[Lexer][SR-1724] Handle hex letters after '.' on hex number literal

The following case used to emit an error:

  extension Int {
    var asUiColor: UIColor { ... }
  }

  0xfff.asUiColor

Author: rintaro

lib/Parse/Lexer.cpp

@@ -862,8 +862,11 @@ void Lexer::lexHexNumber() {
   if (*CurPtr != '.' && *CurPtr != 'p' && *CurPtr != 'P')
     return formToken(tok::integer_literal, TokStart);
 
+  const char *PtrOnDot = nullptr;
+
   // (\.[0-9A-Fa-f][0-9A-Fa-f_]*)?
   if (*CurPtr == '.') {
+    PtrOnDot = CurPtr;
     ++CurPtr;
 
     // If the character after the '.' is not a digit, assume we have an int
@@ -876,6 +879,11 @@ void Lexer::lexHexNumber() {
     while (isHexDigit(*CurPtr) || *CurPtr == '_')
       ++CurPtr;
     if (*CurPtr != 'p' && *CurPtr != 'P') {
+      if (!isDigit(PtrOnDot[1])) {
+        // e.g: 0xff.description
+        CurPtr = PtrOnDot;
+        return formToken(tok::integer_literal, TokStart);
+      }
       diagnose(CurPtr, diag::lex_expected_binary_exponent_in_hex_float_literal);
       return formToken(tok::unknown, TokStart);
     }
@@ -885,10 +893,19 @@ void Lexer::lexHexNumber() {
   assert(*CurPtr == 'p' || *CurPtr == 'P' && "not at a hex float exponent?!");
   ++CurPtr;
 
-  if (*CurPtr == '+' || *CurPtr == '-')
+  bool signedExponent = false;
+  if (*CurPtr == '+' || *CurPtr == '-') {
     ++CurPtr;  // Eat the sign.
+    signedExponent = true;
+  }
 
   if (!isDigit(*CurPtr)) {
+    if (PtrOnDot && !isDigit(PtrOnDot[1]) && !signedExponent) {
+      // e.g: 0xff.fpValue, 0xff.fp
+      CurPtr = PtrOnDot;
+      return formToken(tok::integer_literal, TokStart);
+    }
+    // Note: 0xff.fp+otherExpr can be valid expression. But we don't accept it.
     diagnose(CurPtr, diag::lex_expected_digit_in_fp_exponent);
     return formToken(tok::unknown, TokStart);
   }

test/expr/expressions.swift

@@ -324,6 +324,12 @@ func tuple_of_rvalues(_ a:Int, b:Int) -> Int {
 extension Int {
   func testLexingMethodAfterIntLiteral() {}
   func _0() {}
+  // Hex letters + non hex.
+  func describe() {}
+  // Hex letters + 'p'.
+  func eap() {}
+  // Hex letters + 'p' + non hex.
+  func fpValue() {}
 }
 
 123.testLexingMethodAfterIntLiteral()
@@ -336,6 +342,10 @@ extension Int {
 0o123._0()
 0x1FFF._0()
 
+0x1FFF.describe()
+0x1FFF.eap()
+0x1FFF.fpValue()
+
 var separator1: Int = 1_
 var separator2: Int = 1_000
 var separator4: Int = 0b1111_0000_