[Type checker] Stop coercing patterns while type-checking them.

Detangle the "type check" and "coerce" phases somewhat for pattern type
checking, as a precursor to making the former more functional.

Author: DougGregor

lib/Sema/TypeCheckConstraints.cpp

@@ -3045,7 +3045,6 @@ bool TypeChecker::typeCheckForEachBinding(DeclContext *dc, ForEachStmt *stmt) {
                                            InitType, options)) {
         return nullptr;
       }
-
       Stmt->setPattern(pattern);
 
       // Get the conformance of the sequence type to the Sequence protocol.

lib/Sema/TypeCheckPattern.cpp

@@ -741,23 +741,7 @@ bool TypeChecker::typeCheckPattern(Pattern *P, DeclContext *dc,
   case PatternKind::Typed: {
     auto resolution = TypeResolution::forContextual(dc);
     TypedPattern *TP = cast<TypedPattern>(P);
-    bool hadError = validateTypedPattern(resolution, TP, options);
-
-    // If we have unbound generic types, don't apply them below; instead,
-    // the caller will call typeCheckBinding() later.
-    if (P->getType()->hasUnboundGenericType())
-      return hadError;
-
-    Pattern *subPattern = TP->getSubPattern();
-    if (TypeChecker::coercePatternToType(subPattern, resolution, P->getType(),
-                                         options|TypeResolutionFlags::FromNonInferredPattern,
-                                         TP->getTypeLoc()))
-      hadError = true;
-    else {
-      TP->setSubPattern(subPattern);
-      TP->setType(subPattern->getType());
-    }
-    return hadError;
+    return validateTypedPattern(resolution, TP, options);
   }
 
   // A wildcard or name pattern cannot appear by itself in a context

lib/Sema/TypeCheckStorage.cpp

@@ -251,6 +251,15 @@ PatternBindingEntryRequest::evaluate(Evaluator &eval,
       binding->diagnose(diag::inferred_opaque_type,
                         binding->getInit(entryNumber)->getType());
     }
+  } else {
+    // Coerce the pattern to the computed type.
+    auto resolution = TypeResolution::forContextual(binding->getDeclContext());
+    if (TypeChecker::coercePatternToType(pattern, resolution,
+                                         pattern->getType(), options)) {
+      binding->setInvalid();
+      pattern->setType(ErrorType::get(Context));
+      return &pbe;
+    }
   }
 
   // If the pattern binding appears in a type or library file context, then

test/decl/var/variables.swift

@@ -107,6 +107,7 @@ func test21057425() -> (Int, Int) {
 
 // rdar://problem/21081340
 func test21081340() {
+  func foo() { }
   let (x: a, y: b): () = foo() // expected-error{{tuple pattern has the wrong length for tuple type '()'}}
 }