Fix "path cost" calculation when computing the shortest path to the exit of a decision dag (#47407)

in the face of multiple when clauses, for the purpose of constructing the "not exhaustive" diagnostic.
Fixes #47164

src/Compilers/CSharp/Portable/Binder/PatternExplainer.cs

@@ -52,10 +52,11 @@ int distance(BoundDecisionDagNode x)
                     BoundEvaluationDecisionDagNode e => (distance(e.Next), e.Next),
                     BoundTestDecisionDagNode { Test: BoundDagNonNullTest _ } t when !nullPaths => (1 + distance(t.WhenTrue), t.WhenTrue),
                     BoundTestDecisionDagNode { Test: BoundDagExplicitNullTest _ } t when !nullPaths => (1 + distance(t.WhenFalse), t.WhenFalse),
-                    BoundTestDecisionDagNode t when distance(t.WhenTrue) is var trueDist && distance(t.WhenFalse) is var falseDist =>
-                        (trueDist <= falseDist) ? (1 + trueDist, t.WhenTrue) : (1 + falseDist, t.WhenFalse),
-                    BoundWhenDecisionDagNode w when distance(w.WhenTrue) is var trueDist && distance(w.WhenFalse) is var falseDist =>
-                        (trueDist <= falseDist) ? (1 + trueDist, w.WhenTrue) : (1 + falseDist + nodeCount, w.WhenFalse),
+                    BoundTestDecisionDagNode t when distance(t.WhenTrue) is var trueDist1 && distance(t.WhenFalse) is var falseDist1 =>
+                        (trueDist1 <= falseDist1) ? (1 + trueDist1, t.WhenTrue) : (1 + falseDist1, t.WhenFalse),
+                    BoundWhenDecisionDagNode w when distance(w.WhenTrue) is var trueDist2 && distance(w.WhenFalse) is var falseDist2 =>
+                        // add nodeCount to the distance if we need to flag that the path requires failure of a when clause
+                        (trueDist2 <= falseDist2) ? (1 + trueDist2, w.WhenTrue) : (1 + (falseDist2 < nodeCount ? nodeCount : 0) + falseDist2, w.WhenFalse),
                     // treat the endpoint as distance 1.
                     // treat other nodes as not on the path to the endpoint
                     _ => ((n == node) ? 1 : infinity, null),

src/Compilers/CSharp/Test/Semantic/Semantics/PatternSwitchTests.cs

@@ -3165,5 +3165,83 @@ void M()
                 Diagnostic(ErrorCode.ERR_SwitchCaseSubsumed, "false").WithLocation(49, 16)
                 );
         }
+
+        [Fact, WorkItem(47164, "https://github.com/dotnet/roslyn/issues/47164")]
+        public void MultipleWhenClausesToFailure_01()
+        {
+            var source =
+@"class Sample
+{
+    void M(int q)
+    {
+        _ = q switch
+        {
+            4 => 1,
+            5 => 2,
+            6 => 3,
+            int i when i % 2 == 1 => 4,
+            int i when i % 2 == 0 => 5,
+        };
+    }
+}";
+            var compilation = CreateCompilation(source, options: TestOptions.ReleaseDll);
+            compilation.VerifyEmitDiagnostics(
+                // (5,15): warning CS8846: The switch expression does not handle all possible values of its input type (it is not exhaustive). For example, the pattern '0' is not covered. However, a pattern with a 'when' clause might successfully match this value.
+                //         _ = q switch
+                Diagnostic(ErrorCode.WRN_SwitchExpressionNotExhaustiveWithWhen, "switch").WithArguments("0").WithLocation(5, 15)
+                );
+        }
+
+        [Fact, WorkItem(47164, "https://github.com/dotnet/roslyn/issues/47164")]
+        public void MultipleWhenClausesToFailure_02()
+        {
+            var source =
+@"class Sample
+{
+    void M(int q)
+    {
+        _ = q switch
+        {
+            4 => 1,
+            5 => 2,
+            6 => 3,
+            int i when i % 2 == 1 => 4,
+        };
+    }
+}";
+            var compilation = CreateCompilation(source, options: TestOptions.ReleaseDll);
+            compilation.VerifyEmitDiagnostics(
+                // (5,15): warning CS8846: The switch expression does not handle all possible values of its input type (it is not exhaustive). For example, the pattern '0' is not covered. However, a pattern with a 'when' clause might successfully match this value.
+                //         _ = q switch
+                Diagnostic(ErrorCode.WRN_SwitchExpressionNotExhaustiveWithWhen, "switch").WithArguments("0").WithLocation(5, 15)
+                );
+        }
+
+        [Fact, WorkItem(47164, "https://github.com/dotnet/roslyn/issues/47164")]
+        public void MultipleWhenClausesToFailure_03()
+        {
+            var source =
+@"class Sample
+{
+    void M(int q)
+    {
+        _ = q switch
+        {
+            4 => 1,
+            5 => 2,
+            6 => 3,
+            int i when i % 3 == 0 => 4,
+            int i when i % 3 == 1 => 5,
+            int i when i % 3 == 2 => 6,
+        };
+    }
+}";
+            var compilation = CreateCompilation(source, options: TestOptions.ReleaseDll);
+            compilation.VerifyEmitDiagnostics(
+                // (5,15): warning CS8846: The switch expression does not handle all possible values of its input type (it is not exhaustive). For example, the pattern '0' is not covered. However, a pattern with a 'when' clause might successfully match this value.
+                //         _ = q switch
+                Diagnostic(ErrorCode.WRN_SwitchExpressionNotExhaustiveWithWhen, "switch").WithArguments("0").WithLocation(5, 15)
+                );
+        }
     }
 }