[CSGen] Rework nil-coalescing operator handling in LinkedExprAnalyzer

Allow `LinkedExprAnalyzer` to capture `??` operator and walk into
its arguments because they could have valuable type information,
but don't attempt to favor or link operators if `??` is present in a chain.

Resolves: rdar://85277993

Author: xedin

lib/Sema/CSGen.cpp

@@ -213,14 +213,6 @@ namespace {
       }
 
       if (auto *binaryExpr = dyn_cast<BinaryExpr>(expr)) {
-        if (auto *overload = dyn_cast<OverloadedDeclRefExpr>(binaryExpr->getFn())) {
-          // Don't walk into nil coalescing operators. Attempting to favor
-          // based on operand types is wrong for this operator.
-          auto identifier = overload->getDecls().front()->getBaseIdentifier();
-          if (identifier.isNilCoalescingOperator())
-            return { false, expr };
-        }
-
         LTI.binaryExprs.push_back(dyn_cast<BinaryExpr>(expr));
       }  
 
@@ -292,11 +284,25 @@ namespace {
 
   /// For a given expression, given information that is global to the
   /// expression, attempt to derive a favored type for it.
-  bool computeFavoredTypeForExpr(Expr *expr, ConstraintSystem &CS) {
+  void computeFavoredTypeForExpr(Expr *expr, ConstraintSystem &CS) {
     LinkedTypeInfo lti;
 
     expr->walk(LinkedExprAnalyzer(lti, CS));
 
+    // Check whether we can proceed with favoring.
+    if (llvm::any_of(lti.binaryExprs, [](const BinaryExpr *op) {
+          auto *ODRE = dyn_cast<OverloadedDeclRefExpr>(op->getFn());
+          if (!ODRE)
+            return false;
+
+          // Attempting to favor based on operand types is wrong for
+          // nil-coalescing operator.
+          auto identifier = ODRE->getDecls().front()->getBaseIdentifier();
+          return identifier.isNilCoalescingOperator();
+        })) {
+      return;
+    }
+
     if (lti.collectedTypes.size() == 1) {
       // TODO: Compute the BCT.
 
@@ -371,13 +377,9 @@ namespace {
       };
 
       simplifyBinOpExprTyVars();
-
-      return true;
     }
-
-    return false;
   }
-  
+
   /// Determine whether the given parameter type and argument should be
   /// "favored" because they match exactly.
   bool isFavoredParamAndArg(ConstraintSystem &CS, Type paramTy, Type argTy,

validation-test/Sema/rdar85277993.swift

@@ -0,0 +1,12 @@
+// RUN: %target-typecheck-verify-swift
+// REQUIRES: objc_interop
+// REQUIRES: OS=macosx
+
+import simd
+import CoreGraphics
+
+let m = simd_float3x3(1)
+
+func foo(point: SIMD2<Float>, depth: Float, a: simd_float3x3? = nil) -> SIMD3<Float> {
+  (a ?? m) * float3(point.x, point.y, 1) * depth
+}