[ConstraintSystem] Don't track captures if solver was in diagnostic mode

If solution application is attempted for one of the sub-expressions,
while diagnostics are trying to narrow down where the failure is
located, don't record that captures need to be computed for closures,
because that could fail later on as in such conditions expressions
are not guaranteed to have correct types (e.g. some types could be
set to "unresolved").

Author: xedin

lib/Sema/CSApply.cpp

@@ -7398,7 +7398,11 @@ 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

@@ -840,6 +840,14 @@ 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

@@ -2184,6 +2184,9 @@ 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;