Adjust unsuppressed error check and doc comments.

src/Compilers/Core/Portable/CommandLine/CommonCompiler.cs

@@ -560,18 +560,19 @@ internal bool ReportDiagnostics(IEnumerable<DiagnosticInfo> diagnostics, TextWri
             => ReportDiagnostics(diagnostics.Select(info => Diagnostic.Create(info)), consoleOutput, errorLoggerOpt);
 
         /// <summary>
-        /// Returns true if there are any diagnostics in the bag which have default severity error and are
-        /// not marked "suppressed". Note: does NOT do filtering, so it may return false if a
+        /// Returns true if there are any error diagnostics in the bag which cannot be suppressed and
+        /// are guaranteed to break the build.
+        /// Only diagnostics which have default severity error and are tagged as NotConfigurable fall in this bucket.
+        /// This includes all compiler error diagnostics and specific analyzer error diagnostics that are marked as not configurable by the analyzer author.
+        /// Note: does NOT do filtering, so it may return false if a
         /// non-error diagnostic were later elevated to an error through filtering (e.g., through
-        /// warn-as-error). This is meant to be a check if there are any "real" errors, in the bag
-        /// since diagnostics with default "error" severity can never be suppressed or reduced
-        /// below error severity.
+        /// warn-as-error).
         /// </summary>
         internal static bool HasUnsuppressedErrors(DiagnosticBag diagnostics)
         {
             foreach (var diag in diagnostics.AsEnumerable())
             {
-                if (diag.DefaultSeverity == DiagnosticSeverity.Error && !diag.IsSuppressed)
+                if (diag.IsUnsuppressedError())
                 {
                     return true;
                 }

src/Compilers/Core/Portable/Compilation/Compilation.cs

@@ -1347,7 +1347,7 @@ internal void CompleteCompilationEventQueue_NoLock()
         /// </summary>
         /// <param name="accumulator">Bag to which filtered diagnostics will be added.</param>
         /// <param name="incoming">Diagnostics to be filtered.</param>
-        /// <returns>True if there were no errors.</returns>
+        /// <returns>True if there are no unsuppressed errors (i.e., no errors which fail compilation).</returns>
         internal bool FilterAndAppendAndFreeDiagnostics(DiagnosticBag accumulator, ref DiagnosticBag incoming)
         {
             bool result = FilterAndAppendDiagnostics(accumulator, incoming.AsEnumerableWithoutResolution(), exclude: null);
@@ -1378,8 +1378,7 @@ internal bool FilterAndAppendDiagnostics(DiagnosticBag accumulator, IEnumerable<
                 {
                     continue;
                 }
-                else if (filtered.DefaultSeverity == DiagnosticSeverity.Error &&
-                    !filtered.IsSuppressed)
+                else if (filtered.IsUnsuppressedError())
                 {
                     hasError = true;
                 }

src/Compilers/Core/Portable/Diagnostic/Diagnostic.cs

@@ -559,6 +559,14 @@ internal virtual bool IsNotConfigurable()
         {
             return AnalyzerManager.HasNotConfigurableTag(this.CustomTags);
         }
+
+        /// <summary>
+        /// Returns true if this is an error diagnostic which cannot be suppressed and is guaranteed to break the build.
+        /// Only diagnostics which have default severity error and are tagged as NotConfigurable fall in this bucket.
+        /// This includes all compiler error diagnostics and specific analyzer error diagnostics that are marked as not configurable by the analyzer author.
+        /// </summary>
+        internal bool IsUnsuppressedError()
+            => DefaultSeverity == DiagnosticSeverity.Error && IsNotConfigurable();
     }
 
     /// <summary>