Allow two trailing slashes, more tests

src/compiler/scala/tools/nsc/ast/parser/Scanners.scala

@@ -910,20 +910,14 @@ trait Scanners extends ScannersCommon {
           setStrVal()
           token = STRINGLIT
         }
-      } else if (ch == '\\') {
-        if (multiLine) {
-          putChar(ch)
-          nextRawChar()
-          getStringPart(multiLine)
-        } else {
+      } else if (ch == '\\' && !multiLine) {
+        putChar(ch)
+        nextRawChar()
+        if (ch == '"' || ch == '\\') {
           putChar(ch)
           nextRawChar()
-          if (ch == '"') {
-            putChar(ch)
-            nextRawChar()
-          }
-          getStringPart(multiLine)
         }
+        getStringPart(multiLine)
       } else if (ch == '$') {
         nextRawChar()
         if (ch == '$') {

test/files/neg/t6476.check

@@ -0,0 +1,4 @@
+t6476.scala:8: error: unclosed string literal
+  mimi"\"
+       ^
+1 error

test/files/neg/t6476.scala

@@ -0,0 +1,9 @@
+// only the last one doesn't parse
+class C {
+  mimi"""\ """
+  mimi"""\\"""
+  mimi"""\"""
+  mimi"\ "
+  mimi"\\"
+  mimi"\"
+}

test/files/neg/t6476b.check

@@ -0,0 +1,7 @@
+t6476b.scala:2: error: invalid escape at terminal index 0 in "\". Use \\ for literal \.
+  val sa = s"""\"""
+               ^
+t6476b.scala:4: error: invalid escape '\ ' not one of [\b, \t, \n, \f, \r, \\, \", \', \uxxxx] at index 0 in "\ ". Use \\ for literal \.
+  val sc = s"""\ """
+               ^
+2 errors

test/files/neg/t6476b.scala

@@ -0,0 +1,8 @@
+class C {
+  val sa = s"""\"""
+  val sb = s"""\\"""
+  val sc = s"""\ """
+  val ra = raw"""\"""
+  val rb = raw"""\\"""
+  val rc = raw"""\ """
+}

test/files/run/t6476.check

@@ -0,0 +1,9 @@
+"Hello", Alice
+"Hello", Alice
+"Hello", Alice
+"Hello", Alice
+\"Hello\", Alice
+\"Hello\", Alice
+\TILT\
+\TILT\
+\\TILT\\

test/files/run/t6476.scala

@@ -0,0 +1,17 @@
+object Test {
+  def main(args: Array[String]): Unit = {
+    val person = "Alice"
+    println(s"\"Hello\", $person")
+    println(s"""\"Hello\", $person""")
+
+    println(f"\"Hello\", $person")
+    println(f"""\"Hello\", $person""")
+
+    println(raw"\"Hello\", $person")
+    println(raw"""\"Hello\", $person""")
+
+    println(s"\\TILT\\")
+    println(f"\\TILT\\")
+    println(raw"\\TILT\\")
+  }
+}