[CS] Don't form conversion between switch subject and pattern

This is wrong because there's nowhere to put any
conversion that is introduced, meaning that we'll
likely crash in SILGen. Change the constraint to
equality, which matches what we do outside of the
constraint system.

rdar://107709341

Author: hamishknight

include/swift/AST/DiagnosticsSema.def

@@ -197,6 +197,9 @@ ERROR(cannot_match_expr_tuple_pattern_with_nontuple_value,none,
 ERROR(cannot_match_unresolved_expr_pattern_with_value,none,
       "pattern cannot match values of type %0",
       (Type))
+ERROR(cannot_match_value_with_pattern,none,
+      "pattern of type %1 cannot match %0",
+      (Type, Type))
 
 ERROR(cannot_reference_compare_types,none,
       "cannot check reference equality of functions; operands here have types "

lib/Sema/CSDiagnostics.cpp

@@ -3549,6 +3549,8 @@ ContextualFailure::getDiagnosticFor(ContextualTypePurpose context,
     return diag::cannot_convert_discard_value;
 
   case CTP_CaseStmt:
+    return diag::cannot_match_value_with_pattern;
+
   case CTP_ThrowStmt:
   case CTP_ForEachStmt:
   case CTP_ForEachSequence:

lib/Sema/CSSyntacticElement.cpp

@@ -657,8 +657,11 @@ class SyntacticElementConstraintGenerator
 
     // Convert the contextual type to the pattern, which establishes the
     // bindings.
-    cs.addConstraint(ConstraintKind::Conversion, context.getType(), patternType,
-                     locator);
+    auto *loc = cs.getConstraintLocator(
+        locator, {LocatorPathElt::PatternMatch(pattern),
+                  LocatorPathElt::ContextualType(context.purpose)});
+    cs.addConstraint(ConstraintKind::Equal, context.getType(), patternType,
+                     loc);
 
     // For any pattern variable that has a parent variable (i.e., another
     // pattern variable with the same name in the same case), require that

test/Constraints/rdar107709341.swift

@@ -0,0 +1,14 @@
+// RUN: %target-typecheck-verify-swift
+
+// rdar://107709341 – Make sure we don't crash.
+func foo(_ x: Int) {
+  // FIXME: We ought to have a better diagnostic
+  let _ = { // expected-error {{unable to infer closure type in the current context}}
+    switch x {
+    case Optional<Int>.some(x):
+      break
+    default:
+      break
+    }
+  }
+}