[Fixit] Provide fixit when converting expressions of type T to [T] or UnsafePointer<T>.

Author: nkcsgexi

lib/Sema/CSDiag.cpp

@@ -3477,6 +3477,24 @@ bool FailureDiagnosis::diagnoseContextualConversionError() {
     return true;
   }
 
+  // When converting from T to [T] or UnsafePointer<T>, we can offer fixit to wrap
+  // the expr with brackets.
+  if (auto *contextDecl = contextualType->getAnyNominal()) {
+    if (contextDecl == CS->TC.Context.getArrayDecl() ||
+        contextDecl == CS->TC.Context.getUnsafePointerDecl()) {
+      SmallVector<Type, 4> scratch;
+      for (Type arg : contextualType->getAllGenericArgs(scratch)) {
+        if (arg->isEqual(exprType)) {
+          diagnose(expr->getLoc(), diagID, exprType, contextualType).
+            fixItInsert(expr->getStartLoc(), "[").fixItInsert(
+              Lexer::getLocForEndOfToken(CS->TC.Context.SourceMgr,
+                                         expr->getEndLoc()), "]");
+          return true;
+        }
+      }
+    }
+  }
+
   // When complaining about conversion to a protocol type, complain about
   // conformance instead of "conversion".
   if (contextualType->is<ProtocolType>() ||

test/Sema/diag_type_conversion.swift

@@ -0,0 +1,8 @@
+// RUN: %target-parse-verify-swift
+
+func foo1(_ a: [Int]) {}
+func foo2(_ a : UnsafePointer<Int>) {}
+func foo3 () {
+  foo1(1) // expected-error {{cannot convert value of type 'Int' to expected argument type '[Int]'}} {{8-8=[}} {{9-9=]}}
+  foo2(1) // expected-error {{cannot convert value of type 'Int' to expected argument type 'UnsafePointer<Int>'}} {{8-8=[}} {{9-9=]}}
+}