Report nullness warnings for nullable collections in foreach.

Fixes:#31503

src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs

@@ -2593,6 +2593,14 @@ private TypeWithState VisitCallReceiver(BoundCall node)
                 {
                     checkNullableValueType = true;
                 }
+                else if (method.OriginalDefinition == compilation.GetSpecialTypeMember(SpecialMember.System_Nullable_T_get_Value))
+                {
+                    // call to get_Value may not occur directly in source, but may be inserted as a result of premature lowering. 
+                    // One example where we do it is foreach with nullables. 
+                    // The reason is Dev10 compatibility (see: UnwrapCollectionExpressionIfNullable in ForEachLoopBinder.cs)
+                    // Regardless of the reasons, we know that the method does not tolerate nulls.
+                    checkNullableValueType = true;
+                }
 
                 // https://github.com/dotnet/roslyn/issues/30598: Mark receiver as not null
                 // after arguments have been visited, and only if the receiver has not changed.

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

@@ -79781,8 +79781,15 @@ static void F1(S? s)
     }
 }";
             var comp = CreateCompilation(source, options: WithNonNullTypesTrue());
-            // https://github.com/dotnet/roslyn/issues/31503: Report warning for `.Value.GetEnumerator()` calls.
-            comp.VerifyDiagnostics();
+
+            comp.VerifyDiagnostics(
+                // (11,27): warning CS8629: Nullable value type may be null.
+                //         foreach (var i in s) // 1
+                Diagnostic(ErrorCode.WRN_NullableValueTypeMayBeNull, "s").WithLocation(11, 27),
+                // (18,27): warning CS8629: Nullable value type may be null.
+                //         foreach (var i in t) // 2
+                Diagnostic(ErrorCode.WRN_NullableValueTypeMayBeNull, "t").WithLocation(18, 27)
+            );
         }
 
         [Fact]