[5.1 06/12][CSApply] Don't try to transform editor placeholder if it's type is invalid

lib/Sema/CSApply.cpp

@@ -4041,6 +4041,19 @@ namespace {
       simplifyExprType(E);
       auto valueType = cs.getType(E);
 
+      // TODO(diagnostics): Once all of the diagnostics are moved to
+      // new diagnostics framework this check could be eliminated.
+      //
+      // Only way for this to happen is CSDiag try to re-typecheck
+      // sub-expression which contains this placeholder with
+      // `AllowUnresolvedTypeVariables` flag set.
+      //
+      // A better solution could be to replace placeholders with this
+      // implicit call early on and type-check that call together with
+      // the rest of the constraint system.
+      if (valueType->hasUnresolvedType())
+        return nullptr;
+
       auto &tc = cs.getTypeChecker();
       auto &ctx = tc.Context;
       // Synthesize a call to _undefined() of appropriate type.
@@ -7597,7 +7610,11 @@ namespace {
           ClosuresToTypeCheck.push_back(closure);
         }
 
-        tc.ClosuresWithUncomputedCaptures.push_back(closure);
+        // Don't try to register captures if constraint system is used to
+        // produce diagnostics for one of the sub-expressions.
+        if (!cs.Options.contains(
+                ConstraintSystemFlags::SubExpressionDiagnostics))
+          tc.ClosuresWithUncomputedCaptures.push_back(closure);
 
         return { false, closure };
       }

lib/Sema/ConstraintSystem.h

@@ -838,6 +838,14 @@ enum class ConstraintSystemFlags {
   /// If set, the top-level expression may be able to provide an underlying
   /// type for the contextual opaque archetype.
   UnderlyingTypeForOpaqueReturnType = 0x40,
+
+  /// FIXME(diagnostics): Once diagnostics are completely switched to new
+  /// framework, this flag could be removed as obsolete.
+  ///
+  /// If set, this identifies constraint system as being used to re-typecheck
+  /// one of the sub-expressions as part of the expression diagnostics, which
+  /// is attempting to narrow down failure location.
+  SubExpressionDiagnostics = 0x80,
 };
 
 /// Options that affect the constraint system as a whole.

lib/Sema/TypeCheckConstraints.cpp

@@ -2190,6 +2190,9 @@ Type TypeChecker::typeCheckExpressionImpl(Expr *&expr, DeclContext *dc,
   if (options.contains(TypeCheckExprFlags::ConvertTypeIsOpaqueReturnType))
     csOptions |= ConstraintSystemFlags::UnderlyingTypeForOpaqueReturnType;
 
+  if (options.contains(TypeCheckExprFlags::SubExpressionDiagnostics))
+    csOptions |= ConstraintSystemFlags::SubExpressionDiagnostics;
+
   ConstraintSystem cs(*this, dc, csOptions, expr);
   cs.baseCS = baseCS;
 

validation-test/compiler_crashers_2_fixed/0196-rdar48937223.swift

@@ -0,0 +1,11 @@
+// RUN: %target-typecheck-verify-swift
+
+protocol P {}
+
+func fn<T, U: P>(_ arg1: T, arg2: (T) -> U) {}
+
+func test(str: String) {
+  fn(str) { arg in
+    <#FOO#> // expected-error {{editor placeholder in source file}}
+  }
+}