[Diag] Fixit when trying to pattern match Optional and non-Optional

If the variable to be matched is an optional and the match partner is
non-optional, provide a fixit that inserts a '?' after the match partner

Author: ahoppen

lib/Sema/CSDiag.cpp

@@ -4558,10 +4558,16 @@ bool FailureDiagnosis::visitApplyExpr(ApplyExpr *callExpr) {
 
     if (callExpr->isImplicit() && overloadName == "~=") {
       // This binop was synthesized when typechecking an expression pattern.
-      diagnose(lhsExpr->getLoc(),
-               diag::cannot_match_expr_pattern_with_value, lhsType, rhsType)
-        .highlight(lhsExpr->getSourceRange())
-        .highlight(rhsExpr->getSourceRange());
+      auto diag = diagnose(lhsExpr->getLoc(),
+                    diag::cannot_match_expr_pattern_with_value,
+                           lhsType, rhsType);
+      diag.highlight(lhsExpr->getSourceRange());
+      diag.highlight(rhsExpr->getSourceRange());
+      if (auto optUnwrappedType = rhsType->getOptionalObjectType()) {
+        if (lhsType->isEqual(optUnwrappedType)) {
+          diag.fixItInsert(lhsExpr->getEndLoc(), "?");
+        }
+      }
       return true;
     }
 

test/expr/unary/selector/selector.swift

@@ -126,7 +126,7 @@ case #selector(C1.method1)?:
 }
 
 switch optionalSel {
-case #selector(SR1827.bar): // expected-error{{expression pattern of type 'Selector' cannot match values of type 'Selector?'}}
+case #selector(SR1827.bar): // expected-error{{expression pattern of type 'Selector' cannot match values of type 'Selector?'}} {{26-26=?}}
   break
 case #selector(SR1827.bar)!: // expected-error{{cannot force unwrap value of non-optional type 'Selector'}}
   break