diff --git a/llvm/lib/Support/FileCheck.cpp b/llvm/lib/Support/FileCheck.cpp
--- a/llvm/lib/Support/FileCheck.cpp
+++ b/llvm/lib/Support/FileCheck.cpp
@@ -156,13 +156,19 @@
   // class instance of the corresponding numeric variable definition is stored
   // in GlobalNumericVariableTable in parsePattern. Therefore, the pointer we
   // get below is for the class instance corresponding to the last definition
-  // of this variable use.
+  // of this variable use. If we don't find a variable definition we create a
+  // dummy one so that parsing can continue. All uses of undefined variables,
+  // whether string or numeric, are then diagnosed in printSubstitutions()
+  // after failing to match.
   auto VarTableIter = Context->GlobalNumericVariableTable.find(Name);
-  if (VarTableIter == Context->GlobalNumericVariableTable.end())
-    return FileCheckErrorDiagnostic::get(
-        SM, Name, "using undefined numeric variable '" + Name + "'");
+  FileCheckNumericVariable *NumericVariable;
+  if (VarTableIter != Context->GlobalNumericVariableTable.end())
+    NumericVariable = VarTableIter->second;
+  else {
+    NumericVariable = Context->makeNumericVariable(0, Name);
+    Context->GlobalNumericVariableTable[Name] = NumericVariable;
+  }
 
-  FileCheckNumericVariable *NumericVariable = VarTableIter->second;
   if (!IsPseudo && NumericVariable->getDefLineNumber() == LineNumber)
     return FileCheckErrorDiagnostic::get(
         SM, Name,
diff --git a/llvm/test/FileCheck/numeric-expression.txt b/llvm/test/FileCheck/numeric-expression.txt
--- a/llvm/test/FileCheck/numeric-expression.txt
+++ b/llvm/test/FileCheck/numeric-expression.txt
@@ -76,9 +76,15 @@
 UNDEFVAR: 11
 UNDEF-USE-LABEL: UNDEF VAR USE
 UNDEF-USE-NEXT: UNDEFVAR: [[#UNDEFVAR]]
-UNDEF-USE-MSG: numeric-expression.txt:[[#@LINE-1]]:30: error: using undefined numeric variable 'UNDEFVAR'
+UNDEF-USE-MSG: numeric-expression.txt:[[#@LINE-1]]:17: error: {{U}}NDEF-USE-NEXT: expected string not found in input
 UNDEF-USE-MSG-NEXT: {{U}}NDEF-USE-NEXT: UNDEFVAR: {{\[\[#UNDEFVAR\]\]}}
-UNDEF-USE-MSG-NEXT: {{^                             \^$}}
+UNDEF-USE-MSG-NEXT: {{^                \^$}}
+UNDEF-USE-MSG-NEXT: numeric-expression.txt:[[#@LINE-6]]:1: note: scanning from here
+UNDEF-USE-MSG-NEXT: UNDEFVAR: 11
+UNDEF-USE-MSG-NEXT: {{^\^$}}
+UNDEF-USE-MSG-NEXT: numeric-expression.txt:[[#@LINE-9]]:1: note: uses undefined variable "UNDEFVAR"
+UNDEF-USE-MSG-NEXT: UNDEFVAR: 11
+UNDEF-USE-MSG-NEXT: {{^\^$}}
 
 ; Numeric expression with unsupported operator.
 RUN: not FileCheck -D#NUMVAR=10 --check-prefix INVAL-OP --input-file %s %s 2>&1 \
diff --git a/llvm/unittests/Support/FileCheckTest.cpp b/llvm/unittests/Support/FileCheckTest.cpp
--- a/llvm/unittests/Support/FileCheckTest.cpp
+++ b/llvm/unittests/Support/FileCheckTest.cpp
@@ -257,11 +257,11 @@
   // Unacceptable variable.
   EXPECT_TRUE(Tester.parseSubstExpect("10VAR"));
   EXPECT_TRUE(Tester.parseSubstExpect("@FOO"));
-  EXPECT_TRUE(Tester.parseSubstExpect("UNDEF"));
 
   // Only valid variable.
   EXPECT_FALSE(Tester.parseSubstExpect("@LINE"));
   EXPECT_FALSE(Tester.parseSubstExpect("FOO"));
+  EXPECT_FALSE(Tester.parseSubstExpect("UNDEF"));
 
   // Use variable defined on same line.
   EXPECT_FALSE(Tester.parsePatternExpect("[[#LINE1VAR:]]"));
@@ -471,9 +471,14 @@
   P = FileCheckPattern(Check::CheckPlain, &Cxt, 2);
   Expression = P.parseNumericSubstitutionBlock(LocalNumVarRef,
                                                DefinedNumericVariable, SM);
-  EXPECT_TRUE(errorToBool(Expression.takeError()));
+  EXPECT_TRUE(bool(Expression));
+  ExpressionVal = (*Expression)->eval();
+  EXPECT_TRUE(errorToBool(ExpressionVal.takeError()));
   EmptyVar = Cxt.getPatternVarValue(EmptyVarStr);
   EXPECT_TRUE(errorToBool(EmptyVar.takeError()));
+  // Clear again because parseNumericSubstitutionBlock would have created a
+  // dummy variable and stored it in GlobalNumericVariableTable.
+  Cxt.clearLocalVars();
 
   // Redefine global variables and check variables are defined again.
   GlobalDefines.emplace_back(std::string("$GlobalVar=BAR"));