diff --git a/llvm/docs/CodeGenerator.rst b/llvm/docs/CodeGenerator.rst
--- a/llvm/docs/CodeGenerator.rst
+++ b/llvm/docs/CodeGenerator.rst
@@ -2090,9 +2090,14 @@
 * On ppc32/64 GOT/PIC only module-local calls (visibility = hidden or protected)
   are supported.
 
-On WebAssembly, tail calls are lowered to ``return_call`` and
-``return_call_indirect`` instructions whenever the 'tail-call' target attribute
-is enabled.
+WebAssembly constraints:
+
+* No variable argument lists are used
+
+* The 'tail-call' target attribute is enabled.
+
+* The caller and callee's return types must match. The caller cannot
+  be void unless the callee is, too.
 
 Example:
 
diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp
--- a/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp
@@ -644,13 +644,36 @@
   if (CLI.IsPatchPoint)
     fail(DL, DAG, "WebAssembly doesn't support patch point yet");
 
-  // Fail if tail calls are required but not enabled
-  if (!Subtarget->hasTailCall()) {
-    if ((CallConv == CallingConv::Fast && CLI.IsTailCall &&
-         MF.getTarget().Options.GuaranteedTailCallOpt) ||
-        (CLI.CS && CLI.CS.isMustTailCall()))
-      fail(DL, DAG, "WebAssembly 'tail-call' feature not enabled");
-    CLI.IsTailCall = false;
+  if (CLI.IsTailCall) {
+    bool MustTail = CLI.CS && CLI.CS.isMustTailCall();
+    if (Subtarget->hasTailCall() && !CLI.IsVarArg) {
+      // Do not tail call unless caller and callee return types match
+      const Function &F = MF.getFunction();
+      const TargetMachine &TM = getTargetMachine();
+      Type *RetTy = F.getReturnType();
+      SmallVector<MVT, 4> CallerRetTys;
+      SmallVector<MVT, 4> CalleeRetTys;
+      computeLegalValueVTs(F, TM, RetTy, CallerRetTys);
+      computeLegalValueVTs(F, TM, CLI.RetTy, CalleeRetTys);
+      bool TypesMatch = CallerRetTys.size() == CalleeRetTys.size() &&
+                        std::equal(CallerRetTys.begin(), CallerRetTys.end(),
+                                   CalleeRetTys.begin());
+      if (!TypesMatch) {
+        // musttail in this case would be an LLVM IR validation failure
+        assert(!MustTail);
+        CLI.IsTailCall = false;
+      }
+    } else {
+      CLI.IsTailCall = false;
+      if (MustTail) {
+        if (CLI.IsVarArg) {
+          // The return would pop the argument buffer
+          fail(DL, DAG, "WebAssembly does not support varargs tail calls");
+        } else {
+          fail(DL, DAG, "WebAssembly 'tail-call' feature not enabled");
+        }
+      }
+    }
   }
 
   SmallVectorImpl<ISD::InputArg> &Ins = CLI.Ins;
diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyMCInstLower.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyMCInstLower.cpp
--- a/llvm/lib/Target/WebAssembly/WebAssemblyMCInstLower.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyMCInstLower.cpp
@@ -226,6 +226,17 @@
           if (WebAssembly::isCallIndirect(MI->getOpcode()))
             Params.pop_back();
 
+          // return_call_indirect instructions have the return type of the
+          // caller
+          if (MI->getOpcode() == WebAssembly::RET_CALL_INDIRECT) {
+            const Function &F = MI->getMF()->getFunction();
+            const TargetMachine &TM = MI->getMF()->getTarget();
+            Type *RetTy = F.getReturnType();
+            SmallVector<MVT, 4> CallerRetTys;
+            computeLegalValueVTs(F, TM, RetTy, CallerRetTys);
+            valTypesFromMVTs(CallerRetTys, Returns);
+          }
+
           auto *WasmSym = cast<MCSymbolWasm>(Sym);
           auto Signature = make_unique<wasm::WasmSignature>(std::move(Returns),
                                                             std::move(Params));
diff --git a/llvm/test/CodeGen/WebAssembly/tailcall.ll b/llvm/test/CodeGen/WebAssembly/tailcall.ll
--- a/llvm/test/CodeGen/WebAssembly/tailcall.ll
+++ b/llvm/test/CodeGen/WebAssembly/tailcall.ll
@@ -1,7 +1,8 @@
 ; RUN: llc < %s -asm-verbose=false -verify-machineinstrs -disable-wasm-fallthrough-return-opt -wasm-disable-explicit-locals -wasm-keep-registers -mattr=+tail-call | FileCheck --check-prefixes=CHECK,SLOW %s
 ; RUN: llc < %s -asm-verbose=false -verify-machineinstrs -disable-wasm-fallthrough-return-opt -wasm-disable-explicit-locals -wasm-keep-registers -fast-isel -mattr=+tail-call | FileCheck --check-prefixes=CHECK,FAST %s
+; RUN: llc < %s --filetype=obj -mattr=+tail-call | obj2yaml | FileCheck --check-prefix=YAML %s
 
-; Test that the tail-call attribute is accepted
+; Test that the tail calls lower correctly
 
 target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
 target triple = "wasm32-unknown-unknown"
@@ -124,6 +125,44 @@
   ret i32 %v
 }
 
+; CHECK-LABEL: mismatched_return_void:
+; CHECK: i32.call $drop=, baz, $pop{{[0-9]+}}, $pop{{[0-9]+}}, $pop{{[0-9]+}}{{$}}
+; CHECK: return{{$}}
+define void @mismatched_return_void() {
+  %v = tail call i32 @baz(i32 0, i32 42, i32 6)
+  ret void
+}
+
+; CHECK-LABEL: mismatched_return_f32:
+; CHECK: i32.call $push[[L:[0-9]+]]=, baz, $pop{{[0-9]+}}, $pop{{[0-9]+}}, $pop{{[0-9]+}}{{$}}
+; CHECK: f32.reinterpret_i32 $push[[L1:[0-9]+]]=, $pop[[L]]{{$}}
+; CHECK: return $pop[[L1]]{{$}}
+define float @mismatched_return_f32() {
+  %v = tail call i32 @baz(i32 0, i32 42, i32 6)
+  %u = bitcast i32 %v to float
+  ret float %u
+}
+
+; CHECK-LABEL: mismatched_indirect_void:
+; CHECK: i32.call_indirect $drop=, $0, $1, $2, $0{{$}}
+; CHECK: return{{$}}
+define void @mismatched_indirect_void(%fn %f, i32 %x, i32 %y) {
+  %p = extractvalue %fn %f, 0
+  %v = tail call i32 %p(%fn %f, i32 %x, i32 %y)
+  ret void
+}
+
+; CHECK-LABEL: mismatched_indirect_f32:
+; CHECK: i32.call_indirect $push[[L:[0-9]+]]=, $0, $1, $2, $0{{$}}
+; CHECK: f32.reinterpret_i32 $push[[L1:[0-9]+]]=, $pop[[L]]{{$}}
+; CHECK: return $pop[[L1]]{{$}}
+define float @mismatched_indirect_f32(%fn %f, i32 %x, i32 %y) {
+  %p = extractvalue %fn %f, 0
+  %v = tail call i32 %p(%fn %f, i32 %x, i32 %y)
+  %u = bitcast i32 %v to float
+  ret float %u
+}
+
 ; CHECK-LABEL: mismatched_byval:
 ; CHECK: i32.store
 ; CHECK: return_call quux, $pop{{[0-9]+}}{{$}}
@@ -135,13 +174,59 @@
 
 ; CHECK-LABEL: varargs:
 ; CHECK: i32.store
-; CHECK: return_call var, $1{{$}}
+; CHECK: i32.call $0=, var, $1{{$}}
+; CHECK: return $0{{$}}
 declare i32 @var(...)
 define i32 @varargs(i32 %x) {
   %v = tail call i32 (...) @var(i32 %x)
   ret i32 %v
 }
 
+; Type transformations inhibit tail calls, even when they are nops
+
+; CHECK-LABEL: mismatched_return_zext:
+; CHECK: i32.call
+define i32 @mismatched_return_zext() {
+  %v = tail call i1 @foo(i1 1)
+  %u = zext i1 %v to i32
+  ret i32 %u
+}
+
+; CHECK-LABEL: mismatched_return_sext:
+; CHECK: i32.call
+define i32 @mismatched_return_sext() {
+  %v = tail call i1 @foo(i1 1)
+  %u = sext i1 %v to i32
+  ret i32 %u
+}
+
+; CHECK-LABEL: mismatched_return_trunc:
+; CHECK: i32.call
+declare i32 @int()
+define i1 @mismatched_return_trunc() {
+  %v = tail call i32 @int()
+  %u = trunc i32 %v to i1
+  ret i1 %u
+}
+
+
+
+; Check that the signatures generated for external indirectly
+; return-called functions include the proper return types
+
+; YAML-LABEL: - Index:           8
+; YAML-NEXT:    ReturnType:      I32
+; YAML-NEXT:    ParamTypes:
+; YAML-NEXT:      - I32
+; YAML-NEXT:      - F32
+; YAML-NEXT:      - I64
+; YAML-NEXT:      - F64
+define i32 @unique_caller(i32 (i32, float, i64, double)** %p) {
+  %f = load i32 (i32, float, i64, double)*, i32 (i32, float, i64, double)** %p
+  %v = tail call i32 %f(i32 0, float 0., i64 0, double 0.)
+  ret i32 %v
+}
+
 ; CHECK-LABEL: .section .custom_section.target_features
 ; CHECK-NEXT: .int8 1
 ; CHECK-NEXT: .int8 43