[CodeCompletion] Fix a crash in context type analysis for tuple expr

When completing inside tuple expressions, the context tuple type may
have fewer number of elements. In such cases, we cannot provide the
expected type.

rdar://problem/61668779

Author: rintaro

lib/IDE/ExprContextAnalysis.cpp

@@ -762,7 +762,9 @@ class ExprContextAnalyzer {
       unsigned Position = 0;
       bool HasName;
       if (getPositionInArgs(*DC, Parent, ParsedExpr, Position, HasName)) {
-        recordPossibleType(tupleT->getElementType(Position));
+        // The expected type may have fewer number of elements.
+        if (Position < tupleT->getNumElements())
+          recordPossibleType(tupleT->getElementType(Position));
       }
       break;
     }

test/IDE/complete_call_arg.swift

@@ -101,6 +101,8 @@
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=ARG_PARAMFLAG_IUO | %FileCheck %s -check-prefix=ARG_PARAMFLAG_IUO
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=ARG_PARAMFLAG_VARIADIC | %FileCheck %s -check-prefix=ARG_PARAMFLAG_VARIADIC
 
+// RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=TUPLEELEM_1 | %FileCheck %s -check-prefix=TUPLEELEM_1
+// RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=TUPLEELEM_2 | %FileCheck %s -check-prefix=TUPLEELEM_2
 
 var i1 = 1
 var i2 = 2
@@ -816,3 +818,14 @@ func testPamrameterFlags(_: Int, inoutArg: inout Int, autoclosureArg: @autoclosu
 // ARG_PARAMFLAG_VARIADIC-DAG: Pattern/ExprSpecific: {#variadicArg: Int...#}[#Int#];
 // ARG_PARAMFLAG_VARIADIC: End completions
 }
+
+func testTupleElement(arg: (SimpleEnum, SimpleEnum)) {
+  testTupleElement(arg: (.foo, .#^TUPLEELEM_1^#))
+// TUPLEELEM_1: Begin completions, 3 items
+// TUPLEELEM_1-DAG: Decl[EnumElement]/ExprSpecific/TypeRelation[Identical]:     foo[#SimpleEnum#]; name=foo
+// TUPLEELEM_1-DAG: Decl[EnumElement]/ExprSpecific/TypeRelation[Identical]:     bar[#SimpleEnum#]; name=bar
+// TUPLEELEM_1-DAG: Decl[EnumElement]/ExprSpecific/TypeRelation[Identical]:     baz[#SimpleEnum#]; name=baz
+// TUPLEELEM_1: End completions
+  testTupleElement(arg: (.foo, .bar, .#^TUPLEELEM_2^#))
+// TUPLEELEM_2-NOT: Begin completions
+}