Index: llvm/test/FileCheck/defines.txt
===================================================================
--- llvm/test/FileCheck/defines.txt
+++ llvm/test/FileCheck/defines.txt
@@ -3,16 +3,32 @@
 ;
 ; RUN: not FileCheck -DVALUE=10 -check-prefix NOT -input-file %s %s 2>&1 | FileCheck %s -check-prefix NOT-ERRMSG
 ; RUN: FileCheck -DVALUE=20 -check-prefix NOT -input-file %s %s
+; RUN: not FileCheck -DVALUE10 -input-file %s %s 2>&1 | FileCheck %s -check-prefix ERRCLIEQ1
+; RUN: not FileCheck -D -input-file %s %s 2>&1 | FileCheck %s -check-prefix ERRCLIEQ2
+; RUN: not FileCheck -D=10 -input-file %s %s 2>&1 | FileCheck %s -check-prefix ERRCLIVAR1
+; RUN: not FileCheck -D= -input-file %s %s 2>&1 | FileCheck %s -check-prefix ERRCLIVAR2
+; RUN: FileCheck -DVALUE= -check-prefix EMPTY -input-file %s %s 2>&1
 
 Value = 10
 ; CHECK: Value = [[VALUE]]
 ; NOT-NOT: Value = [[VALUE]]
 
-; ERRMSG: defines.txt:8:10: error: CHECK: expected string not found in input
+; ERRMSG: defines.txt:[[@LINE-3]]:10: error: CHECK: expected string not found in input
 ; ERRMSG: defines.txt:1:1: note: scanning from here
 ; ERRMSG: defines.txt:1:1: note: with variable "VALUE" equal to "20"
-; ERRMSG: defines.txt:7:1: note: possible intended match here
+; ERRMSG: defines.txt:[[@LINE-7]]:1: note: possible intended match here
 
-; NOT-ERRMSG: defines.txt:9:12: error: {{NOT}}-NOT: excluded string found in input
-; NOT-ERRMSG: defines.txt:7:1: note: found here
-; NOT-ERRMSG: defines.txt:7:1: note: with variable "VALUE" equal to "10"
\ No newline at end of file
+; NOT-ERRMSG: defines.txt:[[@LINE-7]]:12: error: {{NOT}}-NOT: excluded string found in input
+; NOT-ERRMSG: defines.txt:[[@LINE-10]]:1: note: found here
+; NOT-ERRMSG: defines.txt:[[@LINE-11]]:1: note: with variable "VALUE" equal to "10"
+
+; ERRCLIEQ1: Missing equal sign in command-line definition '-DVALUE10'
+
+; ERRCLIEQ2: FileCheck: for the -D option: requires a value!
+
+; ERRCLIVAR1: Missing pattern variable name in command-line definition '-D=10'
+
+; ERRCLIVAR2: Missing pattern variable name in command-line definition '-D='
+
+Empty value = @@
+; EMPTY: Empty value = @[[VALUE]]@
Index: llvm/utils/FileCheck/FileCheck.cpp
===================================================================
--- llvm/utils/FileCheck/FileCheck.cpp
+++ llvm/utils/FileCheck/FileCheck.cpp
@@ -51,9 +51,10 @@
              "this pattern occur which are not matched by a positive pattern"),
     cl::value_desc("pattern"));
 
-static cl::list<std::string> GlobalDefines("D", cl::Prefix,
-    cl::desc("Define a variable to be used in capture patterns."),
-    cl::value_desc("VAR=VALUE"));
+static cl::list<std::string>
+    GlobalDefines("D", cl::AlwaysPrefix,
+                  cl::desc("Define a variable to be used in capture patterns."),
+                  cl::value_desc("VAR=VALUE"));
 
 static cl::opt<bool> AllowEmptyInput(
     "allow-empty", cl::init(false),
@@ -523,8 +524,25 @@
   for (auto CheckNot : ImplicitCheckNot)
     Req.ImplicitCheckNot.push_back(CheckNot);
 
-  for (auto G : GlobalDefines)
+  bool GlobalDefineError = false;
+  for (auto G : GlobalDefines) {
+    size_t EqIdx = G.find('=');
+    if (EqIdx == std::string::npos) {
+      errs() << "Missing equal sign in command-line definition '-D" << G
+             << "'\n";
+      GlobalDefineError = true;
+      continue;
+    }
+    if (EqIdx == 0) {
+      errs() << "Missing pattern variable name in command-line definition '-D"
+             << G << "'\n";
+      GlobalDefineError = true;
+      continue;
+    }
     Req.GlobalDefines.push_back(G);
+  }
+  if (GlobalDefineError)
+    return 2;
 
   Req.AllowEmptyInput = AllowEmptyInput;
   Req.EnableVarScope = EnableVarScope;