diff --git a/lib/Parse/Lexer.cpp b/lib/Parse/Lexer.cpp
index fef46453c959e..06a949bda90b5 100644
--- a/lib/Parse/Lexer.cpp
+++ b/lib/Parse/Lexer.cpp
@@ -1241,19 +1241,6 @@ static bool diagnoseZeroWidthMatchAndAdvance(char Target, const char *&CurPtr,
   return *CurPtr == Target && CurPtr++;
 }
 
-/// advanceIfMultilineDelimiter - Centralized check for multiline delimiter.
-static bool advanceIfMultilineDelimiter(const char *&CurPtr,
-                                        DiagnosticEngine *Diags) {
-  const char *TmpPtr = CurPtr;
-  if (*(TmpPtr - 1) == '"' &&
-      diagnoseZeroWidthMatchAndAdvance('"', TmpPtr, Diags) &&
-      diagnoseZeroWidthMatchAndAdvance('"', TmpPtr, Diags)) {
-    CurPtr = TmpPtr;
-    return true;
-  }
-  return false;
-}
-
 /// advanceIfCustomDelimiter - Extracts/detects any custom delimiter on
 /// opening a string literal, advances CurPtr if a delimiter is found and
 /// returns a non-zero delimiter length. CurPtr[-1] must be '#' when called.
@@ -1300,6 +1287,37 @@ static bool delimiterMatches(unsigned CustomDelimiterLen, const char *&BytesPtr,
   return true;
 }
 
+/// advanceIfMultilineDelimiter - Centralized check for multiline delimiter.
+static bool advanceIfMultilineDelimiter(unsigned CustomDelimiterLen,
+                                        const char *&CurPtr,
+                                        DiagnosticEngine *Diags,
+                                        bool IsOpening = false) {
+
+  // Test for single-line string literals that resemble multiline delimiter.
+  const char *TmpPtr = CurPtr + 1;
+  if (IsOpening && CustomDelimiterLen) {
+    while (*TmpPtr != '\r' && *TmpPtr != '\n') {
+      if (*TmpPtr == '"') {
+        if (delimiterMatches(CustomDelimiterLen, ++TmpPtr, nullptr)) {
+          return false;
+        }
+        continue;
+      }
+      ++TmpPtr;
+    }
+  }
+
+  TmpPtr = CurPtr;
+  if (*(TmpPtr - 1) == '"' &&
+      diagnoseZeroWidthMatchAndAdvance('"', TmpPtr, Diags) &&
+      diagnoseZeroWidthMatchAndAdvance('"', TmpPtr, Diags)) {
+    CurPtr = TmpPtr;
+    return true;
+  }
+
+  return false;
+}
+
 /// lexCharacter - Read a character and return its UTF32 code.  If this is the
 /// end of enclosing string/character sequence (i.e. the character is equal to
 /// 'StopQuote'), this returns ~0U and advances 'CurPtr' pointing to the end of
@@ -1342,7 +1360,8 @@ unsigned Lexer::lexCharacter(const char *&CurPtr, char StopQuote,
 
       DiagnosticEngine *D = EmitDiagnostics ? Diags : nullptr;
       auto TmpPtr = CurPtr;
-      if (IsMultilineString && !advanceIfMultilineDelimiter(TmpPtr, D))
+      if (IsMultilineString &&
+          !advanceIfMultilineDelimiter(CustomDelimiterLen, TmpPtr, D))
         return '"';
       if (CustomDelimiterLen &&
           !delimiterMatches(CustomDelimiterLen, TmpPtr, D, /*IsClosing=*/true))
@@ -1478,7 +1497,9 @@ static const char *skipToEndOfInterpolatedExpression(const char *CurPtr,
       if (!inStringLiteral()) {
         // Open string literal.
         OpenDelimiters.push_back(CurPtr[-1]);
-        AllowNewline.push_back(advanceIfMultilineDelimiter(CurPtr, nullptr));
+        AllowNewline.push_back(advanceIfMultilineDelimiter(CustomDelimiterLen,
+                                                           CurPtr, nullptr,
+                                                           true));
         CustomDelimiter.push_back(CustomDelimiterLen);
         continue;
       }
@@ -1490,7 +1511,8 @@ static const char *skipToEndOfInterpolatedExpression(const char *CurPtr,
         continue;
 
       // Multi-line string can only be closed by '"""'.
-      if (AllowNewline.back() && !advanceIfMultilineDelimiter(CurPtr, nullptr))
+      if (AllowNewline.back() &&
+          !advanceIfMultilineDelimiter(CustomDelimiterLen, CurPtr, nullptr))
         continue;
 
       // Check whether we have equivalent number of '#'s.
@@ -1827,7 +1849,8 @@ void Lexer::lexStringLiteral(unsigned CustomDelimiterLen) {
   // diagnostics about changing them to double quotes.
   assert((QuoteChar == '"' || QuoteChar == '\'') && "Unexpected start");
 
-  bool IsMultilineString = advanceIfMultilineDelimiter(CurPtr, Diags);
+  bool IsMultilineString = advanceIfMultilineDelimiter(CustomDelimiterLen,
+                                                       CurPtr, Diags, true);
   if (IsMultilineString && *CurPtr != '\n' && *CurPtr != '\r')
     diagnose(CurPtr, diag::lex_illegal_multiline_string_start)
         .fixItInsert(Lexer::getSourceLoc(CurPtr), "\n");
diff --git a/test/Parse/raw_string.swift b/test/Parse/raw_string.swift
index d07bbd24fa54f..cbf721ec9ee31 100644
--- a/test/Parse/raw_string.swift
+++ b/test/Parse/raw_string.swift
@@ -68,6 +68,49 @@ _ = ##"""
       """##
 // CHECK: "a raw string with \"\"\" in it"
 
+// ===---------- False Multiline Delimiters --------===
+
+/// Source code contains zero-width character in this format: `#"[U+200B]"[U+200B]"#`
+/// The check contains zero-width character in this format: `"[U+200B]\"[U+200B]"`
+/// If this check fails after you implement `diagnoseZeroWidthMatchAndAdvance`,
+/// then you may need to tweak how to test for single-line string literals that
+/// resemble a multiline delimiter in `advanceIfMultilineDelimiter` so that it
+/// passes again.
+/// See https://bugs.swift.org/browse/SR-8678
+_ = #"​"​"#
+// CHECK: "​\"​"
+
+_ = #""""#
+// CHECK: "\"\""
+
+_ = #"""""#
+// CHECK: "\"\"\""
+
+_ = #""""""#
+// CHECK: "\"\"\"\""
+
+_ = #"""#
+// CHECK: "\""
+
+_ = ##""" foo # "# "##
+// CHECK: "\"\" foo # \"# "
+
+_ = ###""" "# "## "###
+// CHECK: "\"\" \"# \"## "
+
+_ = ###"""##"###
+// CHECK: "\"\"##"
+
+_ = "interpolating \(#"""false delimiter"#)"
+// CHECK: "interpolating "
+// CHECK: "\"\"false delimiter"
+  
+_ = """
+  interpolating \(#"""false delimiters"""#)
+  """
+// CHECK: "interpolating "
+// CHECK: "\"\"false delimiters\"\""
+
 let foo = "Interpolation"
 _ = #"\b\b \#(foo)\#(foo) Kappa"#
 // CHECK: "\\b\\b "
diff --git a/test/Parse/raw_string_errors.swift b/test/Parse/raw_string_errors.swift
index 0ba693fc79f30..07e5a72cb8410 100644
--- a/test/Parse/raw_string_errors.swift
+++ b/test/Parse/raw_string_errors.swift
@@ -9,6 +9,11 @@ let _ = #"\##("invalid")"#
 // expected-error@-1{{too many '#' characters in delimited escape}}
 // expected-error@-2{{invalid escape sequence in literal}}
 
+let _ = ###"""invalid"######
+// expected-error@-1{{too many '#' characters in closing delimiter}}{{26-29=}}
+// expected-error@-2{{consecutive statements on a line must be separated by ';'}}
+// expected-error@-3{{expected expression}}
+
 let _ = ####"invalid"###
 // expected-error@-1{{unterminated string literal}}
 
@@ -17,8 +22,16 @@ let _ = ###"invalid"######
 // expected-error@-2{{consecutive statements on a line must be separated by ';'}}
 // expected-error@-3{{expected expression}}
 
-let _ = ##"""##
+let _ = ##"""aa
   foobar
-  ##"""##
+  aa"""##
 // expected-error@-3{{multi-line string literal content must begin on a new line}}{{14-14=\n}}
 // expected-error@-2{{multi-line string literal closing delimiter must begin on a new line}}{{5-5=\n}}
+
+let _ = #""" foo "bar" #baz
+  """#
+// expected-error@-2{{multi-line string literal content must begin on a new line}}{{13-13=\n}}
+
+let _ = ###""" "# "##
+  """###
+// expected-error@-2{{multi-line string literal content must begin on a new line}}{{15-15=\n}}