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

`addExplicitConversionConstraint` generates a disjunction to check
whether type could be coerced via bridging. That is not useful for
literal initialization which could use direct equality if the type
of cast is valid because necessary conformance checks have been
performed before the transformation.

(cherry picked from commit 45ebc56)

Author: xedin

lib/Sema/CSGen.cpp

@@ -2542,10 +2542,21 @@ 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)