Merge pull request #63762 from xedin/fix-init-via-coercion-performance

[CSGen] SE-0213: Literal initialization via coercion shouldn't try br…

Author: xedin

lib/Sema/CSGen.cpp

@@ -3284,10 +3284,21 @@ namespace {
       auto fromType = CS.getType(expr->getSubExpr());
       auto locator = CS.getConstraintLocator(expr);
 
-      // Add a conversion constraint for the direct conversion between
-      // types.
-      CS.addExplicitConversionConstraint(fromType, toType, RememberChoice,
-                                         locator);
+      // Literal initialization (e.g. `UInt32(0)`) doesn't require
+      // a conversion because the literal is supposed to assume the
+      // `to` type.
+      //
+      // `to` type could be a type variable if i.e. the repr is invalid,
+      // in such cases a slower conversion path is a better choice to
+      // let it be inferred from the context and/or from the literal itself.
+      if (expr->isLiteralInit() && !toType->isTypeVariableOrMember()) {
+        CS.addConstraint(ConstraintKind::Equal, fromType, toType, locator);
+      } else {
+        // Add a conversion constraint for the direct conversion between
+        // types.
+        CS.addExplicitConversionConstraint(fromType, toType, RememberChoice,
+                                           locator);
+      }
 
       // If the result type was declared IUO, add a disjunction for
       // bindings for the result of the coercion.

test/Constraints/init_literal_via_coercion.swift

@@ -0,0 +1,22 @@
+// RUN: %target-swift-frontend %s -typecheck -debug-constraints -swift-version 4 2>%t.err
+// RUN: %FileCheck %s < %t.err
+
+// SE-0213. `UInt32(0)` and similar expressions that get transformed into
+// `0 as <#Type#>` should get literal bound early via equality constraint.
+
+// CHECK: ---Constraint solving at [{{.*}}:12:1 - line:12:13]---
+// CHECK: (integer_literal_expr type='[[LITERAL_VAR:\$T[0-9]+]]' {{.*}}
+// CHECK: Type Variables:
+// CHECK: [[LITERAL_VAR]] [allows bindings to: {{.*}}] as UInt32 {{.*}}
+// CHECK-NOT: disjunction (remembered) \[\[locator@{{.*}} [Coerce@{{.*}}\]\]]:
+_ = UInt32(0)
+
+// CHECK: ---Constraint solving at [{{.*}}22:1 - line:22:13]---
+// CHECK: (coerce_expr implicit type='[[CAST_TYPE:\$T[0-9]+]]' {{.*}}
+// CHECK-NEXT: (nil_literal_expr type='[[LITERAL_VAR:\$T[0-9]+]]' {{.*}}
+// CHECK: Type Variables:
+// CHECK: [[LITERAL_VAR]] [allows bindings to: {{.*}}] as Int? {{.*}}
+// CHECK: disjunction (remembered) {{.*}}
+// CHECK-NEXT: >  [favored]  [[CAST_TYPE]] bind Int?
+// CHECK-NEXT: >             [[CAST_TYPE]] bind Int
+_ = Int!(nil)