Rename CharSequence.get to charAt

This is done for JVM interoperability. There's still a member function
String.get() and an extension function "get" on CharSequence

 #KT-1730 Fixed
 #KT-5389 Fixed

compiler/testData/builtin-classes.txt

@@ -294,7 +294,7 @@ public final class CharRange : kotlin.Range<kotlin.Char>, kotlin.Progression<kot
 }
 
 public trait CharSequence {
-    public abstract fun get(/*0*/ index: kotlin.Int): kotlin.Char
+    public abstract fun charAt(/*0*/ index: kotlin.Int): kotlin.Char
     public abstract fun length(): kotlin.Int
 }
 
@@ -1428,8 +1428,9 @@ public final class ShortRange : kotlin.Range<kotlin.Short>, kotlin.Progression<k
 
 public final class String : kotlin.Comparable<kotlin.String>, kotlin.CharSequence {
     /*primary*/ public constructor String()
+    public open override /*1*/ fun charAt(/*0*/ index: kotlin.Int): kotlin.Char
     public open override /*1*/ fun compareTo(/*0*/ other: kotlin.String): kotlin.Int
-    public open override /*1*/ fun get(/*0*/ index: kotlin.Int): kotlin.Char
+    public final fun get(/*0*/ index: kotlin.Int): kotlin.Char
     public open override /*1*/ fun length(): kotlin.Int
     public final fun plus(/*0*/ other: kotlin.Any?): kotlin.String
 }

compiler/testData/codegen/boxWithStdlib/strings/kt5389_stringBuilderGet.kt

@@ -0,0 +1,4 @@
+fun box(): String {
+    val sb = StringBuilder("OK")
+    return "${sb.get(0)}${sb[1]}"
+}

compiler/testData/diagnostics/tests/MergePackagesWithJava.txt

@@ -4473,7 +4473,7 @@ package java {
             public open fun append(/*0*/ p0: kotlin.String!): java.lang.AbstractStringBuilder!
             public open fun appendCodePoint(/*0*/ p0: kotlin.Int): java.lang.AbstractStringBuilder!
             public open fun capacity(): kotlin.Int
-            public open fun charAt(/*0*/ p0: kotlin.Int): kotlin.Char
+            public open override /*1*/ fun charAt(/*0*/ index: kotlin.Int): kotlin.Char
             public open fun codePointAt(/*0*/ p0: kotlin.Int): kotlin.Int
             public open fun codePointBefore(/*0*/ p0: kotlin.Int): kotlin.Int
             public open fun codePointCount(/*0*/ p0: kotlin.Int, /*1*/ p1: kotlin.Int): kotlin.Int
@@ -7971,7 +7971,7 @@ package java {
             private final var hash: kotlin.Int
             private final var hash32: kotlin.Int
             private final val value: kotlin.CharArray!
-            public open fun charAt(/*0*/ p0: kotlin.Int): kotlin.Char
+            public open override /*1*/ fun charAt(/*0*/ index: kotlin.Int): kotlin.Char
             public open fun codePointAt(/*0*/ p0: kotlin.Int): kotlin.Int
             public open fun codePointBefore(/*0*/ p0: kotlin.Int): kotlin.Int
             public open fun codePointCount(/*0*/ p0: kotlin.Int, /*1*/ p1: kotlin.Int): kotlin.Int
@@ -8084,7 +8084,7 @@ package java {
             public open override /*1*/ fun append(/*0*/ p0: kotlin.String!): java.lang.StringBuffer!
             public open override /*1*/ fun appendCodePoint(/*0*/ p0: kotlin.Int): java.lang.StringBuffer!
             public open override /*1*/ fun capacity(): kotlin.Int
-            public open override /*1*/ fun charAt(/*0*/ p0: kotlin.Int): kotlin.Char
+            public open override /*2*/ fun charAt(/*0*/ index: kotlin.Int): kotlin.Char
             public open override /*1*/ fun codePointAt(/*0*/ p0: kotlin.Int): kotlin.Int
             public open override /*1*/ fun codePointBefore(/*0*/ p0: kotlin.Int): kotlin.Int
             public open override /*1*/ fun codePointCount(/*0*/ p0: kotlin.Int, /*1*/ p1: kotlin.Int): kotlin.Int
@@ -8155,7 +8155,7 @@ package java {
             public open override /*1*/ fun append(/*0*/ p0: kotlin.String!): java.lang.StringBuilder!
             public open override /*1*/ fun appendCodePoint(/*0*/ p0: kotlin.Int): java.lang.StringBuilder!
             public open override /*1*/ /*fake_override*/ fun capacity(): kotlin.Int
-            public open override /*1*/ /*fake_override*/ fun charAt(/*0*/ p0: kotlin.Int): kotlin.Char
+            public open override /*2*/ /*fake_override*/ fun charAt(/*0*/ index: kotlin.Int): kotlin.Char
             public open override /*1*/ /*fake_override*/ fun codePointAt(/*0*/ p0: kotlin.Int): kotlin.Int
             public open override /*1*/ /*fake_override*/ fun codePointBefore(/*0*/ p0: kotlin.Int): kotlin.Int
             public open override /*1*/ /*fake_override*/ fun codePointCount(/*0*/ p0: kotlin.Int, /*1*/ p1: kotlin.Int): kotlin.Int

compiler/testData/diagnostics/tests/cast/neverSucceeds/MappedSubtypes.txt

@@ -14,8 +14,8 @@ internal final class JSub : java.lang.CharSequence {
 
 internal final class Sub : kotlin.CharSequence {
     public constructor Sub()
+    public abstract override /*1*/ /*fake_override*/ fun charAt(/*0*/ index: kotlin.Int): kotlin.Char
     public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
-    public abstract override /*1*/ /*fake_override*/ fun get(/*0*/ index: kotlin.Int): kotlin.Char
     public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
     public abstract override /*1*/ /*fake_override*/ fun length(): kotlin.Int
     public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String

compiler/testData/diagnostics/tests/j+k/kt1730_implementCharSequence.kt

@@ -0,0 +1,24 @@
+// KT-1730 Method which has been implemented by Java is recognized to be abstract.
+
+// FILE: C.java
+public class C implements java.lang.CharSequence {
+    @Override
+    public int length() {
+        return 3;
+    }
+    @Override
+    public char charAt(int index) {
+        return 48;
+    }
+    @Override
+    public CharSequence subSequence(int start, int end) {
+        return "ab";
+    }
+    @Override
+    public String toString() {
+        return "abc";
+    }
+}
+
+// FILE: T.kt
+class T : C()

compiler/testData/diagnostics/tests/j+k/kt1730_implementCharSequence.txt

@@ -0,0 +1,21 @@
+package
+
+public open class C : kotlin.CharSequence {
+    public constructor C()
+    java.lang.Override() public open override /*1*/ fun charAt(/*0*/ index: kotlin.Int): kotlin.Char
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    java.lang.Override() public open override /*1*/ fun length(): kotlin.Int
+    java.lang.Override() public open fun subSequence(/*0*/ start: kotlin.Int, /*1*/ end: kotlin.Int): kotlin.CharSequence!
+    java.lang.Override() public open override /*1*/ fun toString(): kotlin.String!
+}
+
+internal final class T : C {
+    public constructor T()
+    java.lang.Override() public open override /*1*/ /*fake_override*/ fun charAt(/*0*/ index: kotlin.Int): kotlin.Char
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    java.lang.Override() public open override /*1*/ /*fake_override*/ fun length(): kotlin.Int
+    java.lang.Override() public open override /*1*/ /*fake_override*/ fun subSequence(/*0*/ start: kotlin.Int, /*1*/ end: kotlin.Int): kotlin.CharSequence!
+    java.lang.Override() public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String!
+}

compiler/testData/diagnostics/tests/scopes/kt955.txt

@@ -4478,7 +4478,7 @@ package java {
             public open fun append(/*0*/ p0: kotlin.String!): java.lang.AbstractStringBuilder!
             public open fun appendCodePoint(/*0*/ p0: kotlin.Int): java.lang.AbstractStringBuilder!
             public open fun capacity(): kotlin.Int
-            public open fun charAt(/*0*/ p0: kotlin.Int): kotlin.Char
+            public open override /*1*/ fun charAt(/*0*/ index: kotlin.Int): kotlin.Char
             public open fun codePointAt(/*0*/ p0: kotlin.Int): kotlin.Int
             public open fun codePointBefore(/*0*/ p0: kotlin.Int): kotlin.Int
             public open fun codePointCount(/*0*/ p0: kotlin.Int, /*1*/ p1: kotlin.Int): kotlin.Int
@@ -7976,7 +7976,7 @@ package java {
             private final var hash: kotlin.Int
             private final var hash32: kotlin.Int
             private final val value: kotlin.CharArray!
-            public open fun charAt(/*0*/ p0: kotlin.Int): kotlin.Char
+            public open override /*1*/ fun charAt(/*0*/ index: kotlin.Int): kotlin.Char
             public open fun codePointAt(/*0*/ p0: kotlin.Int): kotlin.Int
             public open fun codePointBefore(/*0*/ p0: kotlin.Int): kotlin.Int
             public open fun codePointCount(/*0*/ p0: kotlin.Int, /*1*/ p1: kotlin.Int): kotlin.Int
@@ -8089,7 +8089,7 @@ package java {
             public open override /*1*/ fun append(/*0*/ p0: kotlin.String!): java.lang.StringBuffer!
             public open override /*1*/ fun appendCodePoint(/*0*/ p0: kotlin.Int): java.lang.StringBuffer!
             public open override /*1*/ fun capacity(): kotlin.Int
-            public open override /*1*/ fun charAt(/*0*/ p0: kotlin.Int): kotlin.Char
+            public open override /*2*/ fun charAt(/*0*/ index: kotlin.Int): kotlin.Char
             public open override /*1*/ fun codePointAt(/*0*/ p0: kotlin.Int): kotlin.Int
             public open override /*1*/ fun codePointBefore(/*0*/ p0: kotlin.Int): kotlin.Int
             public open override /*1*/ fun codePointCount(/*0*/ p0: kotlin.Int, /*1*/ p1: kotlin.Int): kotlin.Int
@@ -8160,7 +8160,7 @@ package java {
             public open override /*1*/ fun append(/*0*/ p0: kotlin.String!): java.lang.StringBuilder!
             public open override /*1*/ fun appendCodePoint(/*0*/ p0: kotlin.Int): java.lang.StringBuilder!
             public open override /*1*/ /*fake_override*/ fun capacity(): kotlin.Int
-            public open override /*1*/ /*fake_override*/ fun charAt(/*0*/ p0: kotlin.Int): kotlin.Char
+            public open override /*2*/ /*fake_override*/ fun charAt(/*0*/ index: kotlin.Int): kotlin.Char
             public open override /*1*/ /*fake_override*/ fun codePointAt(/*0*/ p0: kotlin.Int): kotlin.Int
             public open override /*1*/ /*fake_override*/ fun codePointBefore(/*0*/ p0: kotlin.Int): kotlin.Int
             public open override /*1*/ /*fake_override*/ fun codePointCount(/*0*/ p0: kotlin.Int, /*1*/ p1: kotlin.Int): kotlin.Int