Index: llvm/lib/Support/FileCheck.cpp
===================================================================
--- llvm/lib/Support/FileCheck.cpp
+++ llvm/lib/Support/FileCheck.cpp
@@ -151,7 +151,9 @@
   return Regex::escape(*VarVal);
 }
 
-bool Pattern::isValidVarNameStart(char C) { return C == '_' || isalpha(C); }
+bool Pattern::isValidVarNameStart(char C) {
+  return C == '_' || isalpha((unsigned char)C);
+}
 
 Expected<Pattern::VariableProperties>
 Pattern::parseVariable(StringRef &Str, const SourceMgr &SM) {
@@ -171,7 +173,7 @@
       return ErrorDiagnostic::get(SM, Str, "invalid variable name");
 
     // Variable names are composed of alphanumeric characters and underscores.
-    if (Str[I] != '_' && !isalnum(Str[I]))
+    if (Str[I] != '_' && !isalnum((unsigned char)Str[I]))
       break;
     ParsedOneChar = true;
   }
@@ -1079,7 +1081,7 @@
 }
 
 static bool IsPartOfWord(char c) {
-  return (isalnum(c) || c == '-' || c == '_');
+  return (isalnum((unsigned char)c) || c == '-' || c == '_');
 }
 
 Check::FileCheckType &Check::FileCheckType::setCount(int C) {
Index: llvm/test/FileCheck/bad-char.txt
===================================================================
--- /dev/null
+++ llvm/test/FileCheck/bad-char.txt
@@ -0,0 +1,36 @@
+# This file contains characters that render as spaces (at least for me in vim)
+# but are encoded as 160.  Each is indicated with a "^" on the following line.
+# FileCheck used to call functions like isalnum on each without casting to
+# unsigned char first, so it sign-extended beyond what unsigned char or EOF can
+# represent.  C says that has undefined behavior, and it has caused stack dumps
+# under Windows.
+
+ BEFORE-PREFIX:
+^
+AFTER-PREFIX :
+            ^
+BEFORE-VAR: [[ VAR:]]
+              ^
+AFTER-VAR: [[VAR :]]
+                ^
+
+RUN: %ProtectFileCheckOutput \
+RUN: not FileCheck -check-prefix=BEFORE-PREFIX %s < /dev/null 2>&1 | \
+RUN:   FileCheck -check-prefix=ERR-EMPTY-CHECK %s
+
+RUN: %ProtectFileCheckOutput \
+RUN: not FileCheck -check-prefix=AFTER-PREFIX %s < /dev/null 2>&1 | \
+RUN:   FileCheck -check-prefix=ERR-NO-CHECK %s
+
+RUN: %ProtectFileCheckOutput \
+RUN: not FileCheck -check-prefix=BEFORE-VAR %s < /dev/null 2>&1 | \
+RUN:   FileCheck -check-prefix=ERR-BAD-VAR %s
+
+RUN: %ProtectFileCheckOutput \
+RUN: not FileCheck -check-prefix=AFTER-VAR %s < /dev/null 2>&1 | \
+RUN:   FileCheck -check-prefix=ERR-BAD-STRING-VAR %s
+
+ERR-EMPTY-CHECK: error: found empty check string
+ERR-NO-CHECK: error: no check strings found
+ERR-BAD-VAR: error: invalid variable name
+ERR-BAD-STRING-VAR: error: invalid name in string variable definition