Correct offset for single-string interpolation

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -1245,12 +1245,16 @@ object Parsers {
               }
             })
 
-      if (in.token == STRINGPART)
-        nextSegment(in.offset + (if (isTripleQuoted) 3 else 1))
+      var offsetCorrection = if isTripleQuoted then 3 else 1
+      def offset = {
+        val result = in.offset + offsetCorrection
+        offsetCorrection = 0
+        result
+      }
       while (in.token == STRINGPART)
-        nextSegment(in.offset)
+        nextSegment(offset)
       if (in.token == STRINGLIT)
-        segmentBuf += literal(inPattern = inPattern, negOffset = in.offset, inStringInterpolation = true)
+        segmentBuf += literal(inPattern = inPattern, negOffset = offset, inStringInterpolation = true)
 
       InterpolatedString(interpolator, segmentBuf.toList)
     }

compiler/test/dotty/tools/dotc/parsing/PositionTest.scala

@@ -0,0 +1,43 @@
+package dotty.tools
+package dotc
+package parsing
+
+import ast.untpd._
+import org.junit.Test
+
+class PositionTest extends ParserTest {
+
+  val tq = "\"\"\""
+  val program = s"""
+    |class A {
+    |  val expr = 42
+    |  val s0 = s"string1"
+    |  val s1 = s"string1$${expr}string2"
+    |  val s2 = s"string1$${expr}string2$${expr}string3"
+    |  val s0m = s${tq}string1${tq}
+    |  val s1m = s${tq}string1$${expr}string2${tq}
+    |  val s2m = s${tq}string1$${expr}string2$${expr}string3${tq}
+    |}""".stripMargin
+
+  @Test
+  def interpolationLiteralPosition: Unit = {
+    val t = parseText(program)
+    t match {
+      case PackageDef(_, List(TypeDef(_, Template(_, _, _, statements: List[Tree])))) => {
+        val interpolations = statements.collect{ case ValDef(_, _, InterpolatedString(_, int)) => int }
+        val lits = interpolations.flatten.flatMap {
+          case l @ Literal(_) => List(l)
+          case Thicket(trees) => trees.collect { case l @ Literal(_) => l }
+        }
+        for {
+          lit <- lits
+          Literal(c) = lit
+          str <- List(c.value).collect { case str: String => str}
+        } {
+          val fromPos = program.substring(lit.span.start, lit.span.end)
+          assert(fromPos == str, s"$fromPos == $str")
+        }
+      }
+    }
+  }
+}