and cannot be resolved.
+
+# CHECK: Cannot generate relocation against undefined global: weakGlobal
+
+--- !WASM
+FileHeader:
+  Version:         0x00000001
+Sections:
+  - Type:            TYPE
+    Signatures:
+      - Index:           0
+        ReturnType:      NORESULT
+        ParamTypes:
+  - Type:            IMPORT
+    Imports:
+      - Module:          env
+        Field:           weakGlobal
+        Kind:            GLOBAL
+        GlobalType:      I32
+        GlobalMutable:   true
+  - Type:            FUNCTION
+    FunctionTypes:   [ 0 ]
+  - Type:            CODE
+    Relocations:
+      - Type:            R_WEBASSEMBLY_GLOBAL_INDEX_LEB
+        Index:           0
+        Offset:          0x00000004
+    Functions:
+      - Index:           0
+        Locals:
+        Body:            2380808080001A0F0B
+  - Type:            CUSTOM
+    Name:            linking
+    SymbolTable:
+      - Index:           0
+        Kind:            GLOBAL
+        Name:            weakGlobal
+        Flags:           [ UNDEFINED, BINDING_WEAK ]
+        Global:          0
+      - Index:           1
+        Kind:            FUNCTION
+        Name:            getWeakGlobal
+        Flags:           [  ]
+        Function:        0
Index: wasm/InputFiles.cpp
===================================================================
--- wasm/InputFiles.cpp
+++ wasm/InputFiles.cpp
@@ -81,10 +81,28 @@
     return Reloc.Addend;
   case R_WEBASSEMBLY_TYPE_INDEX_LEB:
     return TypeMap[Reloc.Index];
-  case R_WEBASSEMBLY_FUNCTION_INDEX_LEB:
-    return getFunctionSymbol(Reloc.Index)->getOutputIndex();
-  case R_WEBASSEMBLY_GLOBAL_INDEX_LEB:
-    return getGlobalSymbol(Reloc.Index)->getOutputIndex();
+  case R_WEBASSEMBLY_FUNCTION_INDEX_LEB: {
+    Symbol *Sym = getFunctionSymbol(Reloc.Index);
+    if (!Sym->hasOutputIndex()) {
+      // This happens if someone tries to use a `call` instruction to invoke a
+      // weak undefined function: if it's not found in the final link, then we
+      // must fail because there's nothing for the `call` to target.
+      fatal("Cannot generate relocation against undefined function: " +
+            toString(*Sym));
+    }
+    return Sym->getOutputIndex();
+  }
+  case R_WEBASSEMBLY_GLOBAL_INDEX_LEB: {
+    Symbol *Sym = getGlobalSymbol(Reloc.Index);
+    if (!Sym->hasOutputIndex()) {
+      // This happens if someone tries to use a `get/set_global` instruction to
+      // use a weak undefined global: if it's not found in the final link, then
+      // we must fail because there's nothing for the instruction to target.
+      fatal("Cannot generate relocation against undefined global: " +
+            toString(*Sym));
+    }
+    return Sym->getOutputIndex();
+  }
   default:
     llvm_unreachable("unknown relocation type");
   }